061
Session Assigned: W&Th P
Title: Backbone Dynamics and Refined Solution Structure of the N-terminal Domain of DNA Polymerase B

Institute: University of CT Health Center, Farmington, CT
Keywords:
Abstract:
The N-terminal domain of DNA Polymerase B was originally characterized as a ssDNA binding domain. Recently, the N-terminal domain has been shown to catalyze the removal of 5-terminal deoxyribose 5-phosphate from abasic sites in duplex DNA. To further refine the solution structure additional multidimensional NOE experimnets were recorded. The new NOE data was analyzed along with previously recorded data interactively with the programs Xeasy and Dyana to rapidly determine additional NOE distance constraints and to calculate structures. The structures were then further refined using Xplor. A total of 1369 upper bound and 1075 lower bound distance restraints and 132 torsion angle restraints were used in the final calculation. The rmsd for heavy backbone atoms in the well ordered region of the protein (13-80) is 0.63 +/- 0.12 A and 1.28 +/- 0.18 A for all heavy atoms. The structure consists of four alfa-helices comprising residues 15-28, 35-47, 56-60, and 68-78 with an omega-loop connecting helices 1 and 2 and a hairpin turn connecting helices 3 and 4 which comprises a helix-hairpin-helix motif (HhH). The N- and C- termini are completly disordered.

In addition to the refined structure the backbone dynamics of the N-terminal domain have been analyzed by 2D ¹H-¹⁵N NMR experiments at both 500 and 600 MHz. Longitudinal (R₁) and transverse (R₂) relaxation rates and ¹H-¹⁵N NOEs for 76 of 86 residues were measured. These data were analyzed using the modelfree formalism assuming both isotropic and anisotropic tumbling to determine the generalized order parameter (S²), effective correlation time for internal motions (t_e), and ¹⁵N-linebroadening due to exchange contributions (R_ex), along with a global rotational correlation time (t_m). The data were also analyzed using reduced spectral density mapping to directly measure the spectral density function at J(0), J(w_N), and J(w_HN). A detailed analysis of the refined solution structure and backbone dynamics and their relationship to DNA binding and catalytic activity will be discussed.

062
Session Assigned: W&Th P
Title: Characterizing Heparin and Sucrose Octasulfate Binding to Hepatocyte Growth Factor Isoforms

Institute: ABL-BRP,NCI-FCRDC,Frederick,MD,USA
Keywords:
Abstract:
Hepatocyte growth factor (HGF) is a potent mitogen, motogen and morphogen. Aberrant expression and function of HGF is implicated in a number of human diseases, including cancer. Heparin or heparan sulfate, present in cell membrane or extra cellular matrix, significantly affects signal transduction by HGF through its receptor c-Met. We have identified a heparin-binding site in the amino (N-) terminal domain of HGF, and estimated its binding affinity to heparin. Protein backbone dynamics data indicate that the heparin-binding region is involved in conformational exchange on a microsecond to millisecond time scale. We also studied binding of a short heparin analog, sucrose octasulfate (SOS), to the N domain and a longer HGF isoform NK1. Our data indicate that SOS, like heparin, binds to a highly positively charged surface of the protein, and ligand binding reduces flexibility and increases the stability of the protein. Research sponsored in part by the National Cancer Institute, DHHS, under contract with ABL.

063
Session Assigned: W&Th P
Title: Motional Dynamics of the Catalytic Loop in OPRTase

Institute: Albert Einstein College of Medicine, Bronx, NY
Keywords:
Abstract:
Salmonella typhimurium orotate phosphoribosyltransferase (OPRTase) is a dimeric 2x213 residue enzyme involved in the biosynthesis of pyrimidine nucleotides by catalyzing the Mg²⁺-dependent formation of orotidine monophosphate (OMP) from orotic acid and phosphoribosyl pyrophosphate (PRPP). The enzyme is a target for chemotherapies aimed at slowing cell growth. Recent studies suggested that a flexible loop in the enzyme is involved in the catalytic mechanism by shielding the active site from solvent during catalysis. To better understand the temporal and spatial coupling between loop movement and catalysis, we are characterizing the dynamic and structural features of the loop in both the ligated and unligated forms of OPRTase. To date the backbone dyamics of the histidines in unligated OPRTase, one (H105) of which is located in the loop, have been characterized by measuring the ¹⁵N T₁ and T₂ relaxation times and ¹⁵N{¹H} NOEs on a [alpha-¹⁵N]histidine-labelled OPRTase sample. The data indicate that the backbone at the loop histidine possesses motions of significant amplitude on both the very fast and relatively slow time scales suggesting that the loop in the region of H105 is very flexible. Efforts to characterize the structure and dynamics of the entire loop in ligated and unligated forms of UL-¹⁵N OPRTase will also be discussed.

064
Session Assigned: W&Th P
Title: Solution Structure and Dynamics for Complexes of Calmodulin with Cognate Peptides of the Plasmamembrane Calciumpump

Institute: University of Winconsin-Milwaukee
Keywords:
Abstract:
Calmodulin (CaM) is an ubiquitous Ca²⁺-binding protein. It plays a pivotal role in the regulation of Ca²⁺-dependent processes in eucaryotic cells. CaM changes its conformation upon binding of Ca²⁺-ions and exposes hydrophobic patches for binding target proteins, and thus transduces signalling processes. The binding regions of some of the protein targets of CaM have been identified1 showing amphipathic peptide sequences that upon binding to CaM formed helices like M13 (which is a synthetic peptide derived from the binding domain of the myosin light chain kinase, MLCK) and C20W, C21W or C24W corresponding to versions of different length of the CaM-binding domain of the plasma membrane Ca²⁺-pump^2,3,4,5,6. CaM alone forms a dumbbell-like structure with a flexible linker connecting the C- and N-terminal halves. Upon binding of peptides, normally a collapsed globular structure is formed. As opposed to this we found that C20W to CaM binds only to the C-terminal domain of CaM whereas the N-terminal domain of the protein was hardly affected with the elongated structure of CaM remaining intact. The peptide C21W,on the other hand, appears to bind to both domains of calmodulin to yield a structure similar to the one of the complex with MLCK. The backbone dynamics of the CaM/C20W complex determined from ¹⁵N relaxation data allows to determine the degree of flexibility in the central helix compared with free CaM. CaM was expressed in E. coli. using minimal media, with 13C-labeled glucose and ¹⁵N-labeled ammoniumsulfate as the only carbon and nitrogen sources, respectively. C20W and C21W were synthesized and purified as described by Vorherr et al.⁷, and were complexed to CaM following the procedure described by Ikura et al⁸. Complete sequence assignments of the ¹³C,¹⁵N-labeled CaM/C20W and CaM/C21W peptide complexes were obtained using a number of different multinuclear NMR measurements and structural determination was achieved employing NOEs within CaM, the peptide and using interfacial NOEs. Dynamic parameters were obtained from ¹⁵N T₁ and T₂ relaxation time measurements and heteronuclear ¹H/¹⁵N NOE measurements using the program Modelfree⁹.

Literature:

1. P. James et al. J. Biol. Chem., 263, 2905, 1988;
2. D.B. Heidorn et al. Biochemistry, 28, 6757, 1989;
3. J. Trewhella et al. Biochemistry, 29, 9316, 1990;
4. T. Vorherr et al. Biochemistry 29, 355, 1990;
5. M. Kataoka et al. Biochemistry 30, 6247, 1991;
6. T. Vorherr et al. Biochemistry 31, 8245, 1992;
7. T. Vorherr et al. Biochemistry29, 355, 1990;
8. Ikura et al., Biochemistry 30,5498, 1991;
9. A.G. Palmer et al. J. Am, Chem. Soc. 113, 4371, 1991;

065
Session Assigned: W&Th P
Title: Temperature dependence of intramolecular dynamics of the basic leucine zipper of GCN4

Institute: Columbia University, New York, NY
Keywords:
Abstract:
The bZip domain of the yeast transcription factor GCN4 consists of a C-terminal leucine zipper and an N-terminal basic DNA-binding region that achieves a stable structure only after association with DNA. Backbone dynamics of a peptide encompassing the basic and leucine zipper bZip domain (residues 226-281) are described using NMR spectroscopy. Our results suggest that, for the basic region, lowering the temperature increases the populations of transient helical conformations and concomitantly reduces the amplitude or timescale of conformational fluctuations on picosecond-nanosecond timescales. Changes in the conformational dynamics of the peptide backbone of the basic region that accompany DNA binding contribute to the overall thermodynamics of complex formation. The change in backbone conformational entropy derived from NMR spin relaxation data agrees well with the result calculated from calorimetric measurements. Restriction of the conformational space accessible to the basic region may reduce significantly the entropic cost associated with formation of the basic region helices consequent to DNA binding.

066
Session Assigned: W&Th P
Title: Characterization of Molecular Motion Using 1-D 13C Chemical Shift and Dipolar-Chemical Shift Spectra

Institute: University of Michigan, Ann Arbor, MI
Keywords:
Abstract:
Molecular motions are extremely important in biological molecules, in particular, peptides and proteins. Solid State NMR has proven to be a powerful probe to investigate local as well as global motions in proteins. In this study, we used a simple 1-D technique to characterize motions in samples of metallocenes, beta-calcium formate, and several membrane-bound peptides. Using this technique, a chemical shift tensor is oriented in the molecular frame at low-temperature. Since chemical shift tensors are highly sensitive to molecular motions, their variation with increasing temperature was used to investigate the molecular dynamics of the sample. This work establishes the versatility of this approach and is a valuable step in probing the more complex motions that occur in proteins.

067
Session Assigned: W&Th P
Title: Solid State NMR Investigation of Spider Silk - Relations between Dynamics and Functions in Proteins

Institute: Cornell University, Louisiana State University
Keywords:
Abstract:
One major challenge for materials scientists has been to solve the puzzle of the naturally occurring silk fiber. In particular, the major ampullate gland silk of spider Nephila clavipes shrinks to less than half of its original length when exposed to water. This phenomenon is known as supercontraction. While the silk fibers combine exceptional strength with elasticity, any practical application of silk-like fibers will require a solution to the problem of supercontraction.

We have used solid state ²H NMR techniques to study spider silk selectively labeled with d₃-leucine (mthyl-deuterated) as a probe of the supercontraction phenomenon. ²H lineshape analysis allows us to investigate local dynamics in two distinct regions of the silk. Variable temperature experiments reveal that the energy barrier to reorient the leucine sidechain is much lower than that for similar modes in related materials. More striking is the change in lineshape observed upon exposure to water, i.e., in the supercontracted state. This change in dynamical behavior is confirmed by comparison to ¹³C MAS experiments.

Our results suggest that the amorphous regions containing leucine residues are essential for the unique properties of the spider dragline silk. In the hydrophilic region of the silk, these modes can be identified as with the supercontraction effect. We further suggest modifications that could be the basis for superfibers that are strong, elastic, environmentally friendly, not supercontracting, and therefore are open to a wealth of promising applications.

068
Session Assigned: W&Th P
Title: Design, Synthesis, and Characterization of a BPTI Core Module

Institute: Dept of Chem, Univ of Minnesota, Minneapolis, MN
Keywords:
Abstract:
We are exploring a new strategy for the design and construction of peptide fragments that can achieve defined, native-like secondary and folded tertiary structure. The strategy is based upon the hypothesis that "core elements" of a protein synthesized in a single, cross-linked polypeptide chain will favor native-like structure, and that native-like core structure will dominate the ensemble when more extended conformations are excluded. The slow exchange core is a collective term for the stretches of residues that bracket the slowest exchanging backbone amide protons in the native protein [Woodward, C. Is the slow exchange core the protein folding core? Trends in Biochemical Sciences 18, 359-360 (1993)]. "Core elements" are the elements of packed secondary structure that contain the slow exchange core. The core elements in BPTI are the two long strands of antiparallel beta-sheet (residues 18-24 and 19-35) and the small beta-bridge (residues 43-44). To test our design strategy, we synthesized a core module of BPTI, OXAD[14-38]. This peptide contains the antiparallel strands connected by a modified reverse turn, a natural disulfide cross-link at the open end of the hairpin, and N- and C-termini blocking groups. In the modified reverse turn, Ala-27 is replaced by Asp-27 to improve solubility and to stabilize a type I beta-turn. RDAD[14-38], a peptide with identical sequence but lacking the disulfide cross-link at the open end, has also been synthesized.

The conformational preferences of both peptides, OXAD[14-38] and RDAD[14-38], were examined using proton NMR spectroscopy; long-range NOEs, H/D amide exchange rates, and the C-alpha-H chemical shifts were compared. Spectra of OXAD[14-38] show significant, multiple, long-range NOEs involving residues in the strands and in the turn, slow amide exchange rates, and significant deviations from "random coil" chemical shift values. Compared to OXAD[14-38], the peptide RDAD[14-38] presents weak non-local NOEs among residues in, and close to, the turn, faster amide exchange rates, and smaller deviations from "random coil" chemical shift values for residues in the strands. These data provide evidence that the ensemble of conformations sampled by the OXAD[14-38] peptide are more collapsed and native-like than the RDAD[14-38].

Institute: Abteilung Grenzflächenphysik, Universität Leipzig
Keywords:
Abstract:
Oxygen-17 enriched zeolites Na-ZSM-5, Na-Y, ferrierite, silicalite, Na-A, LSX and the layer silicate Na-Ilerite were studied by MAS, MQ MAS, DOR, CP and REDOR NMR in the field of 17.6 T. It has been shown that multiple-quantum and double rotation techniques are complementary tools for getting highly-resolved ¹⁷O NMR spectra of solids. SiOSi and SiOAl bonds are studied in zeolites Na-ZSM-5, ferrierite and Na-Y, whereas zeolite Na-A and LSX contain only SiOAl bonds. For hydrated zeolite Na-A, the spectrum was fully resolved into three lines by DOR and 5Q MAS. The values of the isotropic chemical shift of the different SiOAl sites in Na-A and Na-LSX are presented as a function of the bond angle. The influence of cations on the isotropic chemical shift was studied in case of the Na-, Cs- and Li-form of LSX. It has been shown that the increase of the basic properties of the zeolite oxygen framework going from the sodium to the caesium form of the zeolite LSX is reflected by an downfield shift of ca. 6 ppm. For the layer silicate Na-Ilerite, a resolution into three lines (SiOH, SiOSi, H₂O ) was achieved by MAS NMR in the field of 17.6 T. The assignment of the SiOH line was proven by CP MAS and REDOR NMR.

070
Session Assigned: W&Th P
Title: Solid State 17O NMR, quadrupolar and chemical shift interaction

Institute: CRMHT-CNRS, Orleans, France
Keywords:
Abstract:
17O is well known in solid state NMR to provide invaluable information on the structure of dielectric crystalline or amorphous materials and their structural evolution during the synthesis process. We report new results obtained in two contrasted cases : polysiloxane gels one hand and TiO2 nano objects on the other hand. Both systems give rise to broad spectra in the disordered amorphous or gel phases hand have been characterized using MQ-MAS and MAS at different spinning rates and principal fields. In the case of the siloxane gels the 17O spectra are broadened by second order quadrupolar interaction and MQ-MAS enables the resolution of T-OT, T-O-Q and Q-O-Q bridging environments. In the case of the TiO2 based nano objects the MQ-MAS clearly shows that most of the broadening is due to chemical shift distribution over hundreds of ppm with minor contribution of the second order quadrupolar broadening.

071
Session Assigned: W&Th P
Title: Silicon Nanoclusters Encapsulated in Y Zeolites studied by ²⁹Si-²⁷Al REAPDOR NMR and Proton MQ NMR

Institute: National Research Council of Canada
Keywords:
Abstract:
Size-controlled and spatially organized silicon nanoclusters were paid great attention in recent years because of their photoluminescent properties and, therefore, potential applications in silicon-compatible optoelectronic devices. Y zeolites, which possess spatially well organized cavities, have been explored as novel encapsulation host for silicon nanoclusters by means of thermal treatment of chemically vapor-deposited disilanes (Si₂H₆). In this presentation, we demonstrate the structural characterization of the silicon nanoclusters by ²⁹Si-²⁷Al Rotational Echo, Adiabatic Passage, DOuble Resonance (REAPDOR) NMR and proton multiple-quantum (MQ) NMR.

As known previously, double resonance NMR has the power to directly determine internuclear geometric structures, and proton MQ NMR is able to detect the coupled spin number of an assemble of clusters. Therefore, the geometric structures of the adsorbed disilanes and the encapsulated silicon clusters in Y zeolites have been studied by the ²⁹Si-²⁷Al REAPDOR NMR experiment via spatially correlating the disilanes and the silicon clusters to the zeolite-framework aluminums. The disilane loading levels in the Y zeolites and the hydrogen numbers in the silicon nanoclusters have been studied by proton MQ NMR.

072
Session Assigned: W&Th P
Title: Solid-State ²⁷Al NMR Spectroscopy of Methylaluminoxane (MAO)

Institute: Albemarle Corporation, Baton Rouge, LA
Keywords:
Abstract:
A variety of ²⁷Al NMR experiments have been done on methylaluminoxane (MAO). MAO is an important co-catalyst for metallocenes in new-generation olefin polymerization. Conventional MAS NMR at 9.5 T is not effective for pristine samples (but does yield limited spectra with minor air exposure). Static NMR methods, including field swept and frequency swept NMR have been used. The variable temperature static NMR of MAO materials shows surprisingly little temperature change to 130 deg C. Quadrupolar coupling constants for most aluminum sites appear to be very large, greater than 10 MHz, possibly indicating five-coordinate structures.

073
Session Assigned: W&Th P
Title: NMR and the thermodynamics of solid solutions

Institute: University of Nebraska at Lincoln
Keywords:
Abstract:
The chemical shifts of NMR nuclei in ionic solid solutions depend on the number and distribution of next nearest neighbor ions. For example, ²⁰⁷Pb NMR spectra of Pb_xSr_1-x(NO₃)₂ and Pb_xSr_1-x(NO₃)₂ display up to 13 lines, arising from lead atoms with 0 to 12 leads in neighboring cationic sites of the unit cell. These lines are further split into multiplets by orientational effects. In favorable cases, anslysis of the sideband intesities in slow-spinning MAS spectra allow determination of the anisotropic effect of a single replacement of an ionic neighbor on the chemical shielding tensor.

The effect is seen in a variety of isomorphous (Pb,Sr,Ba) solid solutions, and also in salts of the larger monovalent ions (K,Rb,Cs,Tl). It appears to be modulated not by the size of the ions but by their polarizability; replacing a less polarizable neighbor ion by a more polarizable one shifts the resonance of the probe nucleus to higher frequency. The effect also seems to be largest for heavier NMR nuclei such as ²⁰⁷Pb and ¹³³Cs.

Analysis of the intensities of the individual signals in these complex spectra allows us to quantify the number of nearest neighbors of each type. We can then simulate these distributions by Monte Carlo methods, extracting the energy difference between like next neighbor pairs and unlike pairs. For example, in the system Pb_xSr_1-x(NO₃)₂, for a next neighbor pair the energy difference .DeltaE = E_Pb-Sr - (E_Pb-Pb +E_Sr-Sr)/2 = 120 J/mol or about 0.05RT. This very small energy difference causes slight clustering of like ions in the solid solution, and is probably due to van der Waals effects.

Hitherto, this system has been assumed to be an ideal solid solution with zero energy of mixing. Interestingly, careful analysis of X-ray powder diffraction data show small-non-idealities in the volume of mixing which interpreted in the conventional way would indicate a net favorable interaction of unlike nearest neighbors. We will demonstrate that the conventional interpretation is thermodynamically unsound and that the NMR results lead to a more rigorous determination of .Delta_mE.

074
Session Assigned: W&Th P
Title: Solid-State P-31 MAS NMR Spectroscopic Investigations of the Structure of Multilayer Langmuir-Blodgett Films

Institute: University of Florida, Gainesville FL 32611-7200
Keywords:
Abstract:
Although NMR is a very powerful spectroscopic tool, it has rarely been used to study the molecular structure and organization of Langmuir-Blodgett (LB) films. Here we demonstrate that P-31 magic angle spinning (MAS) NMR can be used to study octadecylphosphonate multilayer LB films. Results show that it is possible to obtain quality P-31 MAS NMR spectra of these thin films, and both quantitative and qualitative information about the film structure are obtained. More specifically, by comparing the spectra of the films with the analogous solid state materials, a definitive structure of the LB film can be determined. To our knowledge, this is the first NMR study to characterize the structure of a LB film. This method takes advantage of the relatively high NMR sensitivity of phosphorus-31 and the lack of any background signals from the substrate that might complicate the interpretation of the spectra. The method is simple and may find future applications as a general tool for the structural characterization of low surface area thin films containing phosphorus, such as other phosphonate or phospholipid LB films, as well as some spontaneously adsorbed multilayer films.

075
Session Assigned: W&Th P
Title: Water in silicate glasses : a multinuclear high resolution NMR study

Institute: CRMHT-CNRS, Orleans, France
Keywords: Inorganic Solid;MQ-MAS;Double Quantum;Correlation
Abstract:
Water dissolution has a tremendous effect on physical properties (viscosity, conductivity) of silicate and alumino-silicate melts. However, the mechanism of the incorporation of water in this type of glasses is still a matter of discussions, and raises a lot of questions.

To study the short- and medium-range order in water bearing silicate and alumino-silicate (phonolite) glasses synthetized under high pressure . We applied a whole range of high resolution solid state NMR methods to the various nuclei: Multiple Quantum-MAS NMR (for 23Na and 27Al), CP, REDOR, HETCOR, double-quantum filters (for 1H and 29Si). In particular, information about longer range ordering, essential to understand the structure, may be obtained by using sequences that combines double-quantum filters and, i.e. CP or echoes.

076
Session Assigned: W&Th P
Title: ⁹⁵Mo Quadrupole Couplings and Chemical Shielding Anisotropies
for some Inorganic Molybdenum Compounds from High-Field Solid-State ⁹⁵Mo NMR

Institute: Department of Chemistry, University of Aarhus
Keywords: ⁹⁵Mo; Quadrupolar Nuclei; MAS; Single-Crystal NMR;
Abstract:
Solid-state ⁹⁵Mo NMR spectroscopy has the potential of becoming a valuable tool in structural studies of a variety of molybdenum compounds in organometallic and inorganic chemistry. We have a special interest in the use of ⁹⁵Mo NMR in studies of molybdenum-based heterogenous catalysts. Only few solid-state ⁹⁵Mo NMR studies have appeared in the literature which may reflect the low natural abundance (15.9 %), low gyromagnetic ratio, large quadrupole moment, and long spin-lattice relaxation times for ⁹⁵Mo. Even for relatively small ⁹⁵Mo quadrupolar couplings these factors may hamper the observation of the ⁹⁵Mo central transition in powdered solids.

In this work we utilize the combination of a high magnetic field (14.1 T) and magic-angle spinning (MAS) NMR to observe the ⁹⁵Mo central and satellite transitions for a series of inorganic molybdenum compounds. In addition to an increase in sensitivity, the high magnetic field decreases the second-order quadrupolar broadening and effects from acoustic ringing. The principal aim has been to obtain precise values for the ⁹⁵Mo quadrupolar couplings and chemical shielding anisotropies for structurally different molybdenum environments. These data are determined from either the manifold of spinning sidebands, observed from the satellite transitions, or from lineshape simulations of the central transition. These parameters represent not only a potential source of structural information, but may also indicate whether ⁹⁵Mo NMR is a feasible technique for more general studies, for example of industrially important materials such as heterogeneous catalysts. ⁹⁵Mo (and ⁹⁷Mo) spin-lattice relaxation times are reported for some of the samples and found to vary significantly for the studied compounds. In a few cases ⁹⁵Mo single-crystal NMR is employed to further extend the information available from the powder samples. For some of the molybdenum compounds (i.e. Mo(CO)₆, Na₂MoO₄^.2H₂O, and MoO₃) the quadrupole coupling and shielding anisotropy data may be compaired with values reported in the literature.

077
Session Assigned: W&Th P
Title: Sensitivity Enhancement in Single-Crystal NMR Spectroscopy

Institute: Department of Chemistry, University of Aarhus
Keywords:
Abstract:
The construction of a single-crystal NMR probe with a new two-axis geometry for the goniometer has improved the sensitivity of the NMR response from small crystals. This allows studies of the smallest crystals used so far in single-crystal NMR. The goniometer of the probe uses only two different mountings of the crystal, and the rotation axis of the goniometer is tilted 45^o with respect to the direction of the magnetic field. By moving the goniometer outside the rf coil, the filling factor and probe sensitivity increase.

Employing this probe and a conventional three-axis single-crystal NMR probe we have studied a series of small crystals by ²³Na, ²⁷Al, ³¹P, ⁶⁷Zn, ⁶⁹Ga, ⁷¹Ga, ⁸⁷Rb, and ⁹⁵Mo single-crystal NMR. In particular we demonstrate that the high sensitivity and resolution of single-crystal NMR makes this technique an attractive alternative to powder methods for solid-state NMR studies of low-gamma nuclei such as ⁶⁷Zn and ⁹⁵Mo provided good crystals are available.

079
Session Assigned: W&Th P
Title: Investigations on the Properties of Lead-exchanged Zeolites using ²⁰⁷Pb Solid Sate NMR

Institute: University of California, Berkeley, CA, USA
Keywords:
Abstract:
It has long been know that metal-exchanged zeolites show a great variety of catalytic activities and a broad range of applications in industrial processes. Through varying the metal and the type of zeolite one is enabled to influence the reactivity and the catalytic properties in very specific ways. In many cases metal ions otherwise not catalytic active, e.g. lead, show catalytic behavior when incorporated in a zeolites of well defined structure. In order to understand the catalytic properties it is necessary to investigate how the internal structure of the zeolite, the presence of other substances, e.g. water, and temperature are influencing the metal ion.

We report here on solid state NMR (static sample) investigations of lead-exchanged zeolites, which are used industrially as polymerization catalysts. The zeolite types used in following experiments are Linde A, Type RHO, Type X, Type Y, and ZSM-5 while the symmetry of the exchange sites decreases in the former line of order.

Due to its relative high sensitivity (comparable to ¹⁹⁵Pt and ¹²⁹Xe), the easy referencing to lead nitrate (most common), and especially its spin of I = 1/2 the ²⁰⁷Pb nucleus is of great interest for solid state NMR spectroscopy.

In order to determine the dynamics of the metal ion and the influence of the symmetry of the metal positions in the zeolite, the time constants T₁ and T₂, their temperature dependency, and the influence of water on the spetroscopical data have been measured.

To get an idea of symmetry of the environment of the metal in the zeolite and the possibility of multiple crystallographic sites the line width of the powder pattern has to be taken in special account, even though it is hard to determine which effect accounts more for the line broadening.

We acknowledge the support of the National Science Foundation (NSF).

Institute: The Institute of Physical and Chemical Research
Keywords: Solid State NMR;Preferential Enrichment;Racemate;Enantiomer
Abstract:
Recently we have found the first case of accomplishment of an enantiomeric resolution by simple recrystallization of a racemic compound.

Namely (1) remarkable enantiomeric enrichment up to 100% ee occurs in the mother liquor by recrystallization of the racemate or nonracemate with low enantiomeric purities from alcohols and (2) repetition of crystallization of the racemate and each deposited crystals successively results in the alternative enantiomeric enrichment of the two enantiomers in the mother liquor and simultaneously leads to the regular reversal of chirality in the deposited crystals.

We designate this new enantiomeric resolution phenomenon as Preferential Enrichment. Here we report the intermolecular interactions of enantiomers in racemates and enantiomers, which undergo Preferential Enrichment in the solid state by means of X-ray crystallographic analysis and Carbon-13 and Nitrogen-15 CP/MAS NMR, respectively.

081
Session Assigned: W&Th P
Title: Dynamics of Phenol Within a Framework of Triphenylphosphine oxide Studied by Solid-state NMR.

Institute: Durham University
Keywords:
Abstract:
The dynamic behaviour of the phenol molecule within a framework of TPPO has been investigated via the phosphorus chemical shift difference created by hydrogen bond formation to phenol. Disorder in the crystal is created by a 2-site jump motion of the phenol. Phosphorus-31 MAS spectra for bandshape analysis, T1.rho. measurements and Selective Polarisation Inversion experiments, covering a temperature range from 180 to 320 K, were recorded and used to calculate values of activation parameters for the 180 flip motion. The three techniques are most appropriate over different temperature ranges, the T1.rho. minimum giving a high temperature point and the rates of motion towards the low temperature limit being calculated more accurately from SPI experiments.

Phosphorus-31 NMR studies of single crystals, in conjunction with data from X-ray techniques, have enabled the orientation of the shielding tensor components to be determined. Because of the axial symmetry, the unique orientations of .sigma. 11 and .sigma. 22 cannot be determined, apart from their orthogonality to each other and to .sigma. 33 (which has been found to lie along the phosphorus-oxygen double bond).

082
Session Assigned: W&Th P
Title: Testing the Surrogate Probe Strategy Using Solid State ⁶⁷Zn NMR

Institute: Battelle, EMSL-PNNL
Keywords:
Abstract:
In order to understand the function of many metalloproteins and enzymes it is desirable to directly observe the metal in the active site of these systems. However, the native metals often have poor spectroscopic properties (such as low g, low natural abundance, or large quadrupole broadening) which makes data acquisition difficult or even impossible. The past twenty-five years or so have seen the development of a surrogate probe strategy, involving the substitution of Cd⁺² for Zn⁺² and/or Ca⁺² (and more recently for Fe, Mg, Mn, and Cu sites), in the nuclear magnetic resonance (NMR) investigations of metalloproteins. The excellent NMR spectroscopic properties of the cadmium in substituted proteins have allowed valuable information to be extracted from many systems. The large body of cadmium models have been used to develop rules correlating structure and shielding. Based on the similarities of the metal properties and the retention of some biological activity in substituted systems, it has been assumed that the cadmium reflects the behavior of the native metals. However, this assumption has not been verified experimentally. We have therefore undertaken the task of testing the surrogate probe strategy by investigating the solid state NMR lineshapes of zinc systems. Discussed are the experimental details (i.e. overcoming the sensitivity issues of ⁶⁷Zn NMR), lineshape simulations and projections for future work.

083
Session Assigned: W&Th P
Title: Developing Structure-Property-Processing Relationships of Chemical Vapor Deposited Fluorocarbon Films through High Resolution ¹⁹F and ¹³C MAS NMR Spectroscopy

Institute: Dept. of Chemical Engineering, MIT, Cambridge, MA
Keywords:
Abstract:
High resolution solid-state ¹⁹F and ¹³C NMR has been achieved with MAS speeds of up to 25 kHz. This enabled detailed elucidation of the structures of fluorocarbon thin films deposited via pulsed plasma enhanced and pyrolytic CVD pathways. ¹⁹F 25 kHz MAS NMR provided network connectivity description of moieties of up to three carbon sequences. Delayed spin echo and variable temperature experiments determined relative moiety mobilities and the extent of chemical dispersion. Nuclear spin-lattice relaxation measurements showed enhanced relaxation through paramagnetic centers and gave a direct correlation to the concentration of film radicals. ¹³C 10 kHz MAS NMR differentiated between sp³ and sp² carbon states. Further differentiation of sp² carbons in conjugated and in more fluorinated environments was also observed. Direct excitation with simultaneous DEPTH background suppression allowed quantitation of ¹⁹F and ¹³C spectra through a nonlinear least squares regression methodology. Identifying and quantitating film fluorocarbon moieties proved extremely profitable in demonstrating our ability to engineer films over a wide compositional and structural phase space through control of deposition parameters such as plasma pulsing duty cycle, precursor gas chemistry and activation pathway. Films can range from amorphous fluorocarbon networks (a-C:F) with substantial crosslinking and sp² contribution to linear CF₂CF₂ polymer chains virtually indistinguishable from poly(tetrafluoroethylene) (PTFE). Thermal decomposition mechanisms have been traced with respect to films from various deposition parameters, and have pointed to the instability of labile end groups and absorbed low molecular weight molecules. Ultimately, a sound understanding of the structure-property-processing relationships of CVD fluorocarbon films will allow the design of film architecture to suit the desired applications for these materials.

084
Session Assigned: W&Th P
Title: Studies of Molecular Dynamics in Solid Aramide Polymers by Multidimensional Local Field MAS NMR

Institute: University of Illinois - Chicago; Chicago, IL, USA
Keywords:
Abstract:
Temperature-dependent measurements of NMR anisotropies are well established tools for the elucidation of molecular motions in polymers. These usually involve ²H quadrupolar, ¹³C CSA or ¹³C-¹H dipolar coupling determinations, approaches that in spite of their usefulness face the need for synthetic labeling, independent characterizations of ¹³C tensors, or efficient ¹H multipulse decoupling techniques, respectively. It is reported, however, that when sufficiently fast MAS speeds are involved (ca.10 kHz for "real" solids) quantitative sideband patterns of directly bonded ¹³C-¹H spin pairs can be obtained without resorting to ¹H-¹H homonuclear decoupling techniques. This frees the resulting line shapes from scaling factors that may otherwise complicate their quantification, and enables their use in the analysis of motional dynamics. After verifying such features on model systems this fast-MAS ¹³C local field approach was applied to the elucidation of dynamic processes in polynuclear aromatic polyamides. These were detected with the aid of preliminary wide-line and relaxation variable-temperature ¹H NMR measurements, and their librational nature could be quantified from the individual ¹H-¹³C sideband patterns. One of the aramides in the analyzed series, p-phenyleneterephthalamide, had been previously analyzed using specifically deuterated analogs and the type of dynamics revealed by the ¹³C-¹H local field experiments agrees well the behavior reported from ²H NMR.

085
Session Assigned: W&Th P
Title: Solid State NMR of Organolithium Compounds: Structure and Dynamics

Institute: University of Siegen, D-57068 Siegen, Germany
Keywords: quadrupole coupling; lithium NMR; dynamics
Abstract:
Solid state NMR is becoming an important tool for structural investigations also in the field of organometallic chemistry. For organolithium compounds, nuclei like C-13, N-15, Si-29, P-31, and the two lithium isotopes Li-6 and Li-7 are available as nuclear probes for various aspects of structure and dynamic behaviour. In this context, the quadrupolar coupling constant observed for Li-7 has been recognized as diagnostic parameter for the characterization of the local Li⁺ environment in organolithium clusters [1-5]. Examples from various lithium salts of reduced cyclic pi-systems, from Schlenk dimers formed upon reduction of styrene derivatives, and from lithiated amines are discussed.

In addition, the sensitivity of NMR for dynamic processes can be exploited to uncover ligand mobility in the solid [2,4,5]. These aspect will be illustrated with the analysis of methyl rotation as well as pi-flip and ring inversion of the five-membered metallacycle formed by the lithium cation and the tetramethylethylene diamine ligand in lithium fluorenide(tmeda) and other systems.

§ deceased October 29, 1997

[1] Johnels, D., J. Organomet. Chem. 445, 1 (1993).
[2] Baumann, W.; Oprunenko, Y.; Günther, H., Z. Naturforsch. 1995, 50a, 429-438.
[3] Johnels, D.; Anderson, A.; Boman, A.; and Edlund, U., Magn. Reson. Chem. 1996, 34, 908-912
[4] Günther, H.; Focke, A; Hausmann, H., unpublished results.
[5] M. Hartung, H. Günther, J.-P- Amoureux, and C. Fernandéz, Magn. Reson. Chem. 36, S61-S70, (1998).

086
Session Assigned: W&Th P
Title: SOLID-STATE NMR INVESTIGATION OF COMPLEXES OF LEAD (II) DIHALIDES WITH ORGANIC LIGANDS

Institute: University of Delaware, Newark, Delaware
Keywords:
Abstract:
Reactions of lead ion with ligands such as 1,10-phenanthroline, bipyridine and thiourea produce materials, the nature of which has been a topic of study by a variety of structural and spectroscopic techniques.¹ The nature of bonding in these materials can be addressed by NMR spectroscopy through the effects on chemical shifts.

Here we report solid-state ²⁰⁷Pb and ¹³C NMR results for a series of complexes of lead (II) dihalides with organic ligands. The NMR results are interpreted in terms of the effects of ligation and ion interactions on bonding.

Work at the Lawrence Berkeley Laboratory, University of California at Berkeley (DLP and MB) was supported by Contract No. DE-AC03-76F0008 from the U.S. Department of Energy, Office of Biomedical and Environmental Research, Medical Applications and Biophysical Research Division. Work at the University of Delaware (AG, CD and GN) was supported by Grant No. 33633-AC of the Petroleum Research Fund of the American Chemical Society.

^1. L. M. Englehardt, J. M. Harrowfield, H. Miyamae, J. M Patrick, B. W. Skelton, A. A. Soudi and A. H. White, Aust. J. Chem., 1996, 49, 1111; and references therein.

087
Session Assigned: W&Th P
Title: ¹³C CP/MAS NMR Studies of Phase Mixtures of Solid Organic Compounds

Institute: University of Minnesota
Keywords:
Abstract:
Polymorphism is the ability of a substance to exist in two or more distinct forms that differ in the conformation and/or arrangement of molecules in the crystal lattice. Polymorphism is particularly relevant to the pharmaceutical and food industries because different polymorphs can have different solubilities, stabilities, and bioavailabilities. Solid-state NMR spectroscopy has emerged as a powerful analytical technique in the characterization of polymorphism in crystalline organic compounds. Each polymorphic form of a compound will produce unique resonances in the solid-state NMR spectrum. Solid-state NMR spectroscopy is also useful for distinguishing solvates and studying amorphous forms. We have used ¹³C CP/MAS NMR to study three known forms of aspartame (L-aspartyl-L-phenylalanine methyl ester) which is widely used as an artificial sweetener. Currently we are studying phase mixtures of a new sweetener called neotame (N-(3,3-dimethylbutyl)-L-aspartyl-L-phenylalanine methyl ester). We have found that many forms of neotame can be generated by altering the crystallization and drying conditions. At least five different resonances for the quaternary phenyl carbon are observed in the different forms. The most stable form of neotame is a monohydrate which exhibits the most downfield resonance for the quaternary phenyl carbon; no higher hydrates are known to exist. The monohydrate is thought to exist in multiple polymorphic forms which are metastable and have unique resonances for the quaternary phenyl carbon. We have also observed an amorphous anhydrate which exhibits a quaternary phenyl resonance centered downfield at about the same chemical shift as the monohydrate. Unfortunately the conditions used to generate these forms do not result in pure phases. We are investigating the relative merits of using solid-state NMR and powder X-ray diffraction to analyze mixtures of forms. Powder X-ray diffraction is currently the most common method of determining polymorphism, but it is susceptible to particle size and preferred orientation effects, does not reveal site specific information, and is unable to probe amorphous materials. Further, the weak diffraction peaks of minor components in mixtures are often overwhelmed by the strong diffraction peaks of the major component. Solid-state NMR spectroscopy is better suited for investigating polymorphic mixtures since it is not as sensitive to particle size effects, does reveal information on the local environment of nuclei, and can be used to study amorphous materials.

088
Session Assigned: W&Th P
Title: Multi-dimensional Solid State NMR Studies of Natural Product - Taxol

Institute: Institute of Biomedical Sciences, Academia Sinica
Keywords:
Abstract:
Taxol an extract of Pacific Yew inner bark has officially proven to be a cancer drug demonstrating its antitumor activity against breast, ovarian and lung cancers. It is of great interest to identify its structure that may be directly related to biological activity. Although isotope label is necessary for determining molecular structure of sample in solid phase, while due to complexity in synthetic chemistry, a natural abundant rather labeled sample was studied. We have implemented 1D and 2D experiments to obtain 1H and 13C chemical shift spectra. Field dependent doublet splitting observed in taxane ring and side-chain, of both 1H and 13C resonances, strongly suggest there are two preferential molecular structures, and yet not observable in solution phase. Subsequent determination of 13C chemical shift anisotropy has carried out by 2D sideband separation experiment[1], and preceded by Herzfeld and Berger method [2] in analysis. We will present the results determined by Solid State NMR measurements and comparion with x-ray crystallography studies.

1. S. Feaux de Lacroix, J. J. Titman, A. Hagemeyer, and H. W. Spiess, J. Magn. Reson., 97, 435 (1992).
2. J. Herzfeld and A. E. Berger, J. Chem. Phys., 73, 6021 (1980).

089
Session Assigned: W&Th P
Title: Decontamination of Nerve and Blister Agents with Macroporous Dendritic Polymers

Institute: GEO-CENTERS, Inc.
Keywords:
Abstract:
A number of novel dendritic and macroporous polymers are being evaluated for the decontamination of nerve and blister agents. One of several versatile monomers constituting these polymers is glycidyl methacrylate; after synthesis of the dendrimer, the epoxy functionality can be opened by a wide range of available nucleophiles to produce beta-substituted hydroxy functionalities. In-situ solids NMR analysis has shown that gel-like dendritic polymers (minimal cross-linking) with produce beta-substituted hydroxy functionalities are relatively inefficient at nerve agent destruction. However, the more heavily cross-linked, macroporous polymers produce substantially higher rates of nerve agent decontamination, apparently as a result of their highly functionalized, permanent pore structure. Preliminary CP-MAS analysis indicates that at least a portion of the decontamination products are covalently bonded to the polymer backbone.

090
Session Assigned: W&Th P
Title: MAS NMR Studies of VX, GD and HD Reactions with Nano-Size Particles

Institute: Geo-Centers, Inc., Aberdeen Proving Ground, MD
Keywords: mustard; soman; nerve agents; blister agents; decontamination
Abstract:
The room temperature reactions of VX, GD and HD with nano-size MgO have been studied using solid state MAS NMR. All three agents hydrolyze on the surface of the very reactive MgO nano-particles. VX yields ethyl methylphosphonic acid (EMPA) and methylphosphonic acid (MPA), but no toxic S-(2-diisopropylamino)ethyl methylphosphonothioate (EA-2192). GD forms both GD-acid and MPA. For HD, in addition to hydrolysis to thiodiglycol, about 50% elimination to divinyl sulfide occurs. The reaction kinetics for all three agents are characterized by a fast initial reaction followed by gradual slowing to a steady-state reaction with first-order behavior. The fast reaction is consistent with liquid spreading through the porous nano-particle aggregates. The steady-state reaction is identified as a gas-phase reaction, mediated by evaporation, once the liquid achieves its volume in the smallest available pores.