Cross-polarization Magic-angle Spinning NMR Spectra Research Articles

The structures of two scorpionates: thallium tetrakis(3-phenyl-1H-pyrazol-1-yl)borate and potassium tetrakis(3-cyclopropyl-1H-pyrazol-1-yl)borate.

The introduction of poly(1H-pyrazolyl)borate anions, better known as scorpionates, as negatively charged ligands for a great diversity of metal cations has had a tremendous influence in coordination chemistry. The structures of two salts of tetrakispyrazolylborate, namely [tetrakis(3-phenyl-1H-pyrazol-1-yl)borato]thallium(I), [Tl(C36H28BN8)], and catena-poly[potassium-[μ2-tetrakis(3-cyclopropyl-1H-pyrazol-1-yl)borato]], [K(C24H28BN8)]n, have been determined at 296 K in the monoclinic P21/c and C2/c space groups, respectively. In their crystal structures, the thallium salt presents discrete molecular motifs, while the potassium salt shows infinite polymeric chains. The 13C and 15N CPMAS (cross polarization magic angle spinning) NMR spectra of these compounds were recorded and the chemical shifts compared with theoretically calculated ones at the GIAO/B3LYP/6-311++G(d,p) level. Both techniques are complementary and mutually consistent.

One-Pot Synthesis of Zeolitic Strong Solid Bases: A Family of Alkaline-Earth Metal-Containing Silicalite-1.

Fabricating stable strong basic sites in well-preserved crystallized zeolitic frameworks still remains a difficult issue. Here, we reported a family of MFI-type metallosilicate zeolites, AeS-1 (Ae: alkaline-earth metal ions of Mg, Ca, Sr or Ba; S-1: silicalite-1) through a direct one-pot hydrothermal method involving the acidic co-hydrolysis/condensation of the silica precursor with the Ae salts. Step-by-step full characterizations were designed and conducted for in-depth discussion of the Ae status in AeS-1. Strong basicity (H_≈22.5-26.5) was detected in AeS-1. The basicity was further confirmed by CO2 sorption measurements, (13) C NMR spectra of chloroform-adsorbed samples, and (1) H→(13) C and (1) H→(29) Si cross-polarization magic-angle spinning NMR spectra of ethyl cyanoacetate-adsorbed samples. The results of Knoevenagel condensations demonstrated the excellent solid base catalysis of AeS-1, which showed high activity, reusability, and shape-selectivity, all of which are explained by Ae-derived zeolitic intracrystalline strong basic sites.

Solid-state 31P NMR investigation on the status of guanine nucleotides in paclitaxel-stabilized microtubules.

Microtubule dynamics is a target for many chemotherapeutic drugs. In order to understand the biochemical effects of paclitaxel on the GTPase activity of tubulin, the status of guanine nucleotides in microtubules was investigated by (31)P cross-polarization magic angle spinning (CPMAS) NMR. Microtubules were freshly prepared in vitro in the presence of paclitaxel and then lyophilized in sucrose buffer for solid-state NMR experiments. A (31)P CPMAS NMR spectrum with the SNR of 25 was successfully acquired from the lyophilized microtubule sample. The broadness of the (31)P spectral lines in the spectrum indicates that the molecular environments around the guanine nucleotides inside tubulin may not be as crystalline as reported by many diffraction studies. Deconvolution of the spectrum into four spectral components was carried out in comparison with the (31)P NMR spectra obtained from five control samples. The spectral analysis suggested that about 13% of the nucleotides were present as GTP and 37% as GDP in the β-tubulin (E-site) of the microtubules. It was found that most of the GDPs were present as GDP-Pi complex in the microtubules, which seems to be one of the effects of paclitaxel binding.

CO₂ adsorption on amine-functionalized periodic mesoporous benzenesilicas.

CO2 adsorption was investigated on amine-functionalized mesoporous silica (SBA-15) and periodic mesoporous organosilica (PMO) samples. Hexagonally (p6mm) ordered mesoporous SBA-15 and benzene-PMO (BPMO) samples were prepared in the presence of Pluronic P123 block copolymer template under acidic conditions. Three kinds of amine-containing organosilanes and polyethylenimine were used to functionalize SBA-15 and BPMO. Small-angle X-ray scattering and nitrogen adsorption isotherms showed that these samples featured ordered mesostructure, high surface area, and narrow pore size distributions. Solid-state (13)C- and (29)Si cross-polarization magic-angle spinning NMR spectra showed chemical linkage between amine-containing modifiers and the surface of mesoporous materials. The chemically linked amine-containing modifiers were found to be on both the inner and outer surfaces. N-[3-(trimethoxysilyl)propyl]ethylenediamine-modified BPMO (A2-BPMO) sample exhibited the highest CO2 uptake (i.e., ∼3.03 mmol/g measured on a volumetric adsorption analyzer) and the fastest adsorption rate (i.e., ∼13 min to attain 90% of the maximum amount) among all the samples studied. Selectivity and reproducibility measurements for the A2-BPMO sample showed quite good performance in flowing N2 gas at 40 mL/min and CO2 gas of 60 mL/min at 25 °C.

Functionally modified monodisperse core–shell silica nanoparticles: Silane coupling agent as capping and size tuning agent

Vinyltriethoxysilane (VTEOS) has been used as capping and size tuning agent to prepare monodisperse core–shell silica hybrid spheres. When VTEOS was used, the relative standard deviation below 3% for silica hybrid spheres can be produced by changing the concentration of VTEOS and the ammonia catalyst. The obtained hybrid particles were characterized and confirmed by various technologies including Fourier transform infrared (FT-IR) spectroscopy, FT-Raman spectroscopy, 29Si CP (cross-polarization) MAS NMR, thermal analysis, particle size distribution, field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). FT-IR, FT-Raman, 29Si CP MAS NMR spectra and thermal analysis confirm the existence of vinyl groups on hybrid particles. Particle size distribution, FE-SEM and TEM reveal the uniformity in size/shape and the core–shell structure of hybrid particles. The influence of experiment parameters on the resulting particles, the capping and size tuning actions of other silane coupling agents (SCAs) were investigated herein as well.

Hartmann-Hahn 2D-map to optimize the RAMP-CPMAS NMR experiment for pharmaceutical materials

Cross polarization-magic angle spinning (CPMAS) is the most used experiment for solid-state NMR measurements in the pharmaceutical industry, with the well-known variant RAMP-CPMAS its dominant implementation. The experimental work presented in this contribution focuses on the entangled effects of the main parameters of such an experiment. The shape of the RAMP-CP pulse has been considered as well as the contact time duration, and a particular attention also has been devoted to the radio-frequency (RF) field inhomogeneity. (13)C CPMAS NMR spectra have been recorded with a systematic variation of (13)C and (1)H constant radiofrequency field pair values and represented as a Hartmann-Hahn matching two-dimensional map. Such a map yields a rational overview of the intricate optimal conditions necessary to achieve an efficient CP magnetization transfer. The map also highlights the effects of sweeping the RF by the RAMP-CP pulse on the number of Hartmann-Hahn matches crossed and how RF field inhomogeneity helps in increasing the CP efficiency by using a larger fraction of the sample. In the light of the results, strategies for optimal RAMP-CPMAS measurements are suggested, which lead to a much higher efficiency than constant amplitude CP experiment.

Mechanochemically conjugated PMHS/nano-SiO 2 hybrid and subsequent optimum grafting density study

In this paper, we reported the preparation of poly(methylhydrosiloxane) (PMHS)/SiO 2 hybrid particles by mechanochemical method based on high energy ball milling (HEBM). The obtained hybrid particles were characterized by Fourier transform infrared (FT-IR) spectroscopy, 29Si CP (cross-polarization) MAS NMR, viscosity measurement, particle size distribution, thermal analysis (TGA, DSC and DTG), static contact angle (CA), field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). FT-IR and 29Si CP MAS NMR spectra indicate that PMHS is chemically anchored onto the surface of nano-SiO 2. Viscosity measurement, particle size distribution, FE-SEM and TEM demonstrate that an appropriate grafting density optimizes the dispersion of nanoparticles in poly(dimethylsiloxane) (PDMS) matrix, so lower viscosity can be achieved. Too high or too low grafting density may only achieve suboptimal and poor dispersions. The optimum grafting density of PMHS on nano-SiO 2 was determined by thermal analysis, with approximately 0.0531 PMHS/nm 2. Static contact angle measurement indicates that the water contact angle of hybrid particles is modulated by changing the grafting density of PMHS on nano-SiO 2. The CA value of PMHS/SiO 2 hybrid with optimum grafting density is 139.4°, and the highest CA value of PMHS/SiO 2 hybrid is approximately 158.2°.

Participation of the Surface Structure of Pharaonis Phoborhodopsin, ppR and its A149S and A149V mutants, Consisting of the C-terminal α-helix and E-F Loop, in the Complex-formation with the Cognate Transducer pHtrII, as Revealed by Site-directed 13C Solid

We have recorded 13C solid state NMR spectra of [3-13C]Ala-labeled pharaonis phoborhodopsin (ppR) and its mutants, A149S and A149V, complexed with the cognate transducer pharaonis halobacterial transducer II protein (pHtrII) (1-159), to gain insight into a possible role of their cytoplasmic surface structure including the C-terminal alpha-helix and E-F loop for stabilization of the 2:2 complex, by both cross-polarization magic angle spinning (CP-MAS) and dipolar decoupled (DD)-MAS NMR techniques. We found that 13C CP-MAS NMR spectra of [3-13C]Ala-ppR, A149S and A149V complexed with the transducer pHtrII are very similar, reflecting their conformation and dynamics changes caused by mutual interactions through the transmembrane alpha-helical surfaces. In contrast, their DD-MAS NMR spectral features are quite different between [3-13C]Ala-A149S and A149V in the complexes with pHtrII: 13C DD-MAS NMR spectrum of [3-13C]Ala-A149S complex is rather similar to that of the uncomplexed form, while the corresponding spectral feature of A149V complex is similar to that of ppR complex in the C-terminal tip region. This is because more flexible surface structure detected by the DD-MAS NMR spectra are more directly influenced by the dynamics changes than the CP-MAS NMR. It turned out, therefore, that an altered surface structure of A149S resulted in destabilized complex as viewed from the 13C NMR spectrum of the surface areas, probably because of modified conformation at the corner of the helix E in addition to the change of hydropathy. It is, therefore, concluded that the surface structure of ppR including the C-terminal alpha-helix and the E-F loops is directly involved in the stabilization of the complex through conformational stability of the helix E.

Synthesis and characterization of chemically bonded stationary phases on hydride surfaces by hydrosilation of alkynes and dienes

A hydrosilation reaction was used to bind four compounds with one or more alkyne groups or two alkene functionalities. The diffuse reflectance infrared Fourier transform, the 29Si cross-polarization magic-angle spinning (CP-MAS) NMR, and the 13C CP-MAS NMR spectra were used to characterize the various bonded materials. The bonded phase density was determined from carbon elemental analysis. The two ten-carbon hydrophobic stationary phases were characterized chromatographically and static stability tests were run in acidic and basic solutions.

Organic Coprecipitates with Calcite: NMR Spectroscopic Evidence

Dissolved organic ligands are well known to interact strongly with the calcite surface, altering precipitation and dissolution rates, crystal morphology, and possibly the ability of calcite to sequester metal contaminants. We show, using NMR spectroscopic techniques, that some of the citrate molecules present in a solution of precipitating calcite are incorporated structurally into the calcite crystal. Calcite grown by a seeded constant-addition method contains approximately 1 wt % coprecipitated citrate and yields 13C{1H} cross-polarization magic-angle spinning NMR spectra that contain broad peaks for citrate plus a signal from carbonate. Results from 13C{1H} heteronuclear correlation NMR experiments show that citrate is located in close spatial proximity to carbonate groups. In addition, calcite/citrate coprecipitates contain about 0.4 wt % excess water, which is not present as fluid inclusions, and some of which occurs as rigid structural water. These results suggestthat water and hydrogen-bonding interactions play a role in the interface between included organic molecules and the calcite host. Additional NMR data obtained for calcite coprecipitates of aspartic and glutamic acids suggest they are also incorporated structurally but at concentrations much lower than for citrate, whereas no evidence was found for phthalate incorporation.

Solid-state 13C NMR spectroscopy studies of xylans in the cell wall of Palmaria palmata (L. Kuntze, Rhodophyta)

The chemical structure and interactions of the cell wall polysaccharides from the red edible seaweed Palmaria palmata were studied by liquid-like magic-angle-spinning (MAS) and cross-polarization MAS (CPMAS) solid-state 13C NMR spectroscopy. The liquid-like MAS and CPMAS 13C NMR spectra of the rehydrated algal powder revealed the presence of β-(1→4)/β-(1→3)-linked d-xylan with chemical shifts close to those observed in the solution 13C NMR spectrum of the polysaccharide. Observation of mix-linked xylan in the liquid-like MAS 13C NMR spectrum indicated that part of this cell wall polysaccharide is loosely held in the alga. The CPMAS NMR spectrum of the dry algal powder alcohol insoluble residue (AIR) showed broad peaks most of which corresponded to the mix-linked xylan. Hydration of AIR induced a marked increase in the signal resolution also in the CPMAS NMR spectra together with a shift of the C-3 and C-4 signals of the (1→3)- and (1→4)-linked xylose, respectively. Such modifications were present in the spectrum of hydrated (1→3)-linked xylan from the green seaweed Caulerpa taxifolia and absent in that of (1→4)-linked xylan from P. palmata. This result emphasizes the important role of (1→3) linkages on the mix-linked xylan hydration-induced conformational rearrangement. The mix-linked xylan signals were observed in the CPMAS NMR spectrum of hydrated residues obtained after extensive extractions by NaOH or strong chaotropic solutions indicating strong hydrogen bonds or covalent linkages. T 1 ρ relaxations were measured close or above 10 ms for the mix-linked xylan in the dry and hydrated state in AIR and indicated that the overall xylan chains likely remain rigid. Rehydration of the mix-linked xylan lead to a decrease in the motion of protons bounded to the C-1 and C-4 carbons of the (1→4)-linked xylose supporting the re-organization of the xylan chains under hydration involving junction-zones held by hydrogen bonds between adjacent (1→4)-linked xylose blocks. The CPMAS NMR spectrum of both dry and rehydrated residues obtained after NaOH and HCl extractions demonstrated the presence of cellulose and (1→4)-linked xylans. The structures of the different polysaccharides are discussed in relation to their interactions and putative functions on the cell wall mechanical properties in P. palmata.

13C CP MAS NMR and GIAO-CHF calculations of coumarins

13C cross-polarization magic-angle spinning NMR spectra were recorded for a series of solid coumarins. Ab initio calculations of shielding constants were performed with the use of GIAO-CHF method. The combined CPMAS NMR and theoretical approach was successful in characterizing solid-state conformations of coumarins; a relationship σ (ppm)=−1.032× δ+205.28 ( R 2=0.9845) can be used to obtain structural information for coumarins, for which solid-state NMR or crystal structure data are not available.

Optically Active Ortho-Metalated Half-Sandwich Ruthenium Complexes: Solid-State NMR as a Convenient Tool To Analyze Mixtures of Diastereomers

Bulk solid samples of various ratios of the cyclometalated arene ruthenium diastereomers (S)Ru- and (R)Ru-[(eta(6)-C6H6)Ru(C6H4-2-(R)-CH(Me)NMe2)PMe2Ph](+)PF6(-) (3a/3b), of which the configurational stability at the metal center has been established by classical solution techniques, have been analyzed by the 13C cross-polarization magic angle spinning (CP-MAS) and 31P MAS NMR. The spectra obtained allowed us to detect both isomers and to estimate their respective proportions by 31P spectra. This technique was applied to a bulk solid sample of the diastereomers (S)Ru- and (R)Ru-[(eta(6)-C6H6)Ru(C6H4-2-(R)-CH(Me)NMe2)NCMe](+)PF6(-) (1a/1b), which were shown to be configurationally labile by classical solution experiments. Detection of isomer 1a only in the resulting 13C CP-MAS NMR spectrum demonstrated that there has been epimerization of 1b to 1a during crystallization, thus confirming the configurational lability at the metal center.

13C CP MAS NMR, FTIR, X-ray diffraction and PM3 studies of some N-(ω-carboxyalkyl)morpholine hydrohalides

N-(ω-carboxyalkyl)morpholine hydrochlorides, OC4H8N(CH2)nCOOH·HCl, n=1–5, were obtained and analyzed by 13C cross polarization (CP) magic angle spinning (MAS) NMR, FTIR and PM3 calculations. The structure of N-(3-carboxypropyl)morpholine hydrochloride (n=3) has been solved by X-ray diffraction method at 100K and refined to the R=0.031. The crystals are monoclinic, space group P21/c, a=14.307(3), b=9.879(2), c=7.166(1)Å,β=93.20(3)°, V=1011.3(3)Å3,Z=4. In this compound the nitrogen atom is protonated and two molecules form a centrosymmetric dimer, connected by two N+–H⋯Cl− (3.095(1)Å) and two O–H⋯Cl− (3.003(1)Å) hydrogen bonds. 13C CP MAS NMR spectra, contrary to the solution, showed non-equivalence of the ring carbon atoms. The PM3 calculations predict a molecular dimer without proton transfer for an HCl complex, while for an HBr complex an ion pairs with proton transfer, and reproduces correctly the conformation of both dimers but overestimates H-bond distances. Shielding constants calculated from the PM3 geometry of ion pairs gave a linear correlation with the 13C chemical shifts in solids.

Coordination solids derived from Cp*M(CN)3- (M = Rh, Ir).

Solutions of Rh2(OAc)4 and Et4N[Cp*Ir(CN)3] react to afford crystals of the one-dimensional coordination solid [Et4N[Cp*Ir(CN)3][Rh2(OAc)4]]. This reaction is reversed by coordinating solvents such as MeCN. The structure of the polymer consists of helical anionic chains containing Rh2(OAc)4 units linked via two of the three CN ligands of Cp*Ir(CN)3-. Use of the more Lewis acidic Rh2(O2CCF3)4 in place of Rh2(OAc)4 gave purple [(Et4N)2[Cp*Ir(CN)3]2[Rh2(O2CCF3)4]3], whose insolubility is attributed to stronger Rh-NC bonds as well as the presence of cross-linking. The species [[Cp*Rh(CN)3][Ni(en)n](PF6)] (n = 2, 3) crystallized from an aqueous solution of Et4N[Cp*Rh(CN)3] and [Ni(en)3](PF6)2; [[Cp*Rh(CN)3][Ni(en)2](PF6)] consists of helical chains based on cis-Ni(en)(2)2+ units. Aqueous solutions of Et4N[Cp*Ir(CN)3] and AgNO3 afforded the colorless solid Ag-[Cp*Ir(CN)3]. Recrystallization of this polymer from pyridine gave the hemipyridine adduct [Ag[Ag(py)][Cp*Ir(CN)3]2]. The 13C cross-polarization magic-angle spinning NMR spectrum of the pyridine derivative reveals two distinct Cp* groups, while in the pyridine-free precursor, the Cp*'s appear equivalent. The solid-state structure of [Ag[Ag(py)][Cp*Ir(CN)3]2] reveals a three-dimensional coordination polymer consisting of chains of Cp*Ir(CN)3- units linked to alternating Ag+ and Ag(py)+ units. The network structure arises by the linking of these helices through the third cyanide group on each Ir center.

Crystal structures and vibrational and solid-state (CPMAS) NMR spectroscopic studies in the tris(triphenylphosphine)-copper(I) and -silver(I) formate systems

The complexes [Cu(PPh3)3O2CH]·HCO2H (1), [Cu(PPh3)3O2CH]·0.5EtOH (2), [Ag(PPh3)3O2CH]·HCO2H (3) and [Ag(PPh3)3O2CH] (4) have been prepared and studied by infrared and solid-state 13C and 31P cross-polarization magic-angle-spinning (CPMAS) NMR spectroscopy. Crystal structures of 1, 2 and 3 show increasing degrees of disorder, but all are shown to contain [M(PPh3)3O2CH] molecular units containing a four-coordinate metal centre with the three crystallographically independent PPh3 ligands occupying three of the coordination sites and the formate anion coordinated through one oxygen atom to the fourth site. The formic acid molecule in [Cu(PPh3)3O2CH]·HCO2H is hydrogen-bonded to the formate ion (O⋯O = 2.529(5) A) to form an assembly that is best described as a biformate anion [H(HCO2)2]−. Unlike the corresponding silver(I) complex, [Cu(PPh3)3O2CH] is not stable in its unsolvated form; the loss of the solvent of crystallization results in conversion of the complex to [Cu(PPh3)2O2CH] and PPh3. Bands in the IR spectra due to the formate and formic acid groups are assigned, and these are consistent with the silver complexes having structures analogous to those of the corresponding copper complexes. The 13C CPMAS NMR spectra clearly resolve resonances from the formate and formic acid carbon atoms, while the 31P CPMAS NMR spectra exhibit complex, partially resolved multiplets arising from inequivalence of the phosphine ligands and the presence of 1J(M,31P) and 2J(31P,31P) scalar (or spin–spin) coupling. These spectra were interpreted with the aid of the 2D 31P CPCOSY technique.

Polyaniline−Dodecylbenzene Sulfonic Acid Polymerized from Aqueous Medium: A Solid State NMR Characterization

The characterization of the product of a novel polymerization procedure of aniline in aqueous medium in the presence of dodecylbenzene sulfonic acid (DBSA) as a dopant is reported. This polymerization yields stable dispersions of the emeraldine salt, which are attractive starting material for the preparation of conducting polymer blends. The as-synthesized polyaniline (PANI)−DBSA and its dedoped and redoped forms, studied by means of 13C, 15N, and 7Li solid-state NMR and wide-angle X-ray diffraction (WAXD), reveal unusual properties. NMR and weight loss measurements show that a large portion of the dopant acid is retained upon standard dedoping by NH4OH. While this dedoped material is an insulator, NMR shows that deprotonation is incomplete and identifies residual bipolaron states. Also, the dielectric properties of the NH4OH dedoped material deviate significantly from those of fully deprotonated polyaniline and resemble those of a conducting one. Dedoping with LiOH removes a large part of the dopant acid and leads to complete deprotonation. Rotating-frame relaxation measurements indicate that the dedoped forms obtained from polyaniline−DBSA exhibit single-component behavior, implying that DBSA is molecularly mixed within the polyaniline. 15N cross-polarization magic-angle spinning NMR spectra of the dedoped samples resolve two distinct imine−amine chemical species, which are formed during this particular polymerization procedure. One of the two imine−amine chemical species binds the dopant acid much stronger than the other. This is confirmed by 15N{7Li} rotational-echo double-resonance (REDOR) NMR experiments on LiOH-dedoped samples. Li+ cations are shown to substitute for H+'s as counterions at sites from which the acid anions are not removed by deprotonation. The REDOR experiments unambiguously show that the Li+ cations reside preferentially next to the imine species with the stronger DBSA binding, and within a distance of ca. 4 Å. WAXD indicates that the doped as-synthesized and dedoped powders are highly amorphous. This is attributed to the incorporation of the DBSA in the polyaniline structure.

The influence of hydration on the conformation of bovine serum albumin studied by solid-state 13C-NMR spectroscopy.

13C proton-decoupled cross-polarization magic-angle spinning nmr spectra of bovine serum albumin are reported as a function of hydration. Increases in hydration level enhance the resolution of the peak centered at about 40 ppm but has little or no effect on the other spectral peaks. Hydration has little effect on either the rotating frame proton spin-lattice relaxation time or the cross-relaxation time for any of the peaks, suggesting that the efficiency of dipolar coupling is largely preserved on hydration of the protein. Resolution enhancement of the peak at 40 ppm is not understood, but possible sources of the behavior include a decrease in the line width of contributing resonances from lysine epsilon carbons due to increased motional averaging on hydration, reordering of disulfide bridges, and titration shifts induced by hydration. Hydration of bovine serum albumin appears to have little effect on the distribution of conformations sampled by the protein so that the broad distribution of conformations observed in the dry state is also observed in the fully hydrated state. This is in contrast to lysozyme where significant ordering of the conformation is seen on hydration.

The influence of hydration on the conformation of lysozyme studied by solid‐state 13C‐nmr spectroscopy

13C proton decoupled cross-polarization magic-angle spinning nmr spectra of lysozyme are reported as a function of hydration. Increases in hydration level enhance the resolution of the spectra, particularly in the aliphatic region, but has no significant effect on either the rotating frame proton spin-lattice relaxation time or the cross-relaxation time. The enhancement in spectral resolution with hydration is attributed to a decrease in the distribution of isotropic chemical shifts, which reflects a decrease in the distribution of conformational states sampled by the protein. Changes in the distribution of isotropic chemical shifts occur after the addition of water to the charged groups as coverage of the polar side chains and peptide groups takes place. The onset of this behavior occurs at a hydration level of about 0.1-0.2 g water/g protein and is largely complete at about 0.3 g water/g protein, the same hydration range where changes in the heat capacity are observed. That hydrogen exchange of buried protons can occur at hydration levels significantly lower than those at which changes in the distribution of conformational states are first observed suggests that some motions that mediate exchange are already present in the dry protein. The preservation of efficient dipolar coupling indicates that the conformational rearrangements that do occur on hydration are small and do not involve any significant overall expansion of free volume or weakening of interactions that would increase the reorientational freedom of protein groups.

Two-dimensional chemical exchange NMR in the solid: proton dynamics in phthalocyanine

Carbon-13 solid-state cross-polarization magic angle spinning (CP-MAS) has proved to be a powerful method for investigating dynamical processes in solids. In investigations of molecules with several resonances, the intrinsically lower resolution of CP-MAS NMR spectra compared to liquid-state spectra makes the interpretation of the spectra cumbersome due to peak overlap. In some cases, such as porphyrins or phthalocyanines, these problems arising in the /sup 13/C spectra have been overcome by observing /sup 15/N instead of /sup 13/C. This requires considerable synthetic effort because the NMR measurements must be performed on an enriched sample to overcome the low sensitivity. They present here results which show that /sup 13/C 2-D exchange exchange spectroscopy can extend the resolution, allowing the spectroscopist to extract the details of the exchange process from a natural abundance sample even when the /sup 13/C resonances strongly overlap in the one-dimensional spectrum. There is another example in which 2-D exchange spectroscopy has been exploited in a solid to determine a proton exchange rate but it was in a system where resolution was not a problem. The use of 2-D spectroscopy to extend resolution should be applicable to a large class of solid compounds.