NMR Property Research Articles

The indirect mechanism of action of the trifluoroacetyl peptides on elastase. Enzymatic and 19F NMR studies.

Trifluoroacetyl (CF3CO) dipeptide anilides are potent reversible inhibitors of elastase. Their 19F NMR spectra in the presence of the enzyme correspond therefore to slow chemical exchange. Their characteristics are very similar to those previously reported for other CF3CO-peptides. This should correspond to a single binding mode, in which the CF3CO group lies in a specific site in close contact with protein protons. Elastase, irreversibly inhibited by alkylation of Ser-195 with phenylsulfonyl fluoride derivatives, binds the CF3CO-Ala-containing dipeptide anilides still more tightly than the native enzyme. It no longer binds the dipeptide anilides containing a bulky CF3CO-Lys group, suggesting a location of the CF3CO site near the S1 subsite. On the other hand, the chemical shift of the CF3CO 19F resonance in the complex is the only NMR property affected by the enzyme sulfonylation, the T1 and nuclear Overhauser effect values being unmodified as compared to those in the complexes with the native enzyme. The observation of the NMR characteristics of elastases inhibited by phenylsulfonyl groups substituted with fluorine shows that complexation with CF3CO-peptides induces a change of conformation of the catalytic site which should correspond to a displacement of His-57 toward the ortho position of the phenylsulfonyl ring. Such a transconformation is not observed with corresponding acetylated peptides. The phenylsulfonyl fluoride derivatives are still able to react with elastase in the presence of a large excess of CF3CO-peptides. In such conditions the rate of inactivation is much slower but still at least 5% of that measured in the absence of the reversible inhibitors. This residual activity is hardly compatible with the presence of two exclusive modes of interaction of the CF3CO-peptides with native elastase. On the contrary, these observations are better interpreted by a single and identical mode of binding of the peptide to the native and sulfonylated enzymes. According to this mode of binding, the reversible inhibition of elastase by CF3CO-peptides should correspond to an indirect mechanism by which a change of conformation at the active site results in a reduced catalytic activity.

Automatic Assignment of Absolute Configuration from 1D NMR Data

[reaction: see text] Opposite enantiomers exhibit different NMR properties in the presence of an external common chiral element, and a chiral molecule exhibits different NMR properties in the presence of external enantiomeric chiral elements. Automatic prediction of such differences, and comparison with experimental values, leads to the assignment of the absolute configuration. Here two cases are reported, one using a dataset of 80 chiral secondary alcohols esterified with (R)-MTPA and the corresponding (1)H NMR chemical shifts and the other with 94 (13)C NMR chemical shifts of chiral secondary alcohols in two enantiomeric chiral solvents. For the first application, counterpropagation neural networks were trained to predict the sign of the difference between chemical shifts of opposite stereoisomers. The neural networks were trained to process the chirality code of the alcohol as the input, and to give the NMR property as the output. In the second application, similar neural networks were employed, but the property to predict was the difference of chemical shifts in the two enantiomeric solvents. For independent test sets of 20 objects, 100% correct predictions were obtained in both applications concerning the sign of the chemical shifts differences. Additionally, with the second dataset, the difference of chemical shifts in the two enantiomeric solvents was quantitatively predicted, yielding r(2) 0.936 for the test set between the predicted and experimental values.

In vivo17O NMR approaches for brain study at high field

17O is the only stable oxygen isotope that can be detected by NMR. The quadrupolar moment of 17O spin (I = 5/2) can interact with local electric field gradients, resulting in extremely short T1 and T2 relaxation times which are in the range of several milliseconds. One unique NMR property of 17O spin is the independence of 17O relaxation times on the magnetic field strength, and this makes it possible to achieve a large sensitivity gain for in vivo 17O NMR applications at high fields. In vivo 17O NMR has two major applications for studying brain function and cerebral bioenergetics. The first application is to measure the cerebral blood flow (CBF) through monitoring the washout of inert H2 17O tracer in the brain tissue following an intravascular bolus injection of the 17O-labeled water. The second application, perhaps the most important one, is to determine the cerebral metabolic rate of oxygen utilization (CMRO2) through monitoring the dynamic changes of metabolically generated H2 17O from inhaled 17O-labeled oxygen gas in the brain tissue. One great merit of in vivo 17O NMR for the determination of CMRO2 is that only the metabolic H2 17O is detectable. This merit dramatically simplifies both CMRO2 measurement and quantification compared to other established methods. There are two major NMR approaches for monitoring H2 17O in vivo, namely direct approach by using 17O NMR detection (referred as direct in vivo 17O NMR approach) and indirect approach by using 1H NMR detection for measuring the changes in T2- or T1rho-weighted proton NMR signals caused by the 17O-1H scalar coupling and proton chemical exchange (referred as indirect in vivo 17O NMR approach). Both approaches are suitable for CBF measurements. However, recent studies indicated that the direct in vivo 17O NMR approach at high/ultrahigh fields appears to offer significant advantages for quantifying and imaging CMRO2. New developments have further demonstrated the feasibility for establishing a completely noninvasive in vivo 17O NMR approach for imaging CMRO2 in a rat brain during a brief 17O2 inhalation. This approach should be promising for studying the central role of oxidative metabolism in brain function and neurological diseases. Finally, the similar approach could potentially be applied to image CMRO2 noninvasively in human brain.

Study of Vulcanization and Degradation Chemistry in Natural Rubber by Solid-State 13C NMR and Physical Property Measurements

Abstract Sulfur-vulcanization and heat aging of natural rubber was investigated by solid-state 13C NMR along with the measurements of physical properties. 13C NMR detects all of the sulfurization occurring in natural rubber network including inter- and intra-molecular crosslink and pendant linkages, while physical measurements such as modulus at small strain and equilibrium swelling postulate detect only an amount of physically effective crosslinks. By comparing those two approaches, the efficiency of sulfur bonding in contributing to physical strength can be determined. It was found that the efficiency became lower for the samples with higher curing temperature, longer cure time, and also at higher sulfur level. It was assumed by 13C chemical shift analysis that all of the sulfurized isoprene units were cis-to-trans isomerized during the vulcanization and heat aging, even for ineffective sulfurization. Furthermore, it was found that the configuration of those units remained in the trans-1,4 form after the desulfurization at either reversion or heat aging. The nature of the trans-1,4 form in the isoprene series was assumed to be more rigid compared to that of a cis-1,4 form. This resulted in an introduction of chain scission at large deformation for the rubber with higher trans-1,4 content. Comparing several samples with different cure temperatures, cure degrees, and sulfur levels, it was postulated that such cis-to-trans isomerization is a main factor governing a degradation chemistry at elevated cure temperature, overcuring and heat aging.

Biodegradability of polysaccharide/EVA reject blends by high resolution NMR and mechanical property

Relaxation spectroscopy analysis represents a significant advance in dynamic methods to study the molecular response of polymeric materials. The stress-strain properties are also a good source of mechanical response. In the present work the measurements of proton spin-lattice relaxation time in the rotating frame and stress-strain properties have been chosen to understand better the biodegradability process in ethylene- co-vinylacetate industry (EVA) reject. It was observed that after the addition of polysaccharide to the copolymer it becomes more rigid; as a consequence the EVA reject degrades more rapidly due to the good interaction between them.

Cross-polarization magic-angle spinning nuclear magnetic resonance study of platinum complexes containing the cis-P 2PtC 2 fragment

31P and 195Pt cross-polarization magic-angle spinning nuclear magnetic resonance (CP-MAS NMR) spectra of three platinum complexes of formal oxidation state Pt(0) and Pt(II), respectively, are reported. All three complexes, (Ph 3P) 2Pt(C 2H 4) ( 1), (Et 2PCH 2CH 2PEt 2)Pt(CCH) 2 ( 2) and (Ph 2PCH 2CH 2 PPh 2)Pt(CCC(CH 3) CH 2) 2 ( 3) contain the square-planar cis-P 2PtC 2 fragment and show unusual NMR spectroscopic properties insofar that the 195Pt shielding patterns are fairly narrow in relation to what one would generally have to expect for 195Pt in square-planar coordination. Another unexpected NMR property of the cis-P 2PtC 2 fragment in 1–3 is the absence of spinning frequency-dependent second-order effects in this solid-state ABX spin system.

Sorption of water by nylon 66 and kevlar 29. Equilibria and kinetics

AbstractThe sorption of water by nylon 66 and Kevlar 29 fabrics was studied over a range of 20–98% relative humidities at 27°C. Equilibrium and rate relationships were developed from the data in this study and from nylon 66 and nylon 6 data of other studies involving fiber and film. The ratio of sorbed moisture to amide concentration is, on average and at high relative humidities, one water molecule per amide unit. The nylon equilibrium data show that there exist threshold relative humidities above and below which sorption properties differ. One threshold is correlated with the glass transition. Another is identified in the glassy state of nylon 6 in which moisture is tightly bound. The equilibrium constant values are highest and the moisture is most tightly bound at very low humidities, and the values are lowest in the plastic region at high humidities. NMR, DSC, and mechanical property data from others are correlated with the sorption discontinuities observed in this study. Rate data indicate that sorption is a diffusion‐controlled process.

Ortho Cyano Substituted Negative Dielectric Anisotropic Liquid Crystals

Abstract A homologous series of 2′-cyano-4′- n-alkylphenyl-4- n-alkylbiphenylcarboxylates and 4- n-alkylcyclohexyl-p-benzoates were synthesized by treating 2′-cyano-4′-n-alkyl substituted phenols with 4-n-alkyl substituted biphenyl carboxylic acids and cyclohexyl benzoic acids. The method of synthesis was discussed. All these compounds were nematogenic in character and their phase transition temperatures were examined. A wide range of mesomorphism was observed in almost all the homologous series. IR, NMR and dielectric property of these compounds were studied. The negative dielectric anisotropic property of these liquid crystalline compounds makes them suitable for application in positive GH display devices.