Solid-state shifts conformational calculations

So far, we have investigated higher-order structure of polypeptides by solid-state high-resolution NMR not only using experimental but also theoretical methods[2-4]. The chem cal shifts can be characterized by variations in the electronic states of the local conformation as defined by the dihedral angles(4>,W). Ando et al. have calculated contour map for the Cp carbons of an alanine dipeptide by using the FPT INDO method within the semi-empirical MO framework. The calculated map reasonably predicts the experimental version. This shows that the chemical shift behavior of the L-alanine residue Cp-carbonyl carbons in the... 

The solid-state 13C cp/mas for crystalline 39, and the quantum mechanical calculated 13C chemical shifts and JHh coupling constants for a TBC model are listed in Table 15. Comparison of the 13C chemical shift values in the cp/mas spectrum versus corresponding values for those from the quantum mechanical calculation shows them to be similar. Dissolution of either quaternary ammonium salt in CDC13 solution affords two species (88 12) in which the major species has undergone a conformational change to a (reference,S)-TCBtype3 structure, while the... 

More recently, Usuki etal. studied, using NMR techniques, the conformation of spirocyclic oxaziridines 3 derived from substituted cyclohexanones <1995JOC8028>. Depending on substitution and on the stereochemistry, these compounds exhibit a substantial upfield shift of the cyclohexyl methylene or methane protons with a 1,3-diaxial relationship to the oxaziridine N-substituent. This effect is ascribed to a conformation that places an aromatic group over the plane of the cyclohexane ring. This conformation has also been observed in the solid state by X-ray crystallography and is further supported by molecular mechanics calculations. 

Fig. 17.4. (a) The calculated stick spectra of n-Si H2 + 2 (w = ca. 7-15) with various conformations by the FPT CNDO/2 method and (b) the observed stick spectra of poly(methylphenylsi-lane) in the solid state. The chemical shift is converted with respect to that of the TT conformation. The positive sign in the 5-scale means a high frequency shift. The stick height indicates the relative peak intensity. 

Finally, it may be concluded that variable temperature Si CP/MAS NMR confirms that the configurations of CPMPS in the solid state can be assigned and the high frequency shift, exhibited by the copolysilanes, arises from the increase of the populations of the GG and GT conformations in the main chain as the temperature is increased. The FPT CNDO/2 MO calculation is very useful in assigning the observed Si NMR signal to configuration and conformation of copolysilanes. 

It has been demonstrated that the isotropic chemical shifts of systems, such as (Gly) (polyglycine(PG)), (Ala) , (Leu) (poly(L-leucine)), (Ileu) -(poly(L-isoleucine)), (Val) (poly(L-valine)), (Phe) (poly(L-phenylalanine)), (Glu(OMe)) (poly(y-methyl L-glutamate)), (Asp(OBzl)) (poly()8-benzyl l-aspartate)) and (Pro) in the peptide backbone of polypeptides in the solid state, exhibit a significant conformation-dependent change [5,6]. Both experimental observation and theoretical calculations confirm this. The Sjso for the a-helix form (97.0-99.2 ppm) appears to low frequencies by about 1.2-... 

Finally, it can be concluded that the chemical shift contour maps are very useful for predicting the dihedral angles , if/) of polypeptides and proteins in the solid state through the shifts of the amide carbonyl carbons of the Gly, L-Ala, n-Leu, L-Val and L-Asp residues. From the above-mentioned results, it has been demonstrated that the NMR chemical shift is a very powerful NMR parameter for obtaining the conformational characterization of solid polypeptides and proteins, and it is greatly enhanced by combining with chemical shift calculations. It has been described elsewhere... 

Hence, if the magnetic anisotropy of the crystalhne complex is measured experimentally, usually by single crystal measurements using the torsion fibre method, and if its crystal structure is known, then the molecular anisotropy usually can be calculated. Provided that the complex has the same conformation in solution as in the solid state, the solution dipolar shifts can be calculated and compared with the experimental data. This procedure and variations of it may be used to investigate the relative crystal fields of different brands, and to test... 

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