Conformation-dependent 13C chemical shifts

Accurate 13C-15N interatomic distances have been measured by means of rotational echo double resonance (REDOR) experiments for oligopeptides [50]. The interatomic 13C-15N distance in the hydrogen bonded fragment was measured to be 4.5 0.1 A in five different samples studied. This finding is consistent with an a-helix structure, in agreement with conformation-dependent 13C chemical shift data. 

C NMR studies on [3-13C]Ala-labeled bR reveal that the C terminal residues, 226-235, participate in the formation of the C terminal a-helix as manifested from the peak position of 15.91 ppm with reference to the conformation-dependent 13C chemical shifts.68 69 81 82 103 The presence of the a-helix was also proved in view of the corresponding conformation-dependent displacement of peaks from [2-13C] and [l-13C]Ala-bR.81 Only part of this a-helix was visible by X-ray diffraction,25 owing to the presence of motions with correlation times of the order of 10-6s, as judged from the carbon spin-lattice relaxation times, 7jc, and spin-spin relaxation times, T2C, under CP-MAS conditions.81... 

As an NMR methodology for elucidating conformational stability in the polypeptide blends, the conformation-dependent 13C NMR chemical shift for polypeptides in the solid state has been reported. It has been elucidated that the 13C NMR chemical shifts of a number of polypeptides in the solid state, as determined by the 13C CP/MAS method, are significantly displaced, depending on their particular conformations such as a-helix, 3i-helix or p-sheet form.11,17 31... 

Spin-lattice relaxation times and 13C chemical shifts were used to study conformational changes of poly-L-lysine, which undergoes a coil-helix transition in a pH range from 9 to 11. In order to adopt a stable helical structure, a minimum number of residues for the formation of hydrogen bonds between the C = 0 and NH backbone groups is necessary therefore for the polypeptide dodecalysine no helix formation was observed. Comparison of the pH-dependences of the 13C chemical shifts of the carbons of poly-L-lysine and (L-Lys)12 shows very similar values for both compounds therefore downfield shifts of the a, / and peptide carbonyl carbons can only be correlated with caution with helix formation and are mainly due to deprotonation effects. On the other hand, a sharp decrease of the 7] values of the carbonyl and some of the side chain carbons is indicative for helix formation [854]. 

Conformation dependence of 13C chemical shifts have been theoretically studied for saturated cyclic or acyclic hydrocarbons (11-13). It is well known that the, 3C chemical shift of aliphatic carbon significantly shift to upfield when a methyl group is... 

N. K. Kochetkov, O. S. Chizhov, and A. S. Shashkov, Dependence of 13C chemical shifts on the spatial interaction of protons, and its application in structural and conformational studies of oligo- and poly-saccharides, Carbohydr. Res., 133 (1984) 173-185. 

Highly accurate interatomic distances (ultimately 0.05 A) may be obtained from REDOR experiments [49], which are therefore an attractive tool for studies of hydrogen bonding. This technique has been used recently to characterise ex-helix structures in polypeptides by measuring 13C=0---H-15N hydrogen bond lengths [50]. The intrachain 13C- 15N interatomic distances, measured for a number of different samples, were found to be 4.5 0.1 A. This finding was used as evidence for the a-helix structure, which is consistent with the conformation dependent displacements of 13C chemical shifts of the Ca, Cp and carbonyl carbons of the peptide unit [51]. 

Table 9 gives the 13C chemical shifts of the compounds77 (3—40) listed in Table 5. Depending on the nature and orientation of the substituents in the dyes, different conformers, resultant from inner rotation around the C-N bonds, could be identified. The 13C chemical shifts of azobenzene-like compounds and diazonium salts coupling products with heterocycles are reported in Refs. 9, 18, 25, 26, 36, 39, 48, 78-87. The 13C chemical shifts of dyes derived from 3-methyl-l-phenylpyrazol-5-one (of type 78 Table 10),88 pyridone (81),39 and acetoacetanilide (82)65 existing completely in hydrazone forms were assigned.

C chemical shifts of stearic, oleic, linoleic, a-linolenic and arachidonic acids have been assigned (Stoffel et al., 1972) and the data are presented in Table 21. Electric field effects associated with the dipolar head groups have been shown to be effective in influencing the 1 3C chemical shifts of unsaturated fatty acids (Batchelor et al., 1973). This electric field effect (Horsley and Sternlicht, 1968, McFarlane, 1970), predicted to be linear and dependent on distance and orientation, can be applied to problems of conformational analysis. A nondestructive technique utilizing l3Cnmr to determine the relative concentrations of the unsaturated fatty acids present in soyabeans has recently been developed (Schaefer and Stejskal, 1974). 

Chemical shifts are very sensitive probes of the molecular environment of a spin. However, in many cases their dependence on the structure is complicated and either not fully understood or too intricate to allow the derivation of reliable conformational constraints [37, 38]. An exception in this respect are the deviations of 13C (and, to some extent, 1 xf) chemical shifts from their random coil values that are correlated with the local... 

The Suter-Flory RIS model of PP (A 027 is employed to calculate the 13C NMR chemical shifts expected at the 9-Ca and the CH3 carbons and at the 8- and 10-CHj carbons in the various stereoisomers of the PP model compound 3,5,7,9,11,13,15-heptamethylheptadecane. Differences in the chemical shifts of the same carbon atom in the various stereoisomers are assumed to be attributable solely to stereo-sequence dependent differences in the probability that the given carbon atom is involved in three-bond gauche or y interactions with other carbon atoms. The Suter-Flory model provides an accurate description of the conformational characteristics of PP which permits a detailed understanding of its 3C NMR spectrum. On the other hand, the failure of Provasoti and Ferro s calculations [Macromolecules 1977, 10, 874] is directely attributed to the inadequacies of the Boyd and Breitling three-state RIS model of PP (A 022). 

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