KEY WORDSHelix-Coil Transition I Polypeptide I NMR I Poly(y-benzyl L-glutamate) I Chemical Shift I Although much simpler in structure than proteins, synthetic polypeptides have served as models for studying various conformational features of proteins. High resolution nuclear magnetic resonance (NMR) spectroscopy has been extensively applied to the study of conformations and conformational transitions of oligo-and polypeptides, particularly their helix-coil transitions. A wide range of carbon-13 chemical shift is especially useful for obtaining information on carbon atoms in amino acid residues such as those in the backbone and side chains. In NMR studies on conformational changes in proteins and polypeptides, the magnitude and sign of the chemical shift displacements induced by external perturbations provide important information. Thus it is important to find the general rule underlying chemical shift displacements accompanying the helix-coil transition of polypeptides.In preceding papers, the helix-coil transitions of poly(y-benzyl L-glutamate) [poly(Glu(OBzl))],l poly(p-benzyl L-aspartate) [poly(Asp(0Bz1))], 2 and poly(y-methyl L-glutamate) [poly(Giu(OMe)W in trifluoroacetic acid (F 3 AcOH)/deuterated chloroform (CDC1 3 ) solvent mixtures were studied by NMR, and some regularity was found to exist in the order of chemical shift displacements along the side chains of these polypeptides. The solvent-induced shifts of carbons in the model compounds were compared with those in polypeptides, and it was concluded that the conformational changes play a dominant role in the chemical shift displacements of amido, o:, p, and y carbon atoms in polypeptides, while the chemical shift displacements of the remaining side chain carbon atoms may be attributed only to the interaction between the side chain ester group and F 3 AcOH. 3 -5 This paper reports the findings of a 13 C NMR study on the helix-coil transitions of poly(yphenethyl L-glutamate) [poly(Glu(OPE))] and poly[y-(3-phenylpropyi)-L-glutamate][poly-(Glu(OPP))] carried out to check the generality of the above conclusion. These polypeptides differ from poly[Giu(OBzl)] only with respect to the length between the ester and phenyl groups of the side chains. The effect of the side chain length on the helix stability is also discussed on the basis of data obtained for a series of poly(L-glutamate)s.
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