Induced 13C shifts by coil-supporting solvents observed in oligopeptides as model compounds of polypeptides

The origin of the chemical shift differences of carbons in polypeptides which accompany the helix-coil transition has been investigated by using oligopeptides, benzyloxycarbonyl-γ-ethyl-L -glutamyl-diethyl-L -glutamate and benzyloxycarbonyl-di-(γ-ethyl-L -glutamyl)-diethyl-L -glutamate, as models of the backbone of polypeptides. Structures of aggregates in deuterated chloroform were proposed for these oligopeptides on the basis of concentration dependence and temperature dependence of the chemical shifts of protons and carbons, and spin-lattice relaxation times. Antiparallel and/or parallel “in-register” structures for extended forms and “out-of-register” network of extended forms are coexisting in deuterated chloroform solution for these oligopeptides. From the shift for the carbons of the oligopeptides induced by organic acids, it was in ferred that down-field shifts are induced at α and amide carbons in polypeptides by organic acids. By comparing the induced shift of the carbons in the peptides with the chemical shift differences of the carbons in polypeptides which accompany the helix-coil transitions, it was found that the conformational changes play a predominant role in the origin of the chemical shift differences of the carbons in polypeptides which accompany the helix-coil transitions, it was found that the conformational changes play a predominnant role in the orgin of the chemical shift differences of amide, α, β, and γ carbons in polypeptides.