CONFORMATIONAL CHARACTERIZATION BY CIRCULAR-DICHROISM SPECTROSCOPY OF VARIOUS FRAGMENTS AND ANALOGS OF CALCITONIN-GENE-RELATED PEPTIDE

A conformational study by circular-dichroism spectroscopy of calcitonin-gene-related peptide (CGRP) and related fragments and analogs was carried out in structure-promoting solvent mixtures. The structural characterization of rat CGRPalpha and the two isoforms of human CGRP, alpha and beta, revealed that these peptides possess very similar conformational features. The far-ultraviolet circular-dichroism spectra, in pure water, of human CGRPalpha, (hCGRPalpha), [Acm-Cys2,7]hCGRPalpha, various fragments and analogs indicated that these peptides exhibited predominantly a random-coil conformation. The addition of increasing concentrations of 1,1,1,3,3,3-hexafluoro-2-propanol to the peptide solutions resulted in a transition from a random-coil conformation to a stabilized alpha-helical structure. The substantial loss of helical content measured with [Acm-CyS2,7]hCGRPalpha, [Acm-Cys2,7]hCGRP-(1-24)-CONH2 and hCGRP-(8-37), compared to hCGRPalpha, suggested that the N-terminal disulfide bridge of hCGRPalpha is essential for adopting a highly stabilized alpha-helical conformation. Moreover, the lower helical content of hCGRP-(8-37), as compared to [Acm-Cys2,7]hCGRPalpha, as well as spectroscopic results measured with various fragments and analogs of hCGRP-(8-37) revealed that N-terminal residues found in the peptide segment 1-12 are important for the full conservation of the amphiphilic alpha-helix. In addition, the similar alpha-helical content of hCGRP-(8-37) and hCGRP-(8-18) indicated that the C-terminal segment 19-37 is not essential for the stabilization of the alpha-helix structure.