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 CGRP alpha 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 CGRP alpha, (hCGRP alpha), [Acm-Cys2,7]hCGRP alpha, 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]hCGRP alpha, [Acm-Cys2,7]hCGRP-(1-24)-CONH2 and hCGRP-(8-37), compared to hCGRP alpha, suggested that the N-terminal disulfide bridge of hCGRP alpha 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]hCGRP alpha, 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.