Conformational characterization by circular‐dichroism spectroscopy of various fragments and analogs of calcitonin‐gene‐related peptide

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Abstract
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 CGRPα and the two isoforms of human CGRP, α and β, revealed that these peptides possess very similar conformational features. The far‐ultraviolet circular‐dichroism spectra, in pure water, of human CGRPα, (hCGRPα), [Acm‐Cys2,7]hCGRPα, 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 α‐helical structure. The substantial loss of helical content measured with [Acm‐Cys2,7]hCGRPα, [Acm‐Cys2,7]hCGRP‐(1–24)‐CONH2 and hCGRP‐(8–37), compared to hCGRPα, suggested that the N‐terminal disulfide bridge of hCGRPα is essential for adopting a highly stabilized α‐helical conformation. Moreover, the lower helical content of hCGRP‐(8–37), as compared to [Acm‐Cys2,7]hCGRPα, 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 α‐helix. In addition, the similar α‐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 α‐helix structure.