Conformational aspects of gastrin‐related peptides: A circular dichroism study

The conformational properties of a series of gastrin‐related peptides in aqueous solution and in 2,2,2‐trifluoroethanol (TFE) have been investigated by CD measurements. In aqueous solution the peptides Leu 32 ‐HG‐34 (human big gastrin), Nle 15 ‐HG‐17 (human little gastrin), and Nle 11 ‐HG‐13 assume a random‐coil structure in the pH range 3–7. In TFE the three hormones fold into partially ordered structures, consisting of mixtures of α‐helix, β‐form and random coil. Comparison with the CD properties of the shorter gastrin peptides HG‐4 (tetragastrin), N α ‐Boc‐HG‐5 (pentagastrin), and HG‐7 (heptagastrin) indicates that the biologically important C‐terminal sequence Trp‐Met‐Asp‐Phe‐NH 2 in TFE does not maintain the same geometry upon elongation of the chain at the N‐terminus from 4 to 34 residues. Thus, the various conformations in solution of the gastrin peptides examined do not provide a structural explanation for their very similar biological activity. Therefore, we hypothesize that the C‐terminal tetrapeptide amide folds into an “active” structure only upon interaction with the receptor.