Solution conformational dynamics of the C-terminal residues in endothelin-1 and some analogues: a time-resolved...

The rotational relaxation times of the single tryptophan residues in endothelin-1, [Ala1,3,11,15]endothelin-1, human pro-endothelin-1, the linear hexapeptide Ac-His-Leu-Asp-Ile-Ile-Trp which corresponds to the C-terminal residues 16-21 in endothelin-1, the cyclic pentapeptide BQ123, and several di- and tri-peptides possessing C-terminal tryptophan residues have been determined from time-resolved fluorescence anisotropy decays obtained by phase/modulation techniques. Fluorescence lifetime distribution widths have also been examined as predictors of conformational heterogeneity/restriction. A significant contribution from a slow rotational component supports either the persistence, on the nano-second timescale at least, of a non-flexible alpha-helical structure for the C-terminal tail residues of endothelin-1 in water as solvent, as seen in the X-ray crystallographic structure, or the interaction of the C-terminal tail residues 16-21 with the constrained disulfide-bridged core residues 1-15. This slow rotational contribution is less evident in the linear, acyclic tetraalanine analogue but greatly increased in pro-endothelin-1. In BQ123 the fluorescence characteristics support the occurrence of a dominant rotameric form involving the indole sidechain of the D-tryptophan residue (C alpha-C beta torsion angle chi 1 of 60 degrees, as previously determined by NMR.