Design, synthesis, and conformation of a model peptide of endothelin with cystine-stabilized alpha-helix motif.

A model 16-peptide of endothelin-1 (MET-1), which has the minimized sequence homology to the corresponding part of endothelin-1 (ET-1), was designed to confirm the cystine-stabilized alpha-helix motif. The model structure consists of an extended structure, a beta-turn part, and an alpha-helix structure that is stabilized by two disulfide bonds. The alpha-helix segment was designed to emphasize the amphiphilic nature. In order to combine the extended structure and the alpha-helix segment, a D-Ala-Pro sequence was selected to fix the beta-turn. The model endothelin 16-peptide amide was synthesized by solid-phase synthesis on a 4-methylbenzhydrylamine resin. Its conformation was examined by CD and two-dimensional (2D) 1H-nmr measurements. MET-1 showed similar CD patterns to ET-1 in both buffer and 50% aqueous trifluoroethanol solution. The 2D nmr experiments in 50% aqueous ethylene glycol revealed that MET-1 closely resembles the conformation of ET-1 with an extended structure, an alpha-helix, and a beta-turn unit in the same position of the sequence. Furthermore, model peptides without disulfide bond(s) could not assume a stable structure in aqueous solution, while they did have similar alpha-helical content in 50% trifluoroethanol with MET-1. When the two disulfide bridges were simultaneously formed, the peptide with the correct disulfide bonds (MET-1) was obtained in threefold excess to the isomer (apamin type, MET-2). These findings obtained by the modeling of ET-1 showed an important role for the stabilization of peptide conformation with disulfide bonds.