Very short peptides with stable folds: Building on the interrelationship of Trp/Trp, Trp/cation, and Trp/backbone–amide interaction geometries

Abstract

By combining a favorable turn sequence with a turn flanking Trp/Trp interaction and a C‐terminal H‐bonding interaction between a backbone amide and an i‐2 Trp ring, a particularly stable (ΔG_U > 7 kJ/mol) truncated hairpin, Ac‐WI‐(D‐Pro‐D‐Asn)‐KWTG‐NH₂, results. In this construct and others with a W‐(4‐residue turn)‐W motif in severely truncated hairpins, the C‐terminal Trp is the edge residue in a well‐defined face‐to‐edge (FtE) aryl/aryl interaction. Longer hairpins and those with six‐residue turns retain the reversed “edge‐to‐face” (EtF) Trp/Trp geometry first observed for the trpzip peptides. Mutational studies suggest that the W‐(4‐residue turn)‐W interaction provides at least 3 kJ/mol of stabilization in excess of that due to the greater β‐propensity of Trp. The π–cation, and Trp/Gly‐H_N interactions have been defined. The latter can give rise to >3 ppm upfield shifts for the Gly‐H_N in ‐WX_nG‐ units both in turns (n = 2) and at the C‐termini (n = 1) of hairpins. Terminal YTG units result in somewhat smaller shifts (extrapolated to 2 ppm for 100% folding). In peptides with both the EtF and FtE W/W interaction geometries, Trp to Tyr mutations indicate that Trp is the preferred “face” residue in aryl/aryl pairings, presumably because of its greater π basicity. Proteins 2009.