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

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 (Delta G(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) aryllaryl 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 beta-propensity of Trp. The beta-propensity of Trp is context dependent, but for the systems studied, always greater than that of Thr (by 0.4-4.7 kJ/mol). At nonH-bonded positions remote from the turn, two alternative EtF geometries are observed and there is no evidence of additional stabilization due to the Trp/Trp interaction. The NMR structuring shift diagnostics of EtF Trp/Trp, Trp/Lys pi-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(n)G- 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 pi basicity.