A search for the ideal type I beta-turn

In 1968 C. Venkatachalam (Biopolymers, Vol. 6, pp. 1425-1436) predicted the ideal forms of beta-turns (type I, type II, etc.) based entirely on theoretical calculations. Subsequently, over a thousand x-ray structures of different globular proteins have been analyzed, with results suggesting that the most important form among the hairpin conformers is the type I beta-turn. For the latter type of hairpin conformation, the original computations had predicted phi i+I = -60 degrees, psi i+1 = -30 degrees, phi i+2 = -90 degrees, and psi i +2 = 0 degrees as backbone torsion angle values, and these have been used from that time as reference values for the identification of the type I beta-turn. However, it has never been clarified whether these "ideal" backbone torsion angle values exist in real structures, or whether these torsion angles are only "theoretical values." Using the most recent release of the Protein Data Bank (1994), a survey has been made to assign amino acid pairs that approach the ideal form of the type I beta-turn. The analysis resulted in four sequences where the deviation from ideal values for any main-chain torsion angles was less than 2 degrees. In order to determine whether such a backbone fold is possible only in proteins owing to fortuitous cooperation of different folding effects, or whether it occurs even in short peptides, various attempts have been made to design the optimal amino acid sequence. Such a peptide model compound adopting precisely the predicted torsion angle values [phi i+1 = -60 degrees, psi i +1 = -30 degrees, phi i +2 = -90 degrees, and psi i+2 = 0 degrees] could provide valuable information. The solid state conformation of cyclo[(delta)Ava-Gly-Pro-Thr(OtBu)-Gly] reported herein, incorporating the -Pro-Thr- subunit, yields values suggesting that the "ideal" type I beta-turn is even possible for a peptide where there are no major environmental effects present.