A SEARCH FOR THE IDEAL TYPE-I BETA-TURN

In 1968 C. Venkatachalam (Biopolymers, Vol. 6, pp. 1425-1436) predicte d the ideal forms of beta-turns (type I, type II, etc.) based entirely on theoretical calculations. Subsequently, over a thousand x-ray stru ctures 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+1) = -60 degrees, psi(i1) = -30 degrees, phi(i+2) = -90 degrees and psi(i+2) = 0 degrees as b ackbone torsion angle values, and these have been used from that time as reference values for the identification of the type I beta-turn. Ho wever, it has never been clarified whether these ''ideal'' backbone to rsion 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 ami no acid pairs that approach the ideal form of the type I beta-turn. Th e analysis resulted in four sequences where the deviation from ideal v alues for any main-chain torsion angles was less than 2 degrees. In or der to determine whether such a backbone fold is possible only in prot eins owing to fortuitous cooperation of different folding effects, or whether it occurs even in short peptides, various attempts have been m ade to design the optimal amino acid sequence. Such a peptide model co mpound adopting precisely the predicted torsion angle values [phi(i+1) = -60 degrees, psi(i+1) = -30 degrees, phi(i+2) = -90 degrees, and ps i(i+2) = 0 degrees] could provide valuable information. The solid stat e conformation of cyclo[(delta)Ava-Gly-Pro-Thr(O(t)Bu)-Gly] reported h erein, incorporating the -Pro-Thr- subunit, yields values suggesting t hat the ''ideal'' type I beta-turn is even possible for a peptide wher e there are no major environmental effects present. (C) 1996 John Wile y & Sons, Inc.