A REVISED SET OF POTENTIALS FOR BETA-TURN FORMATION IN PROTEINS

Three thousand eight hundred ninety-nine beta-turns have been identifi ed and classified using a nonhomologous data set of 205 protein chains . These were used to derive beta-turn positional potentials for turn t ypes I' and II' for the first time and to provide updated potentials f or formation of the more common types I, II, and VIII. Many of the seq uence preferences for each of the 4 positions in turns can be rational ized. in terms of the formation of stabilizing hydrogen bonds, prefere nces for amino acids to adopt a particular conformation in phi, psi sp ace, and the involvement of turn types I' and II' in beta-hairpins. On ly 1,632 (42%) of the turns occur in isolation; the remainder have at least 1 residue in common with another turn and have hence been classi fied as multiple turns. Several types of multiple turn have been ident ified and analyzed.