Organometallic β-turn mimetics. A structural and spectroscopic study of inter-strand hydrogen bonding in ferrocene and cobaltocenium conjugates of amino acids and dipeptides

Three thousand eight hundred ninety-nine beta-turns have been identified and classified using a nonhomologous data set of 205 protein chains. These were used to derive beta-turn positional potentials for turn types I' and II' for the first time and to provide updated potentials for formation of the more common types I, II, and VIII. Many of the sequence preferences for each of the 4 positions in turns can be rationalized in terms of the formation of stabilizing hydrogen bonds, preferences for amino acids to adopt a particular conformation in phi, psi space, and the involvement of turn types I' and II' in beta-hairpins. Only 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 classified as multiple turns. Several types of multiple turn have been identified and analyzed.