A Crystalline beta-Hairpin Peptide Nucleated by a Type I’ Aib-D-Ala beta-Turn: Evidence for Cross-Strand Aromatic Interactions

Recent progress in the design of beta-hairpin peptides[1] and beta-sheet models has been based on the ability to nucleate reverse turns of the appropriate stereochemistry. D-Pro-Gly[2] and to a lesser extent Asn-Gly[3] segments have been shown to facilitate formation of type I’ and II’ beta-turns, which are most often found at the site of sharp polypeptide chain reversal, that is, beta-hairpins in proteins.[4, 5] The prime turns, I’ and II’, can exert differing influences on the relative twist of the antiparallel strands. The I’ turn has the sense of twist that matches the twisting of adjacent beta-strands in proteins. In contrast, the II’ turn results in a more planar arrangement with the hairpin flattening to a considerable degree.[4a,e] The D-Pro-Xxx segment can in principle adopt both II’ and I’ turn conformations as si (D-Pro) values of +30 deg. and –120 deg. are energetically favorable.[1a, 5]