Structure and Assembly of Designed β-Hairpin Peptides in Crystals as Models for β-Sheet Aggregation

De novo designed β-hairpin peptides have generally been recalcitrant to crystallization. The crystal structures of four synthetic peptide β-hairpins, Boc-Leu-Val-Val-dPro-Gly-Leu-Phe-Val-OMe (1), Boc-Leu-Phe-Val-dPro-Ala-Leu-Phe-Val-OMe (2), Boc-Leu-Val-Val-dPro-Aib-Leu-Val-Val-OMe (3), and Boc-Met-Leu-Phe-Val-dPro-Ala-Leu-Val-Val-Phe-OMe (4), are described. The centrally positioned dPro-Xxx segment promotes prime β-turn formation, thereby nucleating β-hairpin structures. In all four peptides well-defined β-hairpins nucleated by central type II‘ dPro-Xxx β-turns have been characterized by X-ray diffraction, providing a view of eight crystallographically independent hairpins. In peptides 1−3 three intramolecular cross-strand hydrogen bonds stabilized the observed β-hairpin, with some fraying of the structures at the termini. In peptide 4, four intramolecular cross-strand hydrogen bonds stabilized the hairpin. Peptides 1−4 reveal common features of packing of β-hairpins into crystals. Two-dimensional sheet formation mediated by intermolecular hydrogen bonds formed between antiparallel strands of adjacent molecule is a recurrent theme. The packing of two-dimensional sheets into the crystals is mediated in the third dimension by bridging solvents and interactions of projecting side chains, which are oriented on either face of the sheet. In all cases, solvation of the central dPro-Xxx peptide unit β-turn is observed. The hairpins formed in the octapeptides are significantly buckled as compared to the larger hairpin in peptide 4, which is much flatter. The crystal structures provide insights into the possible modes of β-sheet packing in regular crystalline arrays, which may provide a starting point for understanding β-sandwich and cross-β-structures in amyloid fibrils.