Three-dimensional structural control of diastereomeric Leu-Leu-Aib-Leu-Leu-Aib sequences in the solid state

Three diastereomeric -Leu-Leu-Aib-Leu-Leu-Aib- peptides composed of the same numbers of L-Leu, D-Leu, and Aib residues were synthesized: Boc-L-Leu-L-Leu-Aib-D-Leu-D-Leu-Aib-OMe (1), Boc-L-Leu-D-Leu-Aib-L-Leu-D-Leu-Aib-OMe (2), and Boc-L-Leu-D-Leu-Aib-D-Leu-L-Leu-Aib-OMe (3). The crystals of the three peptides were characterized by X-ray crystallographic analysis as follows: (1) orthorhombic, P2(1)2(1)2(1), a = 21.383 A, b = 11.070 A, c = 19.560 A, Z = 4, R(1) = 0.0527, and R(w) = 0.1562; (2) monoclinic, P2(1), a = 9.391 A, b = 21.278 A, c = 11.662 A, beta = 99.125, Z = 2, R(1) = 0.0507, and R(w) = 0.1447; and (3) triclinic, P1, a = 12.545 A, b = 14.913 A, c = 15.330 A, alpha = 77.622, beta = 66.601, gamma = 78.839, Z = 2, R(1) = 0.0775, and R(w) = 0.1971. The three diastereomeric peptides, 1, 2, and 3, showed unique conformations. That is to say, 1 was folded into a left-handed (M) 3(10)-helical structure, 2 was folded into a distorted beta-hairpin nucleated by a type II' beta-turn-like structure, and 3 was folded into an S-shape turn structure based on two type II/III beta-turns.