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, P2₁2₁2₁, a = 21.383 Å, b = 11.070 Å, c = 19.560 Å, Z = 4, R₁ = 0.0527, and R_w = 0.1562; (2) monoclinic, P2₁, a = 9.391 Å, b = 21.278 Å, c = 11.662 Å, β = 99.125, Z = 2, R₁ = 0.0507, and R_w = 0.1447; and (3) triclinic, P1, a = 12.545 Å, b = 14.913 Å, c = 15.330 Å, α = 77.622, β = 66.601, γ = 78.839, Z = 2, R₁ = 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) 3₁₀-helical structure, 2 was folded into a distorted β-hairpin nucleated by a type II′ β-turn-like structure, and 3 was folded into an S-shape turn structure based on two type II/III β-turns.