Disruption of Native β-Turns: Consequence of Folding Competition between Native and Orthanilic Acid Proline-BasedPseudoβ-Turn

Crystal and NMR Structures of a Peptidomimetic β-Turn That Provides Facile Synthesis of 13-Membered Cyclic Tetrapeptides

Herein we report the unique conformations adopted by linear and cyclic tetrapeptides (CTPs) containing 2-aminobenzoic acid (2-Abz) in solution and as single crystals. The crystal structure of the linear tetrapeptide H₂ N-d-Leu-d-Phe-2-Abz-d-Ala-COOH (1) reveals a novel planar peptidomimetic β-turn stabilized by three hydrogen bonds and is in agreement with its NMR structure in solution. While CTPs are often synthetically inaccessible or cyclize in poor yield, both 1 and its N-Me-d-Phe analogue (2) adopt pseudo-cyclic frameworks enabling near quantitative conversion to the corresponding CTPs 3 and 4. The crystal structure of the N-methylated peptide (4) is the first reported for a CTP containing 2-Abz and reveals a distinctly planar 13-membered ring, which is also evident in solution. The N-methylation of d-Phe results in a peptide bond inversion compared to the conformation of 3 in solution.

Residue dependent hydrogen-bonding preferences in orthanilic acid-based short peptide β-turn motifs

This communication describes the competition between native β-turn (C10) and 2-aminobenzenesulfonic acid (^SAnt)(orthanilic acid)-based pseudo β-turn (C11) in their hybrid peptides.