Switching of turn conformation in an aspartate anion peptide fragment by NH . . . O- hydrogen bonds

Aspartic acid protease model peptides Z-Phe-Asp(COOH)-Thr-Gly-Ser-Ala-NHCy (1) and AdCO-Asp(COOH)-Val-Gly-NHBzl (3), and their aspartate anions (NEt4)[Z-Phe-Asp(COO-)-Thr-Gly-Ser-Ala-NHCy] (2) and (NEt4)[AdCO-Asp(COO-)-Val-Gly-NHBzl] (4), having an invariant primary sequence of the Asp-X(Thr,Ser)-Gly fragment, were synthesized and characterized by 1H-NMR, CD, and infrared (IR) spectroscopies. NMR structure analyses indicate that the Asp O(delta) atoms of the aspartate peptide 2 are intramolecularly hydrogen-bonded with Gly, Ser, Ala NH, and Ser OH, supporting the rigid beta-turn-like conformation in acetonitrile solution. The tripeptide in the aspartic acid 3 forms an inverse gamma-turn structure, which is converted to a beta-turn-like conformation because of the formation of the intramolecular NH . . . O- hydrogen bonds with the Asp O(delta) in 4. Such a conformational change is not detected between dipeptides AdCO-Asp(COOH)-Va-NHAd (5) and (NEt4)[AdCO-Asp(COO-)-Val-NHAd] (6). The pK(a) value of side-chain carboxylic acid (5.0) for 3 exhibits a lower shift (0.3 unit) from that of 5 in aqueous polyethyleneglycol lauryl ether micellar solution. NMR structure analyses for 3 in an aqueous micellar solution indicate that the preorganized turn structure, which readily forms the NH . . . O- hydrogen bonds, lowers the pK(a) value and that resulting hydrogen bonds stabilize the rigid conformation in the aspartate anion state. We found that the formation of the NH . . . O- hydrogen bonds involved in the hairpin turn is correlated with the protonation and deprotonation state of the Asp side chain in the conserved amino acid fragments.