Aggregation of apolar peptides in organic solvents. Concentration dependence of ¹H-nmr parameters for peptide NH groups in 3₁₀ helical decapeptide fragment of suzukacillin

Abstract

Peptide NH chemical shifts and their temperature dependences have been monitored as a function of concentration for the decapeptide, Boc-Aib-Pro-Val-Aib-Val-Ala-Aib-Ala-Aib-Aib-OMe in CDCl₃(0.001-0.06M) and (CD₃)₂SO (0.001-0.03M). The chemical shifts and temperature coefficients for all nine NH groups show no significant concentration dependence in (CD₃)₂SO. Seven NH groups yield low values of temperature coefficients over the entire range, while one yields an intermediate value. In CDCl₃, the Aib(1) NH group shows a large concentration dependence of both chemical shift and temperature coefficient, in contrast to the other eight NH groups. The data suggest that in (CD₃)₂SO, the peptide adopts a 3₁₀ helical conformation and is monomeric over the entire concentration range. In CDCl₃, the 3₁₀ helical peptide associates at a concentration of 0.01M, with the Aib(1) NH involved in an intermolecular hydrogen bond. Association does not disrupt the intramolecular hydrogen-bonding pattern in the decapeptide.