Aggregation of Apolar Peptides in Organic Solvents. Concentration Dependence of 1H-NMR Parameters for Peptide NH Groups in $3_{10}$ Helical Decapeptide Fragment of Suzukacillin

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 $CD{Cl}_3$ (0.001-0.06M) and ${({CD}_3)}_2SO$ (0.001-0.03M). The chemical shifts and temperature coefficients for all nine NH groups show no significant concentration dependence in ${({CD}_3)}_2SO$. Seven NH groups yield low values of temperature coefficients over the entire range, while one yields an intermediate value. In $CD{Cl}_3$, 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 ${({CD}_3)}_2SO$, the peptide adopts a $3_{10}$ helical conformation and is monomeric over the entire concentration range. In $CD{Cl}_3$, the $3_{10}$ 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.