Conformational Effects on Peptide Aggregation in Organic Solvents: Spectroscopic Studies of Two Chemotactic Tripeptide Analogs

The aggregation behavior of the chemotactic peptide analogs, Formyl-Met-Leu-Phe-OMe (1) and Formyl-Met-AibPhe-OMe ${(2)}_1$, has been studied in chloroform and dimethylsulfoxide over the concentration range of 0.2-110 mM by 'H-nmr spectroscopy. Both peptides associate in $CD{Cl}_3$, at concentrations \geq 2 mM, while there is no evidence for aggregation in ${({CD}_3)}_2SO$. Analog 1 adopts an extended conformation in both solvents favoring association to form \beta-sheet structures. A folded, \gamma-turn conformation involving a 3 \rightarrow 1 hydrogen bond between Met CO and Phe NH is supported by 1H-, 13Gnmr, and ir studies of analog 2. The influence of backbone conformation on the ease of peptide aggregation is demonstrated by ir studies in $CH{Cl}_3$ and CD studies in dioxane.