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‐Aib‐Phe‐OMe (2), has been studied in chloroform and dimethylsulfoxide over the concentration range of 0.2–110 mM by 1H‐nmr spectroscopy. Both peptides associate in CDCl3 at concentrations ≥ 2 mM, while there is no evidence for aggregation in (CD3)2SO. Analog 1 adopts an extended conformation in both solvents favoring association to form β‐sheet structures. A folded, γ‐turn conformation involving a 3 → 1 hydrogen bond between Met CO and Phe NH is supported by 1H‐, 13C‐nmr, and ir studies of analog 2. The influence of backbone conformation on the ease of peptide aggregation is demonstrated by ir studies in CHCl3 and CD studies in dioxane.