α-Helical versus 3₁₀-Helical Conformation of Alanine-Based Peptides in Aqueous Solution: An Electron Spin Resonance Investigation

Due to the difficulties in experimentally differentiating between the a- and 3₁₀-helical conformations in solution, isolated helical peptides have been assumed to be in the a-helical conformation. However, recent electron spin resonance (ESR) studies have suggested that such peptides, in particular short alanine-based peptides, are 3₁₀-helical (Miick, S. M.; et al. Nature 1992, 359, 653–5). This result prompted us to further investigate the helical conformations of alanine-based peptides in solution using electron spin resonance spectroscopy. Unlike previous investigations with a flexible link connecting the spin-label to the peptide backbone, we used a conformationally constrained spin-label (4-amino-4-carboxy-2, 2, 6, 6-tetramethylpiperidine-1-oxyl, Toac) that is rigidly attached to the peptide backbone. From a combination of molecular modeling and ESR spectroscopy investigations, it was concluded that these alanine-based peptides exist primarily in the a-helical conformation, and not the 3₁₀-form as previously suggested for an analogous set of peptides in aqueous environments. This discrepancy is thought to be due to the differences in flexibility of the spin-labels employed. The conformationally constrained spin-label Toac used in this study should accurately reflect the backbone conformation. Free energy surfaces, or potentials of mean force, for the conformational transition of the spin-label used in previous studies (Miick S. M.; et al. Nature 1992, 359, 653–5) suggest that this spin-label is too flexible to accurately distinguish between the α- and 3₁₀-helical conformations.