ALPHA-HELICAL VERSUS 3(10)-HELICAL CONFORMATION OF ALANINE-BASED PEPTIDES IN AQUEOUS-SOLUTION - AN ELECTRON-SPIN-RESONANCE INVESTIGATION

Due to the difficulties in experimentally differentiating between the alpha- and 3(10)-helical conformations in solution, isolated helical p eptides have been assumed to be in the alpha-helical conformation. How ever, recent electron spin resonance (ESR) studies have suggested that such peptides, in particular short alanine-based peptides, are 3(10)- helical (Miick, S. M.; et al. Nature 1992, 359, 653-5). This result pr ompted 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 sp in-label to the peptide backbone, we used a conformationally constrain ed spin-label no-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 conc luded that these alanine-based peptides exist primarily in the alpha-h elical conformation, and not the 3(10)-form as previously suggested fo r an analogous set of peptides in aqueous environments. This discrepan cy 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. Fr ee energy surfaces, or potentials of mean force, for the conformationa l 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 fl exible to accurately distinguish between the alpha- and 3(10)-helical conformations.