Spectroscopic evidence for backbone desolvation of helical peptides by 2,2,2-trifluoroethanol: An isotope-edited FTIR study

2,2,2-Trifluoroethanol (TFE) is widely used to induce helix formation in peptides and proteins, but the mechanism behind this effect is still poorly understood. Several recent papers have proposed that TFE acts by selectively desolvating the peptide backbone groups of the helix state. Infrared (IR) spectroscopy of the amide I band of polypeptides can be used to probe both secondary structure and backbone solvation, making this technique well suited for addressing the effect of TFE on polypeptide conformation. In this paper, we report the IR spectra as a function of TFE concentration for an alanine-rich peptide based on the repeat (AAKAA)(n). The IR spectra confirm that TFE desolvates the helical state of the peptide to a greater extent than the random coil state. Moreover, using a series of specifically C-13-labeled peptides, the precise residues desolvated in the presence of TFE were identified. The residues most desolvated by TFE are the alanines located at position i - 4 in the sequence, where i is a lysine residue. This pattern of desolvation is consistent with molecular dynamics simulations which predict strong interactions between the lysine side chain at position n. and the backbone carbonyl of the alanine at position i - 4. This is the first direct spectroscopic evidence of specific desolvation of helix backbone atoms in model alanine-rich peptides.