Probing the effect of side chains on the conformation and stability of helical peptides via isotope-edited infrared spectroscopy.

The mechanism of helix stabilization or destabilization by different amino acids has been the subject of several experimental and theoretical studies; these studies suggest that large or bulky side chains may modulate helix stability by altering the hydration of the helix backbone. In this paper, we report a spectroscopic study to determine the effect of alanine to leucine substitutions on the conformation and solvation of specific segments of a model helical peptide. A 25-residue, alanine-rich, helical peptide [Ac-(AAAAK)(4)-AAAAY-NH(2) (AKA)] and its two leucine variants [Ac-LLLLK-(AAAAK)(3)-AAAAY-NH(2) (LKA) and Ac-(AAAAK)(4)-LLLLY-NH(2) (AKL)] were characterized by infrared (IR) and electronic circular dichroism (ECD) spectroscopies. Introduction of (13)C isotopes into specific, consecutive, backbone carbonyls for certain blocks of each of the peptides mentioned above allows the IR spectra to be interpreted in terms of the conformation and solvation of specific residues within the helix. These isotope-edited IR spectra of the leucine peptides do not show evidence of a decrease in the degree of backbone solvation compared to the alanines, but suggest that the peptide may adopt a distorted conformation to accommodate the larger leucine side chains at the N-terminus. These experiments demonstrate the power of isotope-edited IR in dissecting subtle changes in helix conformation at the residue level.