Isotope-Edited FT-IR Spectra of Isotopmers of Helical Hexamers of Alpha-Aminoisobutyric Acid

Abstract

Isotope-edited FT-infrared spectra of the Amide I region of hexamers of alpha-aminoisobutyric acid (Z-Aib6-OtBu) have been collected in order to explore the effects of 13C=O enrichment on the FT-IR spectra in the conformational context of 310 helices. Oligomers of Aib are known to adopt predominantly 310 helical structures, even at short peptide lengths. The Amide I band is sensitive to the details of peptide secondary structure, but the competency of this band to distinguish between alpha- and 310 helical secondary structure remains an open question. The 310 helix is shorter than an alpha-helix of the same number of residues and exhibits an i to i+3 hydrogen bonding pattern, instead of an i to i +4 pattern of the alpha-helix. These differences bring amide oscillators of a 310 helix slightly closer in space and in shorter periodicity of hydrogen-bonding partnership as compared to an alpha-helix. We have collected infrared spectra of isotopomers of hexamers of Aib (e.g., Z-Aib-Aib-Aib-Aib-Aib-Aib-OtBu, Z-Aib∗-Aib-Aib∗-Aib-Aib-Aib-OtBu, and Z-Aib∗-Aib∗-Aib-Aib-Aib-Aib-OtBu, Aib∗ = 13C enrichment at ∗C=O) in dichloromethane (a nonpolar aprotic solvent) and methanol (a polar, protic solvent) to examine the effects of carbon-13 enrichment on the spectra. Differences among the spectra of peptide isotopomers enriched with one or two 13C labels will be examined, detailing the effects of coupling between pairs of 13C-labeled Amide I oscillators.