Vibrational coupling, isotopic editing, and β-sheet structure in a membrane-bound polypeptide

The N-acetylated hexapeptide WLLLLL (AcWL5) partitions into lipid membranes and is believed to assemble into an antiparallel beta-sheet. As a test of this structural assignment, the peptide bonds of residues 2-6 were labeled with C-13 and allowed to adsorb onto a supported lipid membrane. Peptides bound to the membrane were examined for evidence of coupling between the labeled vibrational modes in adjacent beta-strands with internal reflection infrared spectroscopy. Experimental results indicate that the amide I absorption band in D2O (i.e., amide I') attributable to C-13 is selectively enhanced when the label is at any one of several positions along the peptide backbone. Simulations employing an excitonic model with through-bond and through-space interactions were performed on AcWL5 models in parallel and antiparallel beta-sheet configurations. The simulations yield spectra in good agreement with the experimental results, accounting for the enhancement of both C-13 band intensities and band frequencies. They also yield insight into the physical origin and structure selectivity of the distinctive amide I' band shapes that arise in isotopically edited spectra. It is concluded that the P-sheet formed by membrane-bound AcWL5 is indeed antiparallel, and the enhancement of C-13 bands in the infrared spectra of these peptides is caused by both interstrand and intrastrand coupling to C-12 modes.