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Vibrational Coupling, Isotopic Editing, and β-Sheet Structure in a Membrane-Bound Polypeptide

2004; American Chemical Society; Volume: 126; Issue: 18 Linguagem: Inglês

10.1021/ja038869f

1943-2984

Cynthia Paul, Jianping Wang, William C. Wimley, Robin M. Hochstrasser, Paul H. Axelsen,

Mass Spectrometry Techniques and Applications

The N-acetylated hexapeptide WLLLLL (AcWL5) partitions into lipid membranes and is believed to assemble into an antiparallel β-sheet. As a test of this structural assignment, the peptide bonds of residues 2−6 were labeled with 13C 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 β-strands with internal reflection infrared spectroscopy. Experimental results indicate that the amide I absorption band in D2O (i.e., amide I') attributable to 13C 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 β-sheet configurations. The simulations yield spectra in good agreement with the experimental results, accounting for the enhancement of both 13C 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 β-sheet formed by membrane-bound AcWL5 is indeed antiparallel, and the enhancement of 13C bands in the infrared spectra of these peptides is caused by both interstrand and intrastrand coupling to 12C modes.