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

Area of Science:

• Biophysics
• Spectroscopy
• Materials Science

Background:

• The N-acetylated hexapeptide WLLLLL (AcWL5) is known to partition into lipid membranes.
• AcWL5 is hypothesized to self-assemble into an antiparallel beta-sheet structure within these membranes.

Purpose of the Study:

• To experimentally verify the proposed antiparallel beta-sheet structure of membrane-bound AcWL5.
• To investigate the vibrational coupling within the peptide structure using isotopic labeling.

Main Methods:

• (13)C isotopic labeling of peptide bonds at residues 2-6.
• Adsorption of labeled peptides onto supported lipid membranes.
• Internal reflection infrared spectroscopy (IR) to detect vibrational coupling.
• Exciton model simulations for parallel and antiparallel beta-sheet configurations.

Main Results:

• Selective enhancement of the (13)C-labeled amide I' absorption band was observed in IR spectra.
• Experimental results were consistent with simulations of vibrational coupling.
• (13)C band intensities and frequencies were accurately reproduced by the models.

Conclusions:

• The membrane-bound AcWL5 peptide adopts an antiparallel beta-sheet conformation.
• Observed spectral enhancements are attributed to interstrand and intrastrand vibrational coupling to (12)C modes.
• Isotopic editing in IR spectroscopy provides structural insights into membrane-associated peptides.