Two-dimensional infrared spectroscopy displays signatures of structural ordering in peptide aggregates

In the presence of lipid bilayers, the hexapeptide AcWL(5) forms membrane-bound aggregates dominated by beta-secondary structure and is thus a useful model for the onset of peptide aggregation in membrane environments. Two-dimensional infrared (2D IR) spectra in the amide I region for aggregates of AcWL(5) peptides with single isotopic labels provide new insight into the residue-level structural ordering of the aggregated peptides. Separation of spectral information along two axes provides clear indications of the band frequencies and relative intensities, which together are an indication of extended amide coupling networks across structural regions of a particular size. The lowered anharmonicities, relative to free peptide, and the narrow, homogeneous lineshapes of the 2D IR peaks indicate the delocalization of vibrational modes through an ordered structure whose flexibility varies with relative peptide concentration. Crosspeaks between delocalized transitions can be used to estimate the strength of the coupling interactions between neighboring residues. The sensitivity of 2D IR spectra to residue-level structural ordering shows that 2D IR spectroscopy is a powerful technique for probing structures formed during the onset of peptide aggregation.