A theoretical analysis of coherent cross-peaks in polarization selective 2DIR for detection of cross-α fibrils.

ABSTRACT

The coupled amide-I vibrational modes in peptide systems such as fibrillar aggregates can often provide a wealth of structural information, although the associated spectra can be difficult to interpret. Using exciton scattering calculations, we characterized the polarization selective 2DIR peak patterns for cross-α peptide fibrils, a challenging system given the similarity between the monomeric and fibrillar structures, and interpret the results in light of recently collected 2D data on the cross-α peptide phenol soluble modulin α3. We find that stacking of α-helices into fibrils couples the bright modes across helical subunits, generating three new Bloch-like extended excitonic states that we designate A⊥, E∥, and E⊥. Coherent superpositions of these states in broadband 2DIR simulations lead to characteristic signals that are sensitive to fibril length and match the experimental 2DIR spectra.