The anomalous infrared amide I intensity distribution in (13)C isotopically labeled peptide beta-sheets comes from extended, multiple-stranded structures: an ab initio study.

Ab initio based calculations of force fields and atomic polar tensors are used to simulate amide I infrared absorption spectra for a series of isotopically substituted (Ac-A(12)-NH-CH(3))(n)() peptides clustered in an antiparallel beta-sheet conformation having a varying number of strands, n = 2-5. The results demonstrate that the anomalous intensity previously reported for the isotopically shifted amide I in (13)C labeled peptides is due to formation of multistranded beta-sheet structures in this conformation. Computations show that the characteristic widely split amide I mode for beta-sheet polypeptides as well as this anomalous intensity enhancement in isotopically substituted beta-sheet peptides grows with increasing sheet size. For sheets of five strands, qualitative and near quantitative agreement with experimental amide I intensity patterns is obtained for both labeled and unlabeled peptides. The strongest transitions primarily represent in-phase coupled modes of the (13)C labeled, next nearest neighbor amides on the inner strands of the multistranded beta-sheet. Long-range transition dipole coupling interactions do not promote the (13)C amide I intensity enhancement. Understanding of the IR intensity mechanisms with this level of detail for the isotopically labeled peptides permits design of site-specific probes of beta-sheet folding and unfolding dynamics.