Two-dimensional infrared measurements of the coupling between amide modes of an α-helix

The 2D IR spectra of doubly isotopically substituted α-helices with 13C16O and 13C18O labels enabled the couplings between amide-I modes separated by one, two and three residues to be measured. The magnitudes and signs of couplings between the isotopomers and between them and the helix exciton band states confirm that the states of the helix and isotopomers are delocalized. The results are compared with simulations. The off-diagonal anharmonicities agree reasonably with a set of couplings that derive from transition charge interactions except for nearest neighbors for which the coupling is modified by through bond interactions.