Amide vibrations are delocalized across the hydrophobic interface of a transmembrane helix dimer

The tertiary interactions between amide-I vibrators on the separate helices of transmembrane helix dieters were probed by ultrafast 2D vibrational photon echo spectroscopy. The 2D IR approach proves to be a useful structural method for the study of membrane-bound structures. The 27-residue human erythrocyte protein Glycophorin A transmembrane peptide sequence: KKITLIIFG(79)VMAGVIGTILLISWG(94)IKK was labeled at G(79) and G(94) with C-13=O-16 or C-13=O-18. The isotopomers and their 50:50 mixtures formed helical dieters in SIDS micelles whose 2D IR spectra showed components from homodimers when both helices had either C-13=O-16 or C-13=O-18 substitution and a heterodimer when one had C-13=O-16 substitution and the other had C-13=O-18 substitution. The cross-peaks in the pure heterodimer 2D IR difference spectrum and the splitting of the homodimer peaks in the linear IR spectrum show that the amide-I mode is delocalized across a pair of helices. The excitation exchange coupling in the range 4.3-6.3 cm(-1) arises from through-space interactions between amide units on different helices. The angle between the two Gly(79) amide-I transition dipoles, estimated at 103 degrees from linear IR spectroscopy and 110 degrees from 2D IR spectroscopy, combined with the coupling led to a structural picture of the hydrophobic interface that is remarkably consistent with results from NMR on helix dieters. The helix crossing angle in SIDS is estimated at 45 degrees. Two-dimensional IR spectroscopy also sets limits on the range of geometrical parameters for the helix dieters from an analysis of the coupling constant distribution.