title: Amide vibrations are delocalized across the hydrophobic interface of a transmembrane helix dimer.
abstract: The tertiary interactions between amide-I vibrators on the separate helices of transmembrane helix dimers 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 (13)C=(16)O or (13)C=(18)O. The isotopomers and their 50:50 mixtures formed helical dimers in SDS micelles whose 2D IR spectra showed components from homodimers when both helices had either (13)C=(16)O or (13)C=(18)O substitution and a heterodimer when one had (13)C=(16)O substitution and the other had (13)C=(18)O 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 dimers. The helix crossing angle in SDS is estimated at 45 degrees. Two-dimensional IR spectroscopy also sets limits on the range of geometrical parameters for the helix dimers from an analysis of the coupling constant distribution.
authors.affiliation: Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323, USA.
authors.forename: Chong
authors.initials: C
authors.lastname: Fang
authors.forename: Alessandro
authors.initials: A
authors.lastname: Senes
authors.forename: Lidia
authors.initials: L
authors.lastname: Cristian
authors.forename: William F
authors.initials: WF
authors.lastname: DeGrado
authors.forename: Robin M
authors.initials: RM
authors.lastname: Hochstrasser
mesh_terms: D000577:Amides / Q000737:chemistry; D000595:Amino Acid Sequence; D055592:Biophysical Phenomena; D001703:Biophysics; D019281:Dimerization; D006021:Glycophorins / Q000737:chemistry*; D006801:Humans; D057927:Hydrophobic and Hydrophilic Interactions; D066298:In Vitro Techniques; D008565:Membrane Proteins / Q000737:chemistry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000737:chemistry; D020836:Protein Structure, Quaternary; D017433:Protein Structure, Secondary; D017550:Spectroscopy, Fourier Transform Infrared; D013816:Thermodynamics; D014732:Vibration
chemical_list: D000577:Amides; D006021:Glycophorins; D008565:Membrane Proteins; D010446:Peptide Fragments