title: Measurement of the membrane dipole electric field in DMPC vesicles using vibrational shifts of p-cyanophenylalanine and molecular dynamics simulations.
abstract: The magnitude of the membrane dipole field was measured using vibrational Stark effect (VSE) shifts of nitrile oscillators placed on the unnatural amino acid p-cyanophenylalanine (p-CN-Phe) added to a peptide sequence at four unique positions. These peptides, which were based on a repeating alanine-leucine motif, intercalated into small unilamellar DMPC vesicles which formed an α-helix as confirmed by circular dichroic (CD) spectroscopy. Molecular dynamics simulations of the membrane-intercalated helix containing two of the nitrile probes, one near the headgroup region of the lipid (αLAX(25)) and one buried in the interior of the bilayer (αLAX(16)), were used to examine the structure of the nitrile with respect to the membrane normal, the assumed direction of the dipole field, by quantifying both a small tilt of the helix in the bilayer and conformational rotation of the p-CN-Phe side chain at steady state. Vibrational absorption energies of the nitrile oscillator at each position showed a systematic blue shift as the nitrile was stepped toward the membrane interior; for several different concentrations of peptide, the absorption energy of the nitrile located in the middle of the bilayer was ∼3 cm(-1) greater than that of the nitrile closest to the surface of the membrane. Taken together, the measured VSE shifts and nitrile orientations within the membrane resulted in an absolute magnitude of 8-11 MV/cm for the dipole field, at the high end of the range of possible values that have been accumulated from a variety of indirect measurements. Implications for this are discussed.
authors.affiliation: Department of Chemistry, ‡Institute for Cell and Molecular Biology, §Center for Nano- and Molecular Science and Technology, and ∥Institute for Computational Engineering and Sciences, The University of Texas at Austin , Austin, Texas 78712, United States.
authors.forename: Rebika
authors.initials: R
authors.lastname: Shrestha
authors.forename: Alfredo E
authors.initials: AE
authors.lastname: Cardenas
authors.forename: Ron
authors.initials: R
authors.lastname: Elber
authors.forename: Lauren J
authors.initials: LJ
authors.lastname: Webb
mesh_terms: D000409:Alanine / Q000031:analogs & derivatives* / Q000737:chemistry; D002942:Circular Dichroism; D004134:Dimyristoylphosphatidylcholine / Q000737:chemistry*; D008051:Lipid Bilayers / Q000737:chemistry; D056004:Molecular Dynamics Simulation; D009570:Nitriles / Q000737:chemistry*; D017433:Protein Structure, Secondary; D013057:Spectrum Analysis; D055672:Static Electricity; D053835:Unilamellar Liposomes / Q000737:chemistry*; D014732:Vibration; D010649:Phenylalanine / Q000031:analogs & derivatives
chemical_list: D000409:Alanine; D004134:Dimyristoylphosphatidylcholine; D008051:Lipid Bilayers; D009570:Nitriles; D053835:Unilamellar Liposomes; C530634:p-cyanophenylalanine; D010649:Phenylalanine