LETTER pubs.acs.org/JPCL
Direct Measurement of the Membrane Dipole Field in Bicelles Using Vibrational Stark Effect Spectroscopy Wenhui Hu and Lauren J. Webb* Department of Chemistry and Biochemistry, Institute for Cell and Molecular Biology, and Center for Nano- and Molecular Science and Technology, The University of Texas at Austin, Austin, Texas 78712, United States
bS Supporting Information ABSTRACT: Electrostatic fields in lipid bilayer membranes influence the structure and function of membrane-associated proteins. We present here the first direct measurement of the membrane dipole electrostatic field in lipid bicelles using vibrational Stark effect spectroscopy, in which a nitrile oscillator’s vibrational frequency changes in response to its local electrostatic environment. We synthesized R-helical peptides containing the unnatural amino acid p-cyanophenylalanine (CN-Phe) at four locations along the helix. This peptide was intercalated into bicelles 5 and 15 nm in radius composed of mixtures of 1,2-dimyristoyl-snglycero-3-phosphocholine (DMPC) and 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC). Changes in the vibrational absorption energy of the nitrile probe at each position along the helical axis were used to determine changes in the local electrostatic field of the probe. We measured the magnitude of the membrane dipole electrostatic field to be 6 MV/cm, changing rapidly near the membrane surface and more slowly in the low dielectric membrane interior. SECTION: Biophysical Chemistry
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mong the three distinct electrostatic potentials that are well known to be present in the lipid membrane bilayer (the transmembrane potential, surface potential, and dipole potential),1,2 dipole potential is the least well characterized. Because it is located entirely within the membrane, it has been difficult to measure directly, and its influence on the structure, aggregation, and function of transmembrane and other membrane-associated proteins has not been studied as extensively as the transmembrane and surface electrostatic potentials. Despite this, a combination of indirect measurements such as ion transport rates,3,4 nitroxide scanning,5 fluorescence of voltagesensitive dyes,6,7 atomic force microscopy (AFM),8 and other methods,911 combined with computational approaches1214 has been used to estimate the magnitude of the dipole potential at a few hundred millivolts,3,8 significantly larger than either the transmembrane (about 10100 mV) or surface potentials (about 830 mV).1 Because this dipole potential drops across a small distance of
