Direct Measurement of the Membrane Dipole Field in Bicelles Using Vibrational Stark Effect Spectroscopy

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 a-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-sn-glycero-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.