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

A 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), 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, nitroxide scanning, fluorescence of voltagesensitive dyes, atomic force microscopy (AFM), and other methods,9!11 combined with computational approaches12!14 has been used to estimate themagnitude of the dipole potential at a few hundred millivolts, significantly larger than either the transmembrane (about 10!100 mV) or surface potentials (about 8!30 mV). Because this dipole potential drops across a small distance of <4 nm within the low dielectric membrane interior, it generates a large electric field, typically 1!10 MV/cm compared with ∼0.25 MV/cm for the transmembrane field and ∼0.1 MV/cm for the surface field. This is believed to have a considerable influence on the structure and function of membrane-associated proteins. Here we present a direct measurement of the membrane dipole field using vibrational Stark effect (VSE) spectroscopy in which changes in the absorption energy of a probe oscillator molecule are used as a local and directional probe of the electrostatic environment in which that oscillator is immersed. The sensitivity of a vibrational oscillator to its local electrostatic environment is given by eq 1