Electrostatics at the Membrane/Peptide Interface: Utilizing pH Sensitive Spin Labels to Probe the Bilayer Interior

Polarity, electric potentials, and hydration are the major physico-chemical characteristics of lipid membranes that govern membrane-protein and protein-protein interactions as well as small molecule transport. Insertion of transmembrane proteins perturbs the local membrane structure, altering the local dielectric environment and hydration at the membrane-protein interface. The significance of distorting the local membrane structure at the lipid-protein interface for modulating protein-protein interactions should not be overlooked. This work reports on employing pH-sensitive, ionizable spin labels to profile a heterogeneous dielectric environment along the α-helix of a peptide that resides in a lipid bilayer. Spin labels were positioned equidistant from the center of the peptide, thus ensuring symmetric location of the labels with respect to the bilayer center. The change in protonation state of the label was directly observed by continuous wave electron paramagnetic resonance. Q-band double electron-electron resonance experiments were carried out to determine the distance between spin label pairs when embedded in lipid bilayers to provide information about the label location. Thus, for the first time, measurements of local electrostatics at the peptide-bilayer interface were based on direct distance measurements rather than on assumptions about probe location. By titrating two pH sensitive spin labels with intrinsic pKa's differing by approximately two pH units, the electrostatic interaction energy between probes could be calculated, allowing estimation of the effective dielectric constant. By varying the lipid composition, the effect(s) of the lipid surface charge on the dielectric profile at the peptide-membrane interface also were investigated. A comparison of the spin labels' properties and utility for this application are summarized. The principle of calculating the local dielectric permittivity of the lipid bilayer regions, i.e. the interface and the interior, using the spin labels is discussed.