A limiting law for the electrostatics of the binding of polypeptides to phospholipid bilayers

Two cysteine-substituted variants of a peptide derived from the first 25 residues of the presequence for subunit IV of cytochrome c oxidase were synthesized and modified with a nitroxide spin label. The equilibrium partitioning of these spin-labeled peptides into negatively charged phospholipid vesicles was studied with electron paramagnetic resonance (EPR) to investigate the binding energetics. It is found that the binding equilibrium constant is an explicit function of a unique variable, the membrane surface potential psi in the Gouy-Chapman-Stern theory. Moreover, at low psi (< 0.5RT/F) the binding equilibrium is described by the linear dependence of the transfer free energy delta G(el) on psi with a slope equal to the full formal charge of the peptides. However, the partition constant levels off at higher psi, suggesting departure from the ideal limiting behavior.