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 equilib rium partitioning of these spin-labeled peptides into negatively charg ed phospholipid vesicles was studied with electron paramagnetic resona nce (EPR) to investigate the binding energetics. It is found that the binding equilibrium constant is an explicit function of a unique varia ble, the membrane surface potential Psi in the Gouy-Chapman-Stern theo ry. Moreover, at low Psi(<0.5RT/F) the binding equilibrium is describe d 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. Howe ver, the partition constant levels off at higher Psi, suggesting depar ture from the ideal limiting behavior.