Protein Chemistry at Membrane Interfaces: Non-additivity of Electrostatic and Hydrophobic Interactions

Non-specific binding of proteins and peptides to charged membrane interfaces depends upon the combined contributions of hydrophobic (ΔGHΦ) and electrostatic (ΔGES) free energies. If these are simply additive, then the observed free energy of binding (ΔGobs) will be given by ΔGobs=ΔGHΦ+ΔGES, where ΔGHΦ=−σNPANP and ΔGES=zFφ. In these expressions, ANP is the non-polar accessible area, σNP the non-polar solvation parameter, z the formal peptide valence, F the Faraday constant, and φ the membrane surface potential. But several lines of evidence suggest that hydrophobic and electrostatic binding free energies of proteins at membrane interfaces, such as those associated with cell signaling, are not simply additive. In order to explore this issue systematically, we have determined the interfacial partitioning free energies of variants of indolicidin, a cationic proline-rich antimicrobial peptide. The synthesized variants of the 13 residue peptide covered a wide range of hydrophobic free energies, which allowed us to examine the effect of hydrophobicity on electrostatic binding to membranes formed from mixtures of neutral and anionic lipids. Although ΔGobs was always a linear function of ΔGHΦ, the slope depended upon anionic lipid content: the slope was 1.0 for pure, zwitterionic phosphocholine bilayers and 0.3 for pure phosphoglycerol membranes. ΔGobs also varied linearly with surface potential, but the slope was smaller than the expected value, zF. As observed by others, this suggests an effective peptide valence zeff that is smaller than the formal valence z. Because of our systematic approach, we were able to establish a useful rule-of-thumb: zeff is reduced relative to z by about 20 % for each 3 kcal mol−1 (1 kcal=4.184 kJ) favorable increase in ΔGHΦ. For neutral phosphocholine interfaces, we found that ΔGobs could be predicted with remarkable accuracy using the Wimley-White experiment-based interfacial hydrophobicity scale.