Electrical interactions of membrane active peptides at lipid/water interfaces

Vital functions of biological membranes are frequently controlled by amphipathic peptides that are associated with the lipid bilayer. The extent of association is largely determined by influences encountered at the interface between the aqueous and lipid moieties, especially involving electrostatic interactions. A basic thermodynamic analysis is presented in terms of a partitioning equilibrium where the membrane is treated as a non-ideal solution of peptide molecules in a two-dimensional lipid solvent. This may then be employed to interpret experimental association isotherms (i.e. the ratio of associated peptide per lipid plotted versus the free aqueous peptide concentration) in the light of a molecular mechanism. Special emphasis is directed towards the evaluation of original titration data under most general circumstances when the association can be monitored using a suitable linear signal (preferentially an optical one). The experimental approaches as well as the merits regarding possible information about the underlying structural and functional features are discussed with pertinent practical examples.