Collisions between helical peptides in membranes monitored using electron paramagnetic resonance: evidence that alamethicin is monomeric in the absence of a membrane potential.

Alamethicin is a 20-amino-acid peptide that produces a voltage-dependent conductance in membranes. We investigated the state of aggregation of alamethicin in egg phosphatidylcholine and dioleoylphosphatidylcholine membranes by examining the EPR spectra obtained from an active analog of this peptide that is spin-labeled at its C-terminus. The dependence of both the linewidth and signal intensity as a function of peptide concentration exhibit exchange broadening as the peptide concentration is increased; however, the exchange rates are linear with peptide concentration as is expected for the simple diffusion of monomers. In addition, the spin-exchange rates obtained from the linebroadening are consistent with collisional rates that are predicted from free Brownian diffusion. The results provide strong evidence that in the absence of a membrane potential, alamethicin is largely monomeric in these membranes.