Intramembrane water associated with TOAC spin-labeled alamethicin: electron spin-echo envelope modulation by D2O

Alamethicin is a 20-residue, hydrophobic, helical peptide, which forms voltage-sensitive ion channels in lipid membranes. The helicogenic, nitroxyl amino acid TOAC was substituted isosterically for Aib at residue positions 1, 8, or 16 in a F50/5 alamethicin analog to enable EPR studies. Electron spin-echo envelope modulation (ESEEM) spectroscopy was used to investigate the water exposure of TOAC-alamethicin introduced into membranes of saturated or unsaturated diacyl phosphatidylcholines that were dispersed in D2O. Echo-detected EPR spectra were used to assess the degree of assembly of the peptide in the membrane, via the instantaneous diffusion from intermolecular spin-spin interactions. The profile of residue exposure to water differs between membranes of saturated and unsaturated lipids. In monounsaturated dioleoyl phosphatidylcholine, D2O-ESEEM intensities decrease from TOAC(1) to TOAC(8) and TOAC(16) but not uniformly. This is consistent with a transmembrane orientation for the protoassembled state, in which TOAC(16) is located in the bilayer leaflet opposite to that of TOAC(1) and TOAC(8). Relative to the monomer in fluid bilayers, assembled alamethicin is disposed asymmetrically about the bilayer midplane. In saturated dimyristoyl phosphatidylcholine, the D2O-ESEEM intensity is greatest for TOAC(8), indicating a more superficial location for alamethicin, which correlates with the difference in orientation between gel- and fluid-phase membranes found by conventional EPR of TOAC-alamethicin in aligned phosphatidylcholine bilayers. Increasing alamethicin/lipid ratio in saturated phosphatidylcholine shifts the profile of water exposure toward that with unsaturated lipid, consistent with proposals of a critical concentration for switching between the two different membrane-associated states.