Effects of pH-induced variations of the charge of the transmembrane α-helical peptide Ac-K2(LA)12K2-amide on the organization and dynamics of the host dimyristoylphosphatidylcholine bilayer membrane

The effects of the transmembrane α-helical peptide Ac-K2(LA)12K2-amide ((LA)12) on the phase transition and dynamics of saturated dimyristoylphosphatidylcholine (DMPC) membranes were investigated at different pH using conventional and saturation-recovery EPR observations of phosphatidylcholine spin labels. At a peptide-to-DMPC ratio of 1/10, the main phase-transition temperature of the DMPC bilayer is decreased by 4.0 °C when measured at pH 7.0, by 1.6 °C when measured at pH 9.5, and not affected when measured at pH 11.5. This reversible pH effect is due to the subsequent neutralization of the positive charges of lysine side chains at both ends of (LA)12. Apparent pKas of the lysine side chain amino groups of (LA)12 in DMPC bilayer are 8.6 and ∼10.9, as compared with the pKa value of 10.5 for these groups when lysine is dissolved in water. Saturation-recovery curves as a function of oxygen concentration using phosphatidylcholine spin labels in DMPC bilayer containing (LA)12 are always mono-exponential when measured at pH 7.0 and 9.5. This observation is consistent with the hypothesis that the lipid exchange rates among the bulk, boundary, and (LA)12-rich regions are faster than 0.5 μs, the electron spin-lattice relaxation time in the presence of molecular oxygen, suggesting that stable oligomers of (LA)12 do not form. Neutralization of one lysine side chain positive charge on each end of the peptide significantly decreases the ordering effect of (LA)12 on the lipid hydrocarbon chains, while its effect on the reorientational motion of terminal groups of lipid hydrocarbon chains is rather moderate. It does not affect the local diffusion-solubility product of oxygen measured in the DMPC-(LA)12 membrane interior.