The influence of phospholipid polar groups on gramicidin channels

The influence of well-defined changes in the polar part of phospholipid molecules on the properties of black lipid membranes was studied using a series of phospholipids with identical hydrocarbon chains, but systematically changed polar groups. The hydrocarbon tails of the lipids under study were composed of 1,2-dipentadecylmethylidene glycerol. The polar parts differed in the degree of N-methylation and comprised phosphocholine, -N,N-dimethylethanolamine, -N-methylethanolamine and ethanolamine. Stable black lipid membranes could be formed with the solvents octane, decane, dodecane, tetradecane and hexadecane. The properties of gramicidin-induced single ionic channels changed systematically in membranes from the phosphatidylcholine to the phosphatidylethanolamine analogue, as indicated by an increase in the amplitude lambda of the unit conductance step and a decrease in the average channel life-time or duration tau. The series of tau-values was opposite to that expected from hydrocarbon thickness (specific capacitance). It is suggested that the surface tension gamma is a relevant parameter for the prediction of tau-values.