Selective diffusion of neutral amino acids across lipid bilayers

1. 1. Liposomes were formed from mixtures of cholesterol, phosphatidyl serine, and lecithins with differing degrees of acyl chain unsaturation. 2. 2. The efflux of trapped amino acid from such liposomes was determined. The order of permeability was phenylalanine > methionine > leucine > isoleucine, norleucine > alanine, glycine > histidine. 3. 3. The rate of amino acid efflux depended on the type of lecithin used. Amino acids diffuse more quickly from liposomes formed from highly unsaturated lecithins. The increase was greatest for the smaller, hydrophilic amino acids. 4. 4. There was a correlation between the water- n -octanol partition coefficients and the rates of efflux for all amino acids tested except histidine. The obvious features distinguishing histidine from the other amino acids are the positive charge carried on the imidazole ring, and the possibility of an extra hydrogen bond. 5. 5. The lecithin-dependent change in the selectivity shown between glycine and phenylalanine is suggested as being brought about by entropic effects in the hydrocarbon core of the phospholipid bilayer.