GRAMICIDIN CHANNELS IN PHOSPHOLIPID-BILAYERS WITH UNSATURATED ACYL CHAINS

In organic solvents gramicidin A (gA) occurs as a mixture of slowly in terconverting double-stranded dimers. Membrane-spanning gA channels, i n contrast, are almost exclusively single-stranded beta(6.3)-helical d imers. Based on spectroscopic evidence, it has previously been conclud ed that the conformational preference of gA in phospholipid bilayers v aries as a function of the degree of unsaturation of the acyl chains. Double-stranded pi pi(5.6)-helical dimers predominate (over single-str anded beta(6.3)-helical dimers) in lipid bilayer membranes with polyun saturated acyl chains. We therefore examined the characteristics of ch annels formed by gA in 1-palmitoyl-2-oleoylphosphatidylcholine/n-decan e, 1,2-dioleoylphosphatidylcholine/n-decane, and 1,2-dilinoleoylphosph atidylcholine/n-decane bilayers. We did not observe long-lived channel s that could be conducting double-stranded pi pi(5.6)-helical dimers i n any of these different membrane environments. We conclude that the s ingle-stranded beta(6.3)-helical dimer is the only conducting species in these bilayers. Somewhat surprisingly, the average channel duration and channel-forming potency of gA are increased in dilinoleoylphospha tidylcholine/n-decane bilayers compared to 1-palmitoyl-2-oleoylphospha tidylcholine/n and dioleoylphosphatidylcholine/n-decane bilayers. To t est for specific interactions between the aromatic side chains of gA a nd the acyl chains of the bilayer, we examined the properties of chann els formed by gramicidin analogues in which the four tryptophan residu es were replaced with naphthylalanine (gN), tyrosine (gT), and phenyla lanine (gM). The results show that all of these analogue channels expe rience the same relative stabilization when going from dioleoylphospha tidylcholine to dilinoleoylphosphatidylcholine bilayers.