GRAMICIDIN-A SHORT-CHAIN PHOSPHOLIPID DISPERSIONS - CHAIN-LENGTH DEPENDENCE OF GRAMICIDIN CONFORMATION AND LIPID ORGANIZATION

Gramicidin-lipid interactions were investigated using diacylphosphatidylcholines that contained two identical acyl chains of varying length, between 6 and 14 carbons. The gramicidin A (gA) conformation was monitored by circular dichroism (CD) spectroscopy and high-performance size-exclusion chromatography, and the lipid organization was investigated using P-31 and H-1 NMR spectroscopy and negative-stain electron microscopy. Diacylphosphatidylcholine (PC) lipids with chain lengths between 4 and 8 carbons have been previously shown to have a micellar organization in aqueous solution [Lin, T.-L., et al. (1986) J. Am. Chem. Soc. 108, 3499-3507]. CD spectra of aqueous gA/lipid dispersions, at a ratio of 1:28, demonstrated that the channel conformation of gA can be readily obtained when the acyl chain length is greater-than-or-equal-to 10, but not when the chain length is less-than-or-equal-to 7. Size-exclusion chromatography revealed that the fraction of gA that could easily be dissociated into monomers in the dispersions increased with increasing acyl chain length, in agreement with the CD results. For a chain length of 8, the results were intermediate. The formation of the channel structure was found to depend on the ''solvent-history'', the temperature, the gA and lipid concentrations, the gA:lipid ratio, and consequently on the method of sample preparation. H-1 and P-31 NMR results suggest that codispersed gA increases the size of dioctanoyl-PC aggregates, but not of dihexanoyl-PC micelles. Negative-stain electron microscopy directly supports these findings. Dihexanoyl-PC (28 mM) was able to solubilize 1 mM gA in H2O, but the gA was not in the 'channel' conformation. By contrast, dioctanoyl-PC profoundly influenced the conformation of gA, inducing the ''channel'' conformation that is typically observed in (longer-chain) bilayer lipids. At the same time, gA influenced the dioctanoyl-PC, increasing the size of the lipid aggregates, as well as inhibiting its tendency toward phase separation. The gramicidin-lipid interactions are therefore reciprocal.