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

Gramicidin-lipid interactions were investigated using diacylphosphatid ylcholines that contained two identical acyl chains of varying length, between 6 and 14 carbons. The gramicidin A (gA) conformation was moni tored by circular dichroism (CD) spectroscopy and high-performance siz e-exclusion chromatography, and the lipid organization was investigate d using P-31 and H-1 NMR spectroscopy and negative-stain electron micr oscopy. Diacylphosphatidylcholine (PC) lipids with chain lengths betwe en 4 and 8 carbons have been previously shown to have a micellar organ ization in aqueous solution [Lin, T.-L., et al. (1986) J. Am. Chem. So c. 108, 3499-3507]. CD spectra of aqueous gA/lipid dispersions, at a r atio of 1:28, demonstrated that the channel conformation of gA can be readily obtained when the acyl chain length is greater-than-or-equal-t o 10, but not when the chain length is less-than-or-equal-to 7. Size-e xclusion chromatography revealed that the fraction of gA that could ea sily be dissociated into monomers in the dispersions increased with in creasing acyl chain length, in agreement with the CD results. For a ch ain length of 8, the results were intermediate. The formation of the c hannel structure was found to depend on the ''solvent-history'', the t emperature, the gA and lipid concentrations, the gA:lipid ratio, and c onsequently on the method of sample preparation. H-1 and P-31 NMR resu lts suggest that codispersed gA increases the size of dioctanoyl-PC ag gregates, but not of dihexanoyl-PC micelles. Negative-stain electron m icroscopy 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 co nformation of gA, inducing the ''channel'' conformation that is typica lly observed in (longer-chain) bilayer lipids. At the same time, gA in fluenced the dioctanoyl-PC, increasing the size of the lipid aggregate s, as well as inhibiting its tendency toward phase separation. The gra micidin-lipid interactions are therefore reciprocal.