GRAMICIDIN-A AGGREGATION IN SUPPORTED GEL STATE PHOSPHATIDYLCHOLINE BILAYERS

Using an atomic force microscope, supported bilayers of saturated phos phatidylcholine (in the gel state) containing various amounts of grami cidin A (gA) were imaged in aqueous solutions and at room temperature. gA clusters were directly observed for the first time under these con ditions. It was found that, at a lower gA concentration, gA aggregated into domains, composed of small clusters along with a considerable am ount of lipids. This basic aggregation unit. most likely a hexamer, re mained the same for acyl chain lengths from 14 to 18 carbons. These sm all clusters were observed to form elongated aggregates (line type) bu t never into extended pure gA domains. When gA concentrations were inc reased, for bilayers with 16 carbons or less, gA aggregated into large r domains but the basic unit remained separated by lipid molecules. At about 5 mol % gA, a percolation-like transition occurred at which the line type aggregates were connected to each other. However, for bilay ers with more than 16 carbons, multiple lamellar strucutres were forme d at higher gA fractions and the top layer had a ripple-like surface m orphology. The molecular mechanism for the formation of these peculiar structures remains to be elucidated.