No abstract

Using the atomic force microscope (AFM) in situ, we have demonstrated that acyl chain interdigitation can be induced reversibly by alcohol in supported unilamellar phospholipid bilayers. At alcohol concentrations considerably lower than the critical values determined by other experimental techniques, it was found that interdigitated domains can be induced and these domains were stable over a long period of time. The mechanism of such domain formation remains to be elucidated. This work also serves as an example to illustrate the uniqueness of the AFM as a powerful tool in the study of membrane structure and conformation under physiological conditions at high spatial resolution.

A commonly used buffer compound, tris(hydroxymethyl)aminomethane (C4H11NO3), was found to induce a ripple phase in supported unilamellar phospholipid bilayers at room temperature. The ripple structure showed various types of domains that could extend to several micrometers in length with many well-defined bendings of either 120 degrees or 60 degrees. Two different periodic ripples were found to coexist in 1,2-dipentadecanoyl-sn-glycero-3-phosphatidylcholine bilayers. Some intermediate states during the transition to the ripple phase were also observed.

No abstract

Using an atomic force microscope, supported bilayers of saturated phosphatidylcholine (in the gel state) containing various amounts of gramicidin A (gA) were imaged in aqueous solutions and at room temperature. gA clusters were directly observed for the first time under these conditions. It was found that, at a lower gA concentration, gA aggregated into domains, composed of small clusters along with a considerable amount of lipids. This basic aggregation unit, most likely a hexamer, remained the same for acyl chain lengths from 14 to 18 carbons. These small clusters were observed to form elongated aggregates (line type) but never into extended pure gA domains. When gA concentrations were increased, for bilayers with 16 carbons or less, gA aggregated into larger 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 bilayers with more than 16 carbons, multiple lamellar structures were formed at higher gA fractions and the top layer had a ripple-like surface morphology. The molecular mechanism for the formation of these peculiar structures remains to be elucidated.