Visualization of phospholipid particle fusion induced by duramycin

We visualized nanometer-scale phospholipid particle fusion by scanning tunneling microscopy (STM) on an alkanethiol-modified gold substrate, induced by duramycin, a tetracyclic antibiotic peptide with 19 amino residues. Ultrasonic homogenization generated a suspension mainly consisting of minimal lipid particles (MLP) from 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) in a phosphate buffer solution, confirmed by dynamic light scattering (DLS). In situ STM discerned individual MLP as particles (diameter approximately 8 nm) spread on Au(111), modified with alkanethiol, within the suspension. The MLP became fragile by the presence of duramycin, and the MLP were easily scratched by the scanning tip into multilayers along the surface. This process of particle fusion on the gold surface coincides with the aggregation of MLP in the suspension, observed by DLS. It was demonstrated that STM is capable of discerning and monitoring the nanometer-scale features of phospholipid particles altered by antibiotics with biochemical impact. STM might allow in situ, real-space, nanometer-scale observations of minute particles composed of phospholipids within the real cells with the highest magnification ratio.

Synthesis and Surface-Active Properties of New Photosensitive Surfactants Containing the Azobenzene Group

Several water-soluble cationic surfactants, 4-alkylazobenzene-4'-(oxy-2-hydroxypropyl)trimethylammonium methylsulfate (AZMS) (AZMS-0, AZMS-1, AZMS-2, AZMS-4, and AZMS-8), containing alkylglycidylether and azoarene have been synthesized with high yields of 63-78% and their surface-active properties have been investigated upon irradiation with UV/vis light. All of the trans-AZMS surfactants are isomerized to cis-trans mixtures containing 92.5% cis isomer by UV light irradiation at 350 nm. The cis isomers in the mixtures are reverted to trans isomers by visible light irradiation (lambda>445 nm). Such photoisomerization induces changes in the surface activity of each surfactant. The critical micelle concentration (cmc) of the trans form of AZMS-8 surfactant is about 1.28x10(-4) mol/l. At the photostationary state, 92.5% of the trans form is changed to the cis form which exhibits a slightly higher cmc (3.41x10(-4) mol/l). The new cmc of AZMS surfactants upon photoisomerization is similar to that of the ideal mixed micellar system. In particular, the ratio of cmc(cis) to cmc(trans) of AZMS derivatives is about 1.87-2.85 which increases proportionally with the chain length of alkyl group. The minimum average area per molecule (A(min)(a/w)) for the trans and cis isomers of AZMS-8 is 0.60 and 0.74 nm(2), respectively. The difference in the A(min)(a/w) may originate from the structural differences in the two isomers. These values are quite different as compared to those of the conventional azobenzene surfactants. Copyright 2000 Academic Press.

Fluorinated vesicles

Stable fluorinated vesicles--i.e. vesicles with a hydrophobic and lipophobic fluorinated film within their bilayer membranes--have been obtained from a variety of neutral, zwitterionic or anionic fluorinated amphiphiles, including single chain phosphocholine derivatives, double-chain phospholipids, glycolipids and glycophospholipids, as well as from combinations of standard phospholipids with amphiphilic mixed fluorocarbon/hydrocarbon compounds. The strong hydrophobic interactions developed by perfluoroalkyl chains result in increased membrane stability, as strikingly illustrated by the fact that even short single-chain fluorinated amphiphiles can form stable, heat-sterilizable vesicles without the need for any additive. They also result in increased versatility in the aggregation behavior of fluorinated surfactants, as shown by the formation, depending on molecular structure and experimental conditions, of a range of supramolecular assemblies other than vesicles, including disks, helical tubules and fibers, for example from fluorinated glycolipids. The more impermeable, adjustable fluorinated core within the liposomal membrane confers to liposomes added drug and probe encapsulation stability as compared to their hydrogenated analogs, whether in buffer or in serum. Fluorinated vesicles made from perfluoroalkylated phospholipids were also found to have 3 to 6 times longer circulation half-lives in mice than similarly sized conventional DSPC/cholesterol liposomes. Finally, the incorporation of mixed fluorocarbon/hydrocarbon alkanes or alkenes into standard liposomes may provide an alternative, straightforward and cost efficient approach to fluorinated vesicles.

Characterization of the preparation process and the photochemical control of electrical properties of bilayer lipid membranes containing azobenzene chromophores

We prepared photoresponsive bilayer lipid membranes (BLMs) containing azobenzene derivatives (4'-octylazobenzene-4-oxybutyric acid (AZ)) and observed the rapid and reversible changes in their electrical properties when irradiated with light. The BLMs consist of AZ (8 mol%) and glyceryl monooleate. The changes in capacitance and conductance upon irradiation by light were found to be 10 and 20%, respectively. The changes in the electrical properties of the membrane and in the structure of AZ under light irradiation were analyzed simultaneously by in-situ spectroscopic, electrical and microscopic measurements. These measurements showed that the electrical changes induced by exposure to light resulted from reversible changes in the membrane structure initiated by the photoisomerization reaction of AZ. This structural change in the membrane occurred within 1 s.