Microenvironmental differences and changes in bilayers of unilamellar vesicles probed by spectroscopic and kinetic parameters.

The UV/Vis absorption band maximum lambdamax of trans-4,4'-nitrophenylaminoazobenzene, the thermal isomerization rate constant kiso of its cis-isomer, the fluorescence intensity ratio of monomer and excimer, and the fluorescence lifetime of the excimer, respectively, of 1,3-di(1-pyrenyl)propane were determined as probes for polarity, water content, and viscosity, respectively, in unilamellar vesicles of di-n-alkyl-dimethylammonium bromides and 1,2-acyl-sn-glycero-3-phosphocholines. The dependence on vesicle size, the solvent (water or HEPES buffer/NaCl solution, each with H2O or D2O), and the temperature (20-60 degrees C) was studied. Apparent Arrhenius activation energies and kinetic solvent isotope effects (KSIE = kiso,H2O/kiso, D2O) were derived. Size and stability of the vesicles prepared by extrusion were controlled by dynamic light scattering. The probe properties clearly indicate the reversibly decreasing size of didodecyldimethylammonium bromide vesicles with increasing temperature but are insensitive against vesicles size variation in most other cases. In the temperature range of the main phase transition of the bilayers, changes of the microenvironment of the probes, and their changing position in the bilayer, respectively, are reflected by characteristic changes of their properties. Buffer/NaCl solution causes vanishing influence of the lipid chain but remaining difference between cationic and zwitterionic headgroups probed by means of kiso.

Interaction between dextran and human low density lipoproteins (LDL) observed using laser light scattering.

Dextran infusions in humans lead to a reduction of low density lipoproteins (LDL) in the plasma compartment. The interaction of dextran with human LDL was investigated in vitro by static and dynamic light scattering. The experiments were performed with human LDL (apoB concentration 0.75 g l(-1)) and dextran (Mw = 40,000 and 70,000 g mol(-1)) at 25 degrees C. The dextran concentrations after mixing were 10 and 50 g l(-1). The hydrodynamic radius for native LDL was found to be RH = 12.9 nm. The addition of dextran induces the formation of LDL associates with a mean radius of RH approximately 200 nm. These findings show that even non-sulphated polysaccharides interact with LDL. The dextran-dependent formation of LDL associates detected in vitro could be the reason for the in vivo effect of dextran on the lipid metabolism.

Micellization of Graft Copolymers of Alkyl Methacrylates with alpha-Methyl-omega-hydroxypoly(oxyethylene) Methacrylates.

The solution properties of graft copolymers having a poly(methyl methacrylate-co-lauryl methacrylate) backbone and poly(oxyethylene) branches of different compositions were studied by static and dynamic light scattering methods in ethanol-water mixed solvents. The graft copolymers formed multimolecular aggregates in mixed solvents with water content higher than or equal to 10 vol%. The difference in solubility of lauryl and poly(oxyethylene) side chains of copolymers seems to be a dominant factor in determining their association behavior. The micelle formation is preferred in such solvents (water content >/=30 vol%) where the difference in solubilities of LAM and PEO side chains on graft copolymers is higher than a critical one. In conditions where there is a lower difference in solubilities of the side chains (water content 10-20 vol%), the random association of copolymers dominates. Copolymers were fully soluble in ethanol with low water content (1 vol%). The highest association number of micelles and consequently the lowest polydispersity were obtained with copolymers having the longest PEO side chains (Mw = 5000 g mol-1). Copyright 1998 Academic Press.