Cationic Lipid Modulates Phospholipid Adsorption on Silica
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Cationic Lipid Modulates Phospholipid Adsorption on Silica Sérgio P. Moura and Ana M. Carmona-Ribeiro Departamento de Bioquímica, Instituto de Química, Universidade de São Paulo, P. O. Box 26077, 05513-970, São Paulo, SP, Brazil. ABSTRACT The effect of cationic lipid dioctadecyldimethylammonium bromide (DODAB) to dipalmitoylphosphatydilcholine (DPPC) molar ratio (%R) on lipid adsorption from DODAB/DPPC vesicles on silica was investigated over a range of %R (0 –20 %) at pH 6.3 and 7.4 from determination of adsorption isotherms, colloid stability, particle sizing and ζ-potentials. At 2 % DODAB and pH 6.3, in pure water, high affinity adsorption isotherms with limiting adsorption indicative of one bilayer deposition on each silica particle were obtained whereas for the other R values tested, limiting lipid adsorption was either above or below the level expected for bilayer deposition. At R smaller or higher than 2%, adsorption isotherms indicated limiting adsorption above and below bilayer deposition, respectively. This suggested a modulating role for DODAB allowing control of total amount of lipid adsorbed on silica. Silica sedimentation documented from photographs was almost absent over a range of lipid concentrations where bilayer deposition was achieved. INTRODUCTION Membrane-derivatized colloidal particles offer an extensive repertoire of chemical functionality and have been introduced as potential applications to a broad set of problems involving chemical events on cell membrane surfaces [1-3]. However, not always the interaction between bilayer vesicles and mineral surfaces produced the desired spreading of lipid bilayers on the supports [4 –8]. Recently, cryo-TEM provided beautiful images of supported lipid bilayers on silica and adhered liposomes attached to the supported bilayers [9] which confirmed evidences previously obtained either by quantitative physicochemical methods such as determination of lipid adsorption isotherms [4 –8], QCM-D [3, 10], surface plasmon ressonance[1], ellipsometry [11] or atomic force microscopy [12, 13]. In order to avoid the vesicle adhesion onto the supported bilayer, DPPC bilayer adsorption on silica was optimized from a small proportion of cationic lipid included in the DPPC bilayer composition. Thereby the electrostatic repulsion between supported DODAB/DPPC bilayers on silica and free DODAB/DPPC vesicles prevents further vesicle adhesion on the desired bilayer assembly on particles. EXPERIMENTAL DETAILS Materials Dipalmitoylphosphatidylcholine (DPPC), dioctadecyldimethylammonium bromide (DODAB) and tris(hydroxymethyl)aminomethane hydrochloride (TRIS-HCl) were obtained from Sigma. Silica AEROSIL OX-50 was obtained from Degussa with 50 nm
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mean particle diameter and 50 m2/g specific surface area provided by the supplier. From routine measurements by dynamic light scattering, silica particles presented extensive aggregation with mean aggregate diameter around 300 nm and 47 m2/g specific surface area as determined by potentiometric titration. The s
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