Quasi-elastic and electrophoretic light scattering studies of the reorganization of dioleoylphosphatidylcholine vesicle membranes by poly(2-ethylacrylic acid)

Abstract

Quasi-elastic light scattering (QELS) and electrophoretic light scattering (ELS) measurements have been used to investigate the pH-dependent structural reorganization of phospholipid vesicles suspended in aqueous solutions of poly(2-ethylacrylic acid) (PEAA). The average radius of dioleoylphosphatidylcholine (DOPC) vesicles suspended in aqueous PEAA was reduced within 1 h from approximately 600 to 60 nm upon acidification. Size distribution analysis revealed narrow single peaks far from the pH at which the reorganization occurs but bimodal distributions in the transition region. The transition pH was shifted from 7.0 to 6.5 as the ionic strength of the sample solutions was increased from 50 to 500 mM. The conformational transition of PEAA from an expanded form at high pH to a relatively compact coil in acidic solutions was also observed by QELS. ELS measurements showed that, as the pH is lowered from 8.0 to 5.5 in the presence of PEAA, the electrophoretic mobility of DOPC vesicles passes through a minimum at the transition pH. At pH lower than that of the transition, vesicle mobility is greater in PEAA solutions than in simple buffers. We believe the increase in mobility is due to adsorption of PEAA on the vesicle surface. We infer that as the pH is lowered, the collapsed PEAA chain provides a hydrophobic site for solubilization of the phospholipid hydrocarbon tails and in so doing disrupts the structural organization characteristic of the pure phospholipid.