Kinetics of low pH-induced lamellar to bicontinuous cubic phase transition in dioleoylphosphatidylserine/monoolein

Recently, it has been well recognized that the modulation of electrostatic interactions due to surface charges can induce transitions between lamellar liquid-crystalline (L-alpha) and inverse bi-continuous double-diamond cubic (QII(D)) phases in biological lipids. To reveal their kinetic pathway and mechanism, we investigated the low pH-induced L-alpha to QII(D) phase transitions in 20%-dioleoylphosphatidylserine (DOPS)/80%-monoolein (MO) using time-resolved small-angle x-ray scattering and a rapid mixing method. At a final pH of 2.6-2.9, the L-alpha phase was transformed completely into the hexagonal II (IIII) phase within 2-10 s after mixing a low pH buffer with a suspension of multilamellar vesicles of 20%-DOPS/80%-MO (the initial step). Subsequently, the IIII phase slowly converted into the Q(II)D phase and completely disappeared within 15-30 min (the second step). The rate constants of the second step were obtained using the singular value decomposition analysis. On the basis of these data, we discuss the underlying mechanism of the kinetic pathway of the low pH-induced L-alpha a to QII(D) phase transitions. (C) 2011 American Institute of Physics. [doi:10.1063/1.3575240]