Poster Presentation 2014 International Biophysics Congress

Initial step of low pH-induced lamellar to bicontinuous cubic phase transition in dioleoylphosphatidylserine/monoolein (#468)

Toshihiko Oka 1 , Taka-aki Tsuboi 1 , Takahiro Saiki 1 , Tomoki Takahashi 1 , Jahangir Md. Alam 1 , Masahito Yamazaki 1
  1. Shizuoka University, Shizuoka, Japan
Electrostatic interactions (EI) are an important factor for phase transitions between lamellar liquid-crystalline (Lα) and inverse bicontinuous cubic (QII) phases. We investigated the low pH-induced Lα to double-diamond cubic (QIID) phase transition in dioleoylphosphatidylserine (DOPS)/ monoolein (MO) using time-resolved small angle X-ray scattering. At the initial step, the Lα phase was directly transformed into the hexagonal II (HII) phase, and subsequently the HII phase slowly converted into the QIID phase. The rate constant of the initial step increased with a decrease in pH and exhibited a size dependence; for smaller vesicles such as LUVs and smaller MLVs with diameters of ~1 μm, the rate constant was smaller. Furthermore, the results indicate that single MLVs larger than ~1 μm can convert into the HII phase without any interaction with other MLVs. These results provide the first experimental evidence of the total kinetics of EI-induced Lα/QII phase transitions. The transient appearance of an intermediate HII phase during the Lα → QII transition is a new kinetic pathway of phase transitions of lipid membranes. It is indispensable to develop quantitative theories and computer simulations on the kinetics of EI-induced Lα/QII phase transitions.