Activation Energy of the Low-pH-Induced Lamellar to Bicontinuous Cubic Phase Transition in Dioleoylphosphatidylserine/Monoolein

Electrostatic interaction is an important factor for phase transitions between lamellar liquidcrystalline (L-alpha) and inverse bicontinuous cubic (Q(II)) phases. We investigated the effect of temperature on the lowpHinduced L-alpha to double diamond cubic (Q(II)(D)) phase transition in dioleoylphosphatidylserine (DOPS)/monoolein (MO) using timeresolved small angle X-ray scattering with a stoppedflow apparatus. Under all conditions of temperature and pH, the L-alpha phase was directly transformed into an intermediate inverse hexagonal (H-II) phase, and subsequently the Hu phase slowly converted to the Q(II)(D) phase. We obtained the rate constants of the initial step (i.e., the La to}In phase transition) and of the second step (i:e., the H-II to Q111) phase transition) using the nonnegative matrix factorization method. The rate constant of the initial step increased with temperature. By analyzing this result, we obtained the values of its apparent activation energy, Ea (L-alpha -> H-II), which did not change with temperature but increased with an increase in pH. In contrast, the rate constant of the second Step decreased with temperature at pH 2:6, although it increased with temperature at pH 2.7 and 2.8. These results indicate that the value of Ea (H-II -> Q(II)) at pH 2.6"increased with temperature, but the values of Ea (Hu QuD) at pH 2.7 and 2.8 were constant with temperature. The values of Ea (H-II -> 92) were smaller than those of Ea (La HO at the same pH. We analyzed these results using a modified quantitative theory on the activation energy of phase transitions of lipid membranes proposed initially by Squires et al. (Squires, A. M.; Conn, C. E.; Seddon, J. M.; Templer, R. H. SOft Matter 2009,.5, 4773). On the basis of these results, we discuss the mechanism of this phase transition.