Laser-induced temperature jump and time-resolved x-ray powder diffraction on phospholipid phase transitions

Abstract

Temperature-jump x-ray small-angle powder diffraction was used to investigate the kinetics and structural mechanism of the thermotropic phase transitions of a phosphatidylethanolamine (1-stearoyl-2-oleoyl-sn-3-phosphatidylethanolamine)/water system. Am Erbium laser (λ=1.5 µm) with a pulse energy of 2 J delivered in 2 ms was used for rapid heating, leading to a T-jump of about 12°C. The small-angle diffraction pattern was detected with 1 ms resolution during the relaxation following the T-jump. The results showed that the L _β−L _α transition (∼ 29°C) is at least as fast as the heating time (2 ms). The lamellar-to-hexagonal (L _α − H _Π) transition was found to be considerably slower with a half-time in the order of 100 ms. Both transitions were found to be two-state, with the phase structures coexisting during the transition and without detectable nonlamellar intermediates.