KINETICS OF THE BAROTROPIC RIPPLE (P-BETA) LAMELLAR LIQUID-CRYSTAL (L-ALPHA) PHASE-TRANSITION IN FULLY HYDRATED DIMYRISTOYLPHOSPHATIDYLCHOLINE (DMPC) MONITORED BY TIME-RESOLVED X-RAY-DIFFRACTION

We present here the first study of the use of a pressure-jump to induce the ripple (P-beta')/lamellar liquid crystal (L-alpha) phase transition in fully hydrated 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). The transition was monitored by using time-resolved x-ray diffraction (TRXRD). Applying a pressure-jump from atmospheric to 11.3 MPa (1640 psig, 111.6 atm) in 2.5 s induces the L-alpha to P-beta-phase transition which takes place in two stages. The lamellar repeat spacing initially increases from a value of 66.0 +/- 0.1 angstrom (n = 4) to a maximum value of 70.3 +/- 0.8 angstrom (n = 4) after 10 s and after a further 100-1 50 s decreases slightly to 68.5 +/- 0.3 angstrom (n = 4). The reverse transition takes place following a pressure jump in 5.5 s from 11.3 MPa to atmospheric pressure. Again, the transition occurs in two stages with the repeat spacing steadily decreasing from an initial value of 68.5 +/- 0.3 angstrom (n = 3) to a minimum value of 66.6 +/- 0.3 angstrom (n = 3) after 50 s and then increasing by approximately 0.5 angstrom over a period of 100 s. The transition temperature increases linearly with pressure up to 14.1 MPa in accordance with the Clapeyron relation, giving a dT/dP value of 0.285-degrees-C/MPa (28.5-degrees-C/kbar) and an associated volume change of 40-mu-l/g. A dynamic compressibility of 0.13 +/- 0.01 angstrom/MPa has been determined for the L-alpha-phase. This value is compared with the equilibrium compressibilities of bilayer and nonbilayer phases reported in the literature. The results suggest testable mechanisms for the pressure-induced transition involving changes in periodicity, phase hydration, chain order, and orientation. A more complete understanding of the transition mechanism will require improvement in detector spatial resolution and sensitivity, and data on the pressure sensitivity of phase hydration.