Phase transition kinetics of phosphatidic acid bilayers A stopped-flow study of the electrostatically induced transition

Phospholipid phase transitions: kinetics and structural mechanisms

A brief review is given on the principles and methods used to investigate structural phase transitions in phospholipid supramolecular structures. The conceptual differences of approaches close to and far from equilibrium are addressed, and the consequences in terms of the limits of interpretation for different types of methods, in particular referring to jump-relaxation and steady-state techniques, are surveyed. With the emphasis on connecting dynamic and structural information, the results obtained so far from different techniques are reviewed, and the open questions addressed. The more recent advances by millisecond time-resolved X-ray diffraction with synchrotron radiation and their main results obtained for transitions triggered by IR-laser temperature jumps are summarized. As a major novel aspect in the field, the necessity of considering martensitic, diffusionless transformation mechanisms and the occurrence of intermediate structures is highlighted.

Kinetics and mechanism of the lamellar gel/lamellar liquid-crystal and lamellar/inverted hexagonal phase transition in phosphatidylethanolamine: a real-time X-ray diffraction study using synchrotron radiation

A study of the kinetics and mechanism of the thermotropic lamellar gel/lamellar liquid-crystalline and lamellar/inverted hexagonal phase transition in dihexadecylphosphatidylethanolamine (DHPE) at various hydration levels has been carried out. Measurements were made by using a real-time X-ray diffraction method at the Cornell High Energy Synchrotron Source. This represents an extension of an earlier study concerning the lamellar gel/lamellar liquid-crystalline phase transition in dipalmitoylphosphatidylcholine [Caffrey, M., & Bilderback, D. H. (1984) Biophys. J. 45, 627-631]. With DHPE, the chain-melting and the nonbilayer transitions were examined under active heating and passive cooling conditions by using a temperature jump to effect phase transformation. Measurements were made at hydration levels ranging from 0% to 60% (w/w) water, and in all cases, the transitions were found to be repeatable, be reversible, and have an upper bound on the transit times (time required to complete the transition) of less than or equal to 3 s. The shortest transit time recorded for the chain-melting and lamellar/hexagonal transitions was less than 1 s. At 8% (w/w) water, the transit times were still on the order of seconds even though the transition does not involve the intermediate L alpha phase. Note, the measured transit times are gross values incorporating the intrinsic transit time in addition to the time required to heat or cool the sample through the transition temperature range and to supply or remove the latent heat of the transition. Regardless of the direction of the transition, both appear to be two state to within the sensitivity limits of the real-time method. From simultaneous wide- and low-angle measurements at the lamellar chain-melting transition, loss of long-range order in the lamellar gel phase appears to precede the chain-melting process. On the basis of the real-time X-ray diffraction measurements, a mechanism is proposed for the lamellar/hexagonal phase transition. The mechanism does not involve large or energetically expensive molecular rearrangements, leads directly to a hexagonal lattice coplanar with the lamellar phase, incorporates facile reversibility, repeatability, and cooperativity, accounts for an observed, apparent memory in the hexagonal phase of the original lamellar phase orientation, and is consistent with the experimental observation of a predominantly two-state transition. In conjunction with the kinetic measurements, the DHPE/water phase diagram was constructed. At and above 12% (w/w) water, the thermotropic transition sequence is L beta'/L alpha/HII.(ABSTRACT TRUNCATED AT 400 WORDS)