Structural intermediates in phospholipid phase transitions

A new experimental technique to investigate the structural rearrangements associated with thermal phase transitions of phospholipids is outlined. The method utilizes the changes in small‐angle X‐ray powder diffraction patterns, monitored with a time resolution of down to 1 ms, following fast temperature jumps of about 10 K height in 1–2 ms. This heating rate, effected by an erbium infrared laser pulse, provides a synchronous disequilibration of the lipid samples and the ensuing structural relaxation processes are observed by using the high‐flux X‐ray beam from a synchrotron source in combination with a rapid position‐sensitive‐detection and data‐acquisition system. For the two symmetry heterologous phase transitions, the lamellar ( L β' )‐to‐monoclinic ( P β' ) `pretransition' of synthetic phosphatidylcholines and the lamellar ( L α )‐to‐inverted hexagonal ( H II ) transition of phosphatidylethanolamines, this method provides the first evidence for the existence of transient intermediate ordered structures. In the pretransition, the first step is the rapid (< 2 ms) formation of a second lamellar lattice, with a decreased repeat distance, which coexists for up to about 100 ms with the original L β' lattice. In a second, slower process, these two lattices merge and transform to the P β' structure within several seconds. In the L α – H II transition, the first rapid step ( τ ~ 5 ms) consists of a uniform shrinkage of the lamellar lattice; this is followed after about 20 ms by the first appearance of a distorted hexagonal lattice and a slow transformation into the final H II lattice. Comparison of these results with X‐ray diffraction data obtained under near‐equilibrium conditions, where these intermediates cannot be detected, shows that the systems respond to the high thermodynamic driving force which exists under non‐linear non‐equilibrium conditions by formation of correlated intermediate structures which provide efficient pathways for relaxation.