Mechanism and kinetics of the subtransition in hydrated L-dipalmitoylphosphatidylcholine.

The mechanism of the subtransitions (Lc to L beta') in L-dipalmitoylphosphatidylcholine bilayers in excess water has been investigated by time-resolved X-ray diffraction using synchrotron radiation. The temperature dependence of the diffraction patterns closely correlate with the asymmetric excess specific heat variation recorded by differential scanning calorimetry. During the subtransition two prominent wide-angle reflections, characteristic of the low-temperature crystalline phase, Lc, gradually change such that a sharp peak at a spacing of 0.430 nm decreases in intensity and ultimately disappears while a broader peak initially located at 0.375 nm progressively shifts to an eventual spacing of 0.410 nm. This behaviour is interpreted as a lateral deformation of the acyl chain packing subcell as the chains begin to rotate until a state is reached where the chains pack on a regular hexagonal array characteristic of the L beta phase. An increase in lamellar repeat distance from 6.0 to 6.4 nm takes place simultaneously with the acyl chain rearrangement at relatively low (5 K/min) as well as high (6 K/s) heating rates. As judged from the shape of the wide-angle peak, transformation to L beta' phase occurs some minutes after transition to the L beta phase. The X-ray data characterise the subtransition as a continuous (second order) phase transition in which a presumably orthorhombic subcell is transformed into a hexagonal subcell in a gradual process. In temperature jump experiments at 6 K/s between 0 degree C and 80 degrees C the relaxation time of the subtransition was found to be about 5 s while the relaxation time of the main gel to liquid-crystalline transition was about 2 s.