Characterization of the sub-transition of hydrated dipalmitoylphosphatidylcholine bilayers. Kinetic, hydration and structural study

Abstract

Differential scanning calorimetry and fast-recording X-ray diffraction methods are used to study the kinetics, hydration and structural changes of the L β, gel→L c ‘crystal’ conversion at −2°C for less than maximally hydrated (27.2 wt% H 2O) and maximally hydrated (48.9 wt% H 2O) 1,2-dipalmitoyl- l-phosphatidylcholine (DPPC) dispersions. Equilibration of DPPC dispersions at −2°C for increasing time periods results in a progressive increase in the sub-transition temperature to a limiting value of 17–19°C, while the enthalpy of the sub-transition also increases on reaching an enthalpy maximum ΔH =5.6–5.8 kcal/mol DPPC after 2 days equilibration. Corresponding X-ray diffraction experiments demonstrate two time domains involving structural alterations. An initial time domain involves a rapid shift of the two characteristic L β, wide angle reflections, 1 4.18 A ̊ −1 and 1 4.08 A ̊ −1 , to 1 4.3 A ̊ −1 and 1 4.0 A ̊ −1 , respectively, while there is no significant change in the lamellar periodicity. A slower structural alteration subsequently occurs involving a progressive decrease in lamellar periodicity to its limiting dimension, d ≅ 59.5 A ̊ and further shifts in the two wide angle reflections to final values of 1 4.4 A ̊ −1 and 1 3.86 A ̊ −1 . These changes are indicative of alterations in both the bilayer organization and the hydrocarbon chain packing. Hydration studies over the range 10.1 to 48.9 wt% H 2O demonstrate that at 4°C the L c bilayer phase has a reduced hydration limit of 11 mol H 2O/mol DPPC compared to 19 mol H 2O/mol DPPC and 25 mol H 2O/mol DPPC for the L β′ and L α bilayer phases, respectively. It is concluded that the L β′ → L c conversion involves dehydration and hydrocarbon chain ordering. The data suggest a crystallization of DPPC and 11 H 2O molecules presumably involving tightly bound water molecules in an interbilayer matrix characterized by water-water and water-DPPC hydrogen bonding. A structural interpretation of the changes occurring in the two-dimensional hydrocarbon chain packing modes during the transitions between the hydrated L c, L β′, P β′ and L α bilayer forms of DPPC is proposed.