Hydration of single crystals of dipalmitoylphosphatidylcholine.

Abstract

Optical birefringence and x-ray measurements are reported for single crystals (0.5 mm x 2 mm X 5 ,m) of dipalmitoylphosphatidylcholine as a function of temperature and relative humidity. Large single crystals grown in CHC1jacetone at a low water concentration undergo an irreversible phase transformation at 20'C and 70% relative humidity or at 60°C and 65% relative humidity. After this transformation occurs, the crystals can be made to undergo a reversible transformation into and out of the L. phase. The x-ray-determined mosaic spread in the hydrated crystal is less than :0.04°both above and below.the main transition.A number of different approaches have been made to prepare oriented lecithin/water samples without ever obtaining the same degree of alignment in high-water samples that can be obtained in low-water single crystals (1)(2)(3)(4)(5)(6)(7)(8)(9). In this manuscript, we describe optical and x-ray measurements on monodomain single-crystal samples of dipalmitoylphosphatidylcholine (Pam2PtdCho) that have a mosaic spread of <0.040 at different temperatures and water contents. They were obtained by exposing crystals grown from an acetone/chloroform solution containing <0.5% water at =160C to relative humidities of up to 95% and temperatures up to 75TC. Optical and x-ray studies of the as-grown crystals are described elsewhere (10).X-ray diffraction studies of lecithin/water phases have been carried out by a number of workers (11-22), and the general properties of the phase diagrams are well established. There exists a transition temperature T, (see, for example, ref. 12) that is a monotonically decreasing function of water content such that, for T > Tc, the equilibrium phase consists ofplanar arrays of "fluid bilayers." This is classified as the La phase in the lipid literature and as a smectic A phase in the liquid crystal literature (23). For T < T, and low water concentrations, the equilibrium phases are crystalline-i.e., periodic in.three dimensions. At higher water concentrations, for T < Tc, x-ray diffraction measurements on partially oriented samples indicate rows of scattering along the (h,0,l) axis, h (4.2 A)-1, suggestive ofphases that have well-developed periodicity within each-bilayer but in which successive bilayers are not correlated (19,20). Thus, these phases can be simply interpreted as two-dimensional crystals stacked on top of one another but without any correlation in their relative positions. Although phases with these properties have been discussed in the liquid crystal literature, it is now generally agreed that, if such phases do exist, the periodicity within the bilayer must have a finite correlation length (24,25). This would result in Bragg-like peaks that have shapes different from those oftrue Bragg peaks from three-dimensional crystals.Attempts have been made to observe this phenomenon in thermotropic liquid crystals by using high resolution x-ray diffraction techniques on well-oriented monodomain samples. In one case, a system previously believed to have this property w...

Interaction of benzene with bilayers. Thermal and structural studies

“…Third, the freeze-fracture images (Figures 1 and 2) show regular striations in the hydrophobic interior of the bilayer. Similar striations have been seen in DMPC, DPPC, and DPPG bilayers at pH 8.0 (Luna & McConnell, 1977;McIntosh & Costello, 1981;Watts et al, 1978) and DPPCcholesterol (x < 0.05) bilayers (Lentz et al, 1980). The replicas of hydrophobic fracture faces shown in Figures 1 and 2 do not exhibit lenses of benzene between lipid monolayers.…”

“…Various lipid samples contained from one to three waters per lipid. In regard to the variation in water content, we note that Urabe et al (1981) have shown that DPPC crystals exposed to atmospheric humidity will rapidly hydrate.…”