Direct determination of layer packing for a phospholipid solid solution at 0.32-nm resolution.

Abstract

Electron diffraction intensity data were collected from a 2:3 binary solid solution of two homologous phosphatidylethanolamines (1,2-dimyristoyl-sn-glycerophosphoethanolamine and 1,2-dlpalmitoyl-sn-glycerophosphoethanolamine) epitaxially oriented by cocrystaliation with naphthalene. The layer pcng was determined directiy by predicting the value of 12 of the 17 phases from Z,-and £2-triplet invariants in space group P1. A reverse Fourier transform of the resulting potential maps provides estimates for three other phases and the two remaining ones were found by generating maps for the 22 = 4 possible phase combinations and then testing the smoothness of the potential prorfle of the hydrocarbon chain packing. The same phase solution can be found by translating a molecular model (based on the known x-ray crystal structure of a shorter homologue) past the unit cell origin. The solid solution is found to retain a stable polar group packing while the statistical occupancy of two terminal-chain carbons is expressed by a reduced potential profile at the nonpolar interface at the bilayer center.Although the crystal structures of pure phospholipids (1) are very important for the understanding of how different headgroup classes and chain substitutions will influence molecular packing in a lipid bilayer, it must be realized that polydispersity of components is the major factor that regulates the physical behavior of biomembrane lipid. Thus it is necessary to obtain structural parameters from bilayers composed of various combinations of lipid species. For example, cholesterol-phospholipid interactions have been studied in the lyotropic smectic phase as oriented multilayer arrays, yielding some structural information about the relative placement of the respective species along the bilayer profile (2, 3). Phospholipid comixtures themselves have received considerable attention since it is well known that changes in relative chain length (4-8), chain unsaturation (6, 7), head-group species (8), and possibly, degree of solvation (9-12) can also play a significant role in regulating bilayer phase behavior. Attempts to study molecular interactions of such comixtures in terms of single crystal structures, however, are frustrated by actually increasing the difficulty of an already demanding task, i.e., growing suitable samples for data collection. Obviously, for dissimilar molecules, such crystal growth may not be possible unless a molecular complex can be formed. Even for a solid solution of nearly identical chain lengths for a single head-group class of a phospholipid, the merest amount of impurity can lead to appreciable crystallization problems (13). Nevertheless structural parameters from such binary combinations are necessary for understanding their phase diagrams, e.g., in terms of the molecular volume rules formulated by Kitaigorodskii (14) for the stability of solid solutions or to model the molecular associations in the liquid crystalline state.As has been shown in an earlier study (15), it is possible to grow highly...

Crystallography of Linear Molecule Binary Solids. X-ray Structure of a Cholesteryl Myristate/Cholesteryl Pentadecanoate Solid Solution

“…Therefore, the increased sensitivity to volume change, compared to the n -paraffins, is caused by the fixed packing of the sterol nuclei in the layer. A similar result from the constrained polar group packing in phospholipid solid solutions has been demonstrated …”

“…A similar result from the constrained polar group packing in phospholipid solid solutions has been demonstrated. 18 For the cholesteryl esters, no quantitative description has been given yet for a solid solution where strict constraints are also placed on the packing of the acyl chains. For this reason, the crystal structure of a nearly 1:1 cholesteryl myristate/cholesteryl pentadecanoate binary solid in the interdigitated bilayer packing was determined.…”