Chlorophyll a in bilayer membranes. I. The thermal phase diagram with distearoylphosphatidylcholine

Phases and phase transitions of the phosphatidylcholines

LIPIDAT (http://www.lipidat.chemistry.ohio-state.edu) is an Internet accessible, computerized relational database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior and molecular structures. Here, a review of the data subset referring to phosphatidylcholines is presented together with an analysis of these data. This subset represents ca. 60% of all LIPIDAT records. It includes data collected over a 43-year period and consists of 12,208 records obtained from 1573 articles in 106 different journals. An analysis of the data in the subset identifies trends in phosphatidylcholine phase behavior reflecting changes in lipid chain length, unsaturation (number, isomeric type and position of double bonds), asymmetry and branching, type of chain-glycerol linkage (ester, ether, amide), position of chain attachment to the glycerol backbone (1,2- vs. 1,3-) and head group modification. Also included is a summary of the data concerning the effect of pressure, pH, stereochemical purity, and different additives such as salts, saccharides, amino acids and alcohols, on phosphatidylcholine phase behavior. Information on the phase behavior of biologically derived phosphatidylcholines is also presented. This review includes 651 references.

Effect of chlorophyll a on the phase behavior of hydrated monogalactosyldiacylglycerols

We have studied the effect of chlorophyll a (chl a) on the X-ray diffraction patterns and the appearance of freeze-fracture electron micrographs of aqueous dispersions of monogalactosyldiacylglycerols (MGDG), the most abundant lipid in the thylakoid membrane. In MGDG systems containing 0-18 mol% of chl a, the diffraction patterns indicate the presence of a well-ordered reverse hexagonal phase. When 30 mol% of chl a was incorporated into the MGDG, the low-angle X-ray diffraction lines of the hexagonal lattice were slightly broadened and were accompanied by additional weak lines. With higher mol percents of chl a, the low-angle lines could no longer be indexed on a hexagonal or lamellar lattice. The freeze-fracture electron micrographs of similar samples showed that the patterns characteristic of the reverse hexagonal phase of an aqueous dispersion of pure MGDG were replaced by large liposomes, the fracture pattern of which is circular. We conclude that chl a in excess of 20 mol% destabilized the orderly reverse hexagonal phase of aqueous MGDG dispersions and disturbed the long-range order of the lipid array. These results are summarized in a temperature-composition isobaric phase diagram over a temperature range of -60 degrees C to 60 degrees C.

Organization of chlorophyll a in bilayer membranes. The chlorophyll a/dimyristoylphosphatidylcholine system

In order to investigate the relative importance of the hydrophobic and headgroup interactions of chlorophyll a in phospholipid bilayers, we have carried out differential scanning calorimetry (DSC) and deuterium (2H) and phosphorus (31P) nuclear magnetic resonance (NMR) experiments on the multilamellar system of chlorophyll a in dimyristoylphosphatidylcholine (DMPC). Compared to the phytol chain of chlorophyll and the previously reported distearoylphosphatidylcholine (DSPC), the acyl chains of DMPC are shorter in length by three and four carbons, respectively. A lowering in the phase-transition temperature was observed for the DMPC multilayers in the presence of chlorophyll a in the DSC thermograms and in the 31P chemical shift anisotropy measurements. These results, together with data on hydrophobic interactions as measured by 2H-NMR and on headgroup interactions as evidenced from 31P-NMR, suggest a phase diagram for the chlorophyll a/DMPC system in which phase separation readily occurs between a chlorophyll-rich compound phase and a chlorophyll-poor phospholipid phase. Compound formation appears to be important in the stabilization of chlorophyll a in bilayers with shorter chains.

Simultaneous modeling of phase and calorimetric behavior in an amphiphilic peptide/phospholipid model membrane

Differential scanning calorimetry (DSC) experiments have been performed on the amphiphilic peptide/1,2-bis(perdeuteriopalmitoyl)-sn-glycero-3-phosphocholine system for which partial phase diagrams have been measured by deuterium nuclear magnetic resonance. The solute concentration dependence of the transition enthalpy in such systems is often interpreted in terms of an annulus of lipid withdrawn, by the solvent, from participation in the transition while the bulk lipid melts with its fully enthalpy. This idea is equivalent to postulating ideal mixing between the lipid and the peptide/lipid complex, and there is little justification for such an assumption. Adaptation of regular solution theory to this system demonstrates that the peptide concentration dependence of the transition enthalpies can be incorporated into a thermodynamic model which reproduces the observed phase behavior fairly well without postulating that a complexing annulus of lipid around the peptide be withdrawn from participating in the chain-melting transition. The model parameters determined by simultaneous fitting of the phase behavior and transition enthalpies are used to simulate the DSC scan shapes. The asymmetry of the calorimetric scans for chi 2 less than or equal to 0.02 is reproduced by the model, but a broad component observed for higher concentration is not. In light of the results presented here, previous analyses of the calorimetric behavior of two-component systems in terms of symmetric transitions which do not account for the possible extent of a region of two-phase equilibrium must be questioned.