W. Patrick Williams

State 1/State 2 changes in higher plants and algae

Current ideas regarding the molecular basis of State 1/State 2 transitions in higher plants and green algae are mainly centered around the view that excitation energy distribution is controlled by phosphorylation of the light-harvesting complex of photosystem II (LHC-II). The evidence supporting this view is examined and the relationship of the transitions occurring in these systems to the corresponding transitions seen in red and blue-green algae is explored.

A Simple Method for the Preparation of Liposomes for Pharmaceutical Applications: Characterization of the Liposomes

Abstract— A new method for the preparation of liposomes is described that avoids the use of pharmaceutically unacceptable solvents and energy‐expensive procedures such as sonication. The method is based on the initial formation of a proliposome mixture containing lipid, ethanol and water, which is converted to lipsomes by a simple dilution step. Measurements using 6‐carboxyfluorescein as a marker indicate that water‐soluble drugs can be trapped with extremely high efficiency (65–80% depending on lipid composition). The structural organization of the proliposome mixture and the final liposomes were characterized using electron microscopy and 31P‐NMR.

Increased thermal stability of pigment-protein complexes of pea thylakoids following catalytic hydrogenation of membrane lipids

The membrane lipids of pea thylakoids were hydrogenated in situ using the homogeneous catalyst palladiumdi-(sodium alizazine monosulphonate). Following hydrogenation, particle-free patches corresponding to phase-separated gel-phase lipids were observed in the fracture-faces of thylakoid membranes. Freeze-fracture studies on samples of hydrogenated thylakoids incubated at elevated temperatures indicated that hydrogenation reduces the tendency of the heated membranes to destack and vesiculate at higher temperatures. Measurements of chlorophyll a fluorescence emission and the thermal properties of hydrogenated thylakoids suggest that the hydrogenation process also leads to an increase in the thermal stability of pigment-protein complexes of the Photosystem II light-harvesting apparatus.

The Hofmeister effect in relation to membrane lipid phase stability

The phase behaviour of L-alpha-1-palmitoyl-2-oleoyl-phosphatidylethanolamine (POPE) was examined in aqueous dispersions containing a range of sodium salts. The phase properties of the lipid exhibited a graded response to the presence of simple anions analogous to that of the Hofmeister series encountered in the study of the solution properties of proteins. Salts early in the series (such as Na2SO4 and NaCl) gave rise to substantial decreases in the temperature (Th) of the transition from the lamellar liquid-crystal to inverted hexagonal phase (L alpha----HII) and small increases in the temperature (Tm) of the transition from the lamellar gel to lamellar liquid-crystal phase (L beta----L alpha). Salts towards the end of this series (such as NaI and NaSCN) led to increases in Th and decreases in Tm. Similar effects were seen in lipid dispersions containing a series of non-ionic co-solutes. In both cases, the relative efficiency of perturbation of the lipid phase properties reflected the relative ability of the anion or co-solute to influence the structure of the bulk water. X-ray diffraction measurements indicated that these effects were probably mediated through the ability of the co-solutes to bring about changes in the extent of the lipid/water interface.

The Physical Properties of Thylakoid Membrane Lipids and Their Relation to Photosynthesis

The three main areas in which research into the physical properties of thylakoid membrane lipids has impacted on photosynthetic research are: lipid phase behavior and its relevance to membrane phase separations; lipidprotein interactions in the context of the structure and function of the main photosynthetic membrane complexes; and the fluidity properties of membranes in terms of thermal adaptation and the factors limiting rates of electron transport.

Phase behaviour of the membrane lipids of the thermophilic blue-green alga Anacystis nidulans

Abstract The phase behaviour of total membrane lipid extracts of the blue-green alga Anacystis nidulans is compared with that of the individual lipid classes present in such extracts using fluorescence probe, differential scanning calorimetry, wide-angle X-ray diffraction and freeze-fracture techniques. Marked differences are observed in the properties of the isolated lipids as compared to the total lipid extracts. In particular, purified samples of monogalactosyldiacylglycerol and phosphatidylglycerol form complex high melting-point gel phases on storage which are not found in the membrane extracts. Addition of Mg 2+ ions to the extracts is also shown to lead to an extensive phase separation of monogalactosyldiacylglycerol from the extracts. The enthalpy changes associated with phase separations occurring in the lipid extracts are found to be approx. 30% higher than those for the corresponding membranes, suggesting that the presence of other components, such as membrane proteins, may influence the phase behaviour of the lipids. The significance of these observations is discussed in terms of the factors limiting the stability of membrane systems.

X‐ray diffraction studies of the structural organisation of prolamellar bodies isolated from Zea mays

Transmission electron microscopy (TEM) indicates that maize prolamellar bodies (PLBs) are built up of tetrapodal units based on a highly convoluted but continuous lipid bilayer exhibiting diamond cubic (Fd3m) symmetry. Such lattices are often described in terms of infinite periodic minimal surfaces (IMPS) exhibiting zero net curvature and dividing the system into two identical subvolumes. If so, X‐ray diffraction measurements would be expected to index on a double‐diamond (Pn3m) lattice with a unit cell length half that of the TEM lattice. Our measurements index on a Fd3m lattice with a similar repeat distance to the TEM images. The PLB membrane is thus inherently asymmetric, probably as the result of the distribution of membrane protein.

Factors influencing PS II particle array formation in Arabidopsis thaliana chloroplasts and the relationship of such arrays to the thermostability of PS II

Chloroplasts of the thermal-stable fatty acid desaturase mutants JB67 and LK3 of Arabidopsis thaliana are characterised by the presence of regular arrays of freeze-fracture particles associated with the core and light-harvesting antennae of Photosystem II (Tsvetkova et al. (1994) Biochim. Biophys. Acta 1192, 263–271). Similar arrays were found to be induced in the membranes of chloroplasts isolated from wild-type plants by resuspending the chloroplasts in media containing Tricine and/or high concentrations of compatible co-solutes such as sorbitol. The thermal stability of their chloroplasts was also increased under such conditions. The increased tendency to form PS II particle arrays, and the enhanced thermal stability of PS II, in chloroplasts isolated from the mutants and wild-type chloroplasts suspended in different media, appear to be reflections of the increased stability of protein-protein interactions between and within PS II units, respectively. The role of lipids in determining the formation of freeze-fracture particle arrays in the mutants is discussed in terms of the observed changes in lipid composition and their possible role in the control of lipid/protein synthesis.

Low-temperature phase behaviour of the major plant leaf lipid monogalactosyldiacylglycerol

Heating and cooling thermograms of unsaturated MGDG samples isolated from the leaves of Vicia faba are surprisingly featureless. This reflects the low enthalpies associated with phase transitions in highly unsaturated lipids and the fact that these transitions, in the case of MGDG, are to a large extent masked by those associated with the freezing and melting of ice. Careful choice of thermal heating/cooling regimes, combined with the use of real-time X-ray diffraction and freeze-fracture measurements, permits a detailed analysis of the phase behaviour of the system. The phase behaviour of unsaturated MGDG samples is shown to be basically similar to that seen in saturated MGDG samples. The lipid which exists in the inverted hexagonal (HexII) liquid crystal phase at room temperature forms a highly disordered lamellar gel (L beta) phase on cooling to temperatures below about -15 degrees C. On reheating, this first reorganizes at a temperature of about -10 degrees C to form a well-defined Lc1 phase. Above about -2 degrees C, this melts to re-form the HexII phase. Samples re-cooled from temperatures between -2 degrees C and 14 degrees C revert directly to the Lc1 phase while samples cooled from higher temperatures form the L beta phase. This reflects the fact that the former samples contain small amounts of unmelted Lc1 phase lipid. The implications of these observations are discussed in terms of the general problems associated with the measurement of low-temperature phase behaviour of membrane lipids.

The effect of ice on membrane lipid phase behaviour.

The effects of ice on the lipid phase behaviour of di-18:1 PE and di-18:2 PE were studied by comparing the behaviour of these lipids in supercooled and frozen dispersions. The presence of ice raised the onset temperature of the L alpha--> L beta phase transition of di-18:1 PE from -10 degrees C to -6.5 degrees C and increased its molar enthalpy from 6.1 to 8.5 kcal/mol but had little effect on the co-operativity of the transition. Real-time X-ray diffraction measurements of the HII--> L alpha phase transition of di-18:2 PE suggested that this transition could take place in the presence of ice but that the corresponding L alpha--> HII phase transition could not take place until the ice melted. Measurements of the temperature dependence of the d-spacing of di-18:1 PE and di-18:2 PC in frozen dispersions indicated that the amounts of unfrozen water in such dispersions changes significantly with temperature. It was concluded that the increases in onset temperature and molar enthalpy seen for the L alpha--> L beta transition of di-18:1 PE probably reflected the effects of osmotic dehydration. The main effect of ice in the case of the HII--> L alpha phase transition, however, appeared to be to limit the ability of the lipid to undergo structural reorganisation.