Aziz A. Durrani

A difference infrared spectroscopic study of gramicidin A, alamethicin and bacteriorhodopsin in perdeuterated dimyristoylphosphatidylcholine

Difference infrared spectroscopy has been used to study the way in which the intrinsic molecules gramicidin A, alamethicin and bacteriorhodopsin perturb their environment when present within a lipid bilayer structure. Dimyristoylphosphatidylcholine containing perdeuterated chains has been used to enable the lipid chain C-2H stretching absorption band to be separated from the C-H bands arising from the intrinsic polypeptide or protein. The C-2H stretching bands of the phospholipid are sensitive to two different types of chain conformation. The C-2H stretching frequency provides information about the static order of the lipid chains, whilst the half-maximum bandwidth provides a measure of chain librational and torsional motion. From the measurements it is concluded that: (1) Above the lipid phase transition temperature tc, low concentrations of either gramicidin A or alamethicin cause a small decrease in lipid chain gauche isomers whilst bacteriorhodopsin in the lipid bilayer has no effect. At higher concentrations each intrinsic molecule causes an increase to occur in lipid chain gauche isomers. (2) The lipid acyl chain motion, as deduced from the bandwidths is increased by the presence of a low concentration of gramicidin A within the lipid bilayer. The presence of the other intrinsic molecules studied have little effect. A higher concentration of alamethicin causes a decrease in chain motion whilst gramicidin A and bacteriorhodopsin have no effect. (3) Below tc each of the intrinsic molecules when present in the lipid bilayer causes an increase in gauche isomers to occur as well as an increase in the lipid chain motion. A broadening of the lipid phase transition occurs as the concentration of the polypeptide increases.

Biomembrnanes as models for polymer surfaces: II. The syntheses of reactive species for covalent coupling of phosphorylcholine to polymer surfaces☆

We have proposed previously that the biocompatibility of biomaterial surfaces might be improved by mimicking the phospholipid components which are present on the external faces of cellular plasma membranes. This approach may have particular relevance to the generation of haemocompatible surfaces, since the distributional asymmetry of phospholipids in the plasma membranes of blood cells is critical to the maintenance of haemostasis. The simplest feature common to the external faces of essentially all the mammalian plasma membranes investigated so far is the high content of the electrically neutral, but zwitterionic, phosphorylcholine. The present communication describes the syntheses of a group of novel reactive species capable of covalently linking phosphorylcholine to a variety of polymer surfaces.

The physical properties and photopolymerization of diacetylene-containing phospholipid liposomes

Abstract (a) The physical properties and photopolymerization of diacetylene-containing phosphatidylcholines with acyl chains of different length and in liposome form have been studied. (b) The structure of these liposomes and their stability during polymerization have been examined using electron microscopy and glucose trapping. (c) Photopolymerization of the diacetylene groupings has been followed by monitoring the conversion of monomer and the formation of coloured polymer and the optimum conditions for polymerization have been established. (d) Changes induced by irradiation on the phase transition behaviour of these lipids were determined by differential scanning calorimetry. Polymerization decreases both the transition temperature and the enthalpy of the main endothermic transition. (e) The permeability of liposomes to glycerol is changed as a result of the polymerization process.

Biomembpanes as models for polymer surfaces: III. Characterization of a phosphorylcholine surface covalenay bound to glass

A surface layer of phosphorylcholine has been chemically linked with the surface hydroxyl groups present on glass and silica by reaction with mono- and bifunctional reagents. Evidence for the structural integrity of the deposited group was provided by the equimolar association of phosphorus and choline with the reacted surfaces. Modified glass surfaces yielded contact angles which are consistent with those found previously for other models of biological membranes. Covalent modification of the treated surfaces was demonstrated by i.r. spectroscopy via the removal of surface hydroxyl groups. The modified surfaces were thermostable at temperatures up to 375 degrees C for extended periods. The relevance of these results to the generation of new biomaterials is discussed.

The biosynthetic incorporation of diacetylenic fatty acids into the biomembranes of Acholeplasma laidlawii A cells and polymerisation of the biomembranes by irradiation with ultraviolet light.

Acholeplasma laidlawii A has been grown in media containing synthetic, long chain C20- and C23-fatty acids possessing a diacetylene group in their acyl chains. Growth on the C23 diacetylenic acid was poor but was good on the C20 acid. Biosynthetic incorporation of the fatty acids occurs; as much as 90% of the membrane lipid fatty acyl chains consisting of the C20-diacetylenic fatty acid, the remainder being shorter chain, saturated fatty acids. The thermal phase transition of this biomembrane has been studied and a differential scanning calorimetry heating curve shows the presence of an endotherm corresponding to a membrane lipid phase transition occurring at about 26 degrees C. The lipid class composition of membranes containing the C20-diacetylene lipids was examined and found to be similar to membranes from cells grown on oleic acid-containing medium. (The ratio of monoglucosyl- to diglucosyldiacylglycerols was the same but the ratio of glycolipid to phosphatidylglycerol was higher in the cells grown with diacetylene fatty acids). Upon irradiation with ultraviolet light the cells and isolated biomembranes become coloured, either red or yellow depending upon their thermal history. The colour change indicates that extensive cross-linking of the lipids of the biomembranes of A. laidlawii has occurred and that a conjugated polymeric structure has been formed. Analysis of the extracted lipids from the biomembranes by GLC indicates that extensive cross-linking of the lipid chains within the biomembrane of a natural cell system has been achieved. The monoglucosyldiacylglycerols cross-link more readily that do the phosphatidylglycerol lipids. The effect of such lipid cross-linking or polymerisation on the activity at 35 degrees C of an intrinsic membrane-bound enzyme, NADH oxidase, and ribonuclease, an extrinsic membrane-bound enzyme, was studied. The NADH oxidase activity decreased rapidly upon cross-linking of the lipid environment whereas ribonuclease activity was unaffected. The potential for future studies of polymerised model and natural biomembranes is discussed.