Phosphocholine Group Research Articles

Normal and shear forces between surfaces bearing phosphocholinated polystyrene nanoparticles

Extending earlier studies where phosphocholinated brushes and phosphocholine‐exposing phosphatidylcholine lipid bilayer vesicles (PC‐liposomes) were shown to be very efficient boundary lubricants in aqueous media by virtue of the highly‐hydrated phophocholine groups, we examined interactions between surfaces bearing phosphocholinated polystyrene nanoparticles (pc‐PS‐NPs). We synthesized such particles by incorporation of alkyl chains terminated with phosphocholine groups in the PS‐NP, with the hydrophilic phosphocholines exposed at the NP surface. These were then allowed to adsorb onto mica surfaces, and the normal and shear interactions between them were examined in a surface force balance. On moderate compressions (contact pressures > 9 atm) the pc‐PS‐NPs were squeezed out, leaving a single layer between the surfaces. Shear of the surfaces revealed a large frictional dissipation, with a friction coefficient μ ≈ 0.2, in contrast to our expectations that such phosphocholinated NPs would provide highly lubricating analogues of PC‐liposomes (for which μ ≈ 10−3 – 10−4). This is attributed to a number of factors, including the inherent roughness of the NP multilayers at low compressions and the relatively low areal density of the phosphocholine groups on the PS‐NP surfaces. In particular, at higher compressions, sliding results in energy‐dissipating breaking and reforming of phosphocholine/mica (dipole/charge) bonds at the mica surfaces (and thus high friction), because of bridging of the surfaces by a single layer of pc‐PS‐NPs. Copyright © 2016 John Wiley & Sons, Ltd.

Tuning Surface Charge and Morphology for the Efficient Detection of Dopamine under the Interferences of Uric Acid, Ascorbic Acid, and Protein Adsorption.

In this research, we aimed to evaluate the impact of the surface charges and morphologies of electrodes on electrochemically detecting dopamine (DA) in the presence of protein adsorption, uric acid (UA), and ascorbic acid (AA). Through the electropolymerization of functionalized 3,4-ethylenedioxythiophenes (EDOT) directly on Au electrodes, we successfully created PEDOT-coated electrodes with three different functional groups and nanostructures. Negatively charged carboxylic acid groups attracted DA while reducing the interferences of UA and AA due to electrostatic effect. We used charge-free tetra(ethylene glycol) and zwitterionic phosphocholine groups are used to evaluate the interference of protein adsorption on DA sensing because they both can effectively prevent the nonspecific adsorption of proteins. These two electrodes can avoid protein adsorption, yet proved ineffective for DA sensing: both tetra(ethylene glycol) and the phosphocholine groups are electroneutral and have minimal electrostatic interactions with DA. We also used three proteins of different isoelectric points - bovine serum albumin, lysozyme, and fibrinogen - to evaluate the influence of protein adsorption on DA detection. We found that for an electrode coated with carboxylic acid-functionalized PEDOT, the adsorption of positively charged lysozyme can promote the detection sensitivity of AA and UA, and that all protein adsorption lowers the sensitivity of DA. In contrast, nanostructures promote the detection sensitivity of all three molecules. All of our tested functionalized PEDOT-coated electrodes demonstrated good stability and functionality in buffers.

Adsorbed film of n-tetradecylphosphocholine at the tetradecane/water interface studied by interfacial tensiometry and X-ray reflection

Abstract The adsorbed film of n-tetradecylphosphocholine (C14PC) at the tetradecane (C14)/aqueous solution (W) interface was studied by interfacial tensiometry and X-ray reflection (XR). The interfacial tension γ was measured as a function of molality m C 14 PC and temperature T under atmospheric pressure. The interfacial density of C14PC Γ C 14 PC H increases with increasing m C 14 PC and converges into 3 μmol m−2 (first saturation) at m C 14 PC ≈ 0.08 mmol kg − 1 and into 4.2 μmol m−2 (second saturation) at the critical micelle concentration (CMC). The electron density profile determined by XR indicated that C14PC molecules form monolayer with relatively loose packing at the first saturation. In the second saturation, on the other hand, the molecules form bilayer in which charge separated phosphocholine groups take upside-down arrangement to interact attractively between neighbors. The positive entropy changes associated with adsorption both from monomeric and from micellar states result from that the contribution of dehydration around C14PC molecules, especially around PC groups, by the adsorption from aqueous solution overcomes that of a comparatively ordered molecular orientation at the interface. By comparing the results at the C14/W interface with those at the Air (A)/W one, it was shown that in the monolayer and bilayer states, hydrophobic chains of C14PC molecules are more ordered at the C14/W than at the A/W interface because of attractive interaction between C14PC and C14 molecules. This leads to smaller value of the partial molar entropy and energy changes associated with the adsorption at the C14/W than at the A/W interface. Furthermore the entropy of micelle formation changes from positive to negative with increasing T, suggesting that the entropy gain due to the dehydration exceeds the entropy loss by the aggregation at low temperatures, while the gain is less than the loss at high temperature.

Recognition Functions of Pentameric C-Reactive Protein in Cardiovascular Disease

C-reactive protein (CRP) performs two recognition functions that are relevant to cardiovascular disease. First, in its native pentameric conformation, CRP recognizes molecules and cells with exposed phosphocholine (PCh) groups, such as microbial pathogens and damaged cells. PCh-containing ligand-bound CRP activates the complement system to destroy the ligand. Thus, the PCh-binding function of CRP is defensive if it occurs on foreign pathogens because it results in the killing of the pathogen via complement activation. On the other hand, the PCh-binding function of CRP is detrimental if it occurs on injured host cells because it causes more damage to the tissue via complement activation; this is how CRP worsens acute myocardial infarction and ischemia/reperfusion injury. Second, in its nonnative pentameric conformation, CRP also recognizes atherogenic low-density lipoprotein (LDL). Recent data suggest that the LDL-binding function of CRP is beneficial because it prevents formation of macrophage foam cells, attenuates inflammatory effects of LDL, inhibits LDL oxidation, and reduces proatherogenic effects of macrophages, raising the possibility that nonnative CRP may show atheroprotective effects in experimental animals. In conclusion, temporarily inhibiting the PCh-binding function of CRP along with facilitating localized presence of nonnative pentameric CRP could be a promising approach to treat atherosclerosis and myocardial infarction. There is no need to stop the biosynthesis of CRP.

Structural Basis for the Specific Recognition of Dual Receptors by the Homopolymeric Psa Fimbriae of Yersinia Pestis

The pH 6 antigen or Psa fimbriae of Yersinia pestis bind to two receptors, β1-linked galactosyl residues in glycosphingolipids and phosphocholine group in phospholipids. Despite the ubiquitous presence of either moiety on the surface of many mammalian cells, Y. pestis appears to prefer interacting with certain types of human cells such as macrophages and alveolar epithelial cells of the lung. The molecular mechanism of this apparent selectivity is not clear. Site-directed mutagenesis of the consensus choline-binding motif in the sequence of PsaA, the subunit of the Psa fimbrial homopolymer, identified residues that abolish either galactosylceramide or phosphatidylcholine binding or both. The crystal structure of the ternary complex of an in cis donor-strand complemented PsaA, galactose and phosphocholine reveals separate galactose and phosphocholine binding sites that share a common structural motif, thus suggesting potential interaction between the two sites. Mutagenesis of this shared structural motif identified Tyr126, which is part of the choline-binding consensus sequence but is found in direct contact with the galactose in the structure of PsaA, important for both receptor binding. This is the first structural resolution of a fimbrial subunit that forms a polymeric polyadhesin describing a unique arrangement of dual receptor binding sites. These findings move the field forward by providing insights into new types of multiple receptor-ligand interactions and should steer research into the synthesis of dual receptor inhibitor molecules to slow down the rapid progression of plague.

Structural basis for the specific recognition of dual receptors by the homopolymeric pH 6 antigen (Psa) fimbriae of Yersinia pestis

The pH 6 antigen (Psa) of Yersinia pestis consists of fimbriae that bind to two receptors: β1-linked galactosyl residues in glycosphingolipids and the phosphocholine group in phospholipids. Despite the ubiquitous presence of either moiety on the surface of many mammalian cells, Y. pestis appears to prefer interacting with certain types of human cells, such as macrophages and alveolar epithelial cells of the lung. The molecular mechanism of this apparent selectivity is not clear. Site-directed mutagenesis of the consensus choline-binding motif in the sequence of PsaA, the subunit of the Psa fimbrial homopolymer, identified residues that abolish galactosylceramide binding, phosphatidylcholine binding, or both. The crystal structure of PsaA in complex with both galactose and phosphocholine reveals separate receptor binding sites that share a common structural motif, thus suggesting a potential interaction between the two sites. Mutagenesis of this shared structural motif identified Tyr126, which is part of the choline-binding consensus sequence but is found in direct contact with the galactose in the structure of PsaA, important for both receptor binding. Thus, this structure depicts a fimbrial subunit that forms a polymeric adhesin with a unique arrangement of dual receptor binding sites. These findings move the field forward by providing insights into unique types of multiple receptor-ligand interactions and should steer research into the synthesis of dual receptor inhibitor molecules to slow down the rapid progression of plague.

Sensing hydration and behavior of pyrene in POPC and POPC/cholesterol bilayers: A molecular dynamics study

Molecular dynamics (MD) simulations of bilayers of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) with varying amounts of cholesterol (0, 5, 20, and 40mol%) were carried out in the absence and presence of inserted pyrene molecules. Both fluorophore and bilayer parameters were computed, for characterization of probe location and dynamics, as well as its effects on the host bilayer. In agreement with previous studies in fluid disordered bilayers, pyrene prefers to be located in the hydrophobic acyl chain region of POPC bilayers, close to the glycerol group of lipid molecules and causes ordering of the lipid acyl chains. However, incorporation of pyrene in binary POPC/cholesterol bilayers decreases the acyl chain order parameter (especially near the end of the chains), opposing the ordering effect of cholesterol. These effects are modest and mainly felt locally. Significantly, as the bilayer is enriched with cholesterol, the relative position of pyrene and the POPC carbonyl and phosphocholine groups is invariant, and the local water density around the probe decreases. This work clarifies and supports the cautious use of pyrene Ham effect to effectively measure equivalent polarity in lipid bilayers. Within the time scale of the MD simulations, which is of the magnitude of the fluorescence lifetime of pyrene, the thermally averaged polarity of lipid bilayers is nearly out of influence of spurious uncertainty in the transverse location of pyrene in the bilayers. This renders the values of equivalent polarity measurements through the pyrene Ham effect more reliable and reproducible than previously expected.

Chemical and physical requirements for lipid extraction by bovine binder of sperm BSP1

The bovine seminal plasma contains phosphocholine-binding proteins, which associate to sperm membranes upon ejaculation. These binder-of-sperm (BSP) proteins then induce a phospholipid and cholesterol efflux from these membranes. In this work, we determined physical and chemical parameters controlling this efflux by characterizing the lipid extraction induced by BSP1, the most abundant of BSP protein in bull seminal plasma, from model membranes with different composition. The model membranes were formed from binary mixtures of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) with 1-palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine (Lyso-PC), 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoserine (POPS) or cholesterol. The modulation of BSP1-induced lipid extraction from membranes by their chemical composition and their physical properties brings us to propose a 3-step extraction mechanism. First, the protein associates with membranes via specific binding to phosphocholine groups. Second, BSP1 penetrates in the membrane, essentially in the external lipid leaflet. Third, BSP1 molecules solubilize a lipid patch coming essentially from the outer lipid leaflet, without any lipid specificity, to ultimately form small lipid/protein auto-assemblies. The stoichiometry of these complexes corresponds to 10–15 lipids per protein. It is also shown that fluid-phase membranes are more prone to BSP1-induced lipid extraction than gel-phase ones. The inhibition of the lipid extraction in this case appears to be related to the inhibition of the protein penetration in the membrane (step 2) and not to the protein association with PC head groups (step 1). These findings contribute to our understanding of the mechanism by which BSP1 modify the lipid composition of sperm membranes, a key event in sperm capacitation.

Dimerization and Anion Binding of a Fluorescent Phospholipid Analogue

A diarylacetylene fluorophore featuring spatially separated urea and phosphocholine (PC) groups forms a macrocyclic "head-to-tail" dimer stabilized by NH(urea)···OP(PC) hydrogen bonds. At concentrations above ~2 × 10(-5) M in CH(2)Cl(2), the emission intensity of the dimer is quenched by HCO(3)(-) and H(2)PO(4)(-) but not by Cl(-) and NO(3)(-). Under more dilute conditions, all four anions are bound unselectively with association constants on the order of 10(5) M(-1).

An easy α-glycosylation methodology for the synthesis and stereochemistry of mycoplasma α-glycolipid antigens.

Mycoplasma fermentans possesses unique α-glycolipid antigens (GGPL-I and GGPL-III) at the cytoplasm membrane, which carry a phosphocholine group at the sugar primary (6-OH) position. This paper describes a practical synthetic pathway to a GGPL-I homologue (C16:0) and its diastereomer, in which our one-pot α-glycosylation method was effectively applied. The synthetic GGPL-I isomers were characterized with 1H NMR spectroscopy to determine the equilibrium among the three conformers (gg, gt, tg) at the acyclic glycerol moiety. The natural GGPL-I isomer was found to prefer gt (54%) and gg (39%) conformers around the lipid tail, while adopting all of the three conformers with equal probability around the sugar position. This property was very close to what we have observed with respect to the conformation of phosphatidylcholine (DPPC), suggesting that the Mycoplasma glycolipids GGPLs may constitute the cytoplasm fluid membrane together with ubiquitous phospholipids, without inducing stereochemical stress.

Reversible phosphocholination of Rab proteins byLegionella pneumophilaeffector proteins

The Legionella pneumophila protein AnkX that is injected into infected cells by a Type IV secretion system transfers a phosphocholine group from CDP-choline to a serine in the Rab1 and Rab35 GTPase Switch II regions. We show here that the consequences of phosphocholination on the interaction of Rab1/Rab35 with various partner proteins are quite distinct. Activation of phosphocholinated Rabs by GTP/GDP exchange factors (GEFs) and binding to the GDP dissociation inhibitor (GDI) are strongly inhibited, whereas deactivation by GTPase activating proteins (GAPs) and interactions with Rab-effector proteins (such as LidA and MICAL-3) are only slightly inhibited. We show that the Legionella protein lpg0696 has the ability to remove the phosphocholine group from Rab1. We present a model in which the action of AnkX occurs as an alternative to GTP/GDP exchange, stabilizing phosphocholinated Rabs in membranes in the GDP form because of loss of GDI binding ability, preventing interactions with cellular GTPase effectors, which require the GTP-bound form. Generation of the GTP form of phosphocholinated Rab proteins cannot occur due to loss of interaction with cellular GEFs.

Covalent Coercion by Legionella pneumophila

Adenylylation of Rab proteins appears to be an intriguing mechanism that Legionella pneumophila uses to modulate their activity during infection. Now the reverse reaction (deadenylylation) (Neunuebel etal., 2011; Tan and Luo, 2011) and a new posttranslational modification (phosphocholination) of Rab1 (Mukherjee etal., 2011) have been reported.

Structural characterization and MHCII-dependent immunological properties of the zwitterionic O-chain antigen of Morganella morganii

Morganella morganii is a commensal Gram-negative bacterium that has long been known to produce an antigen bearing phosphocholine groups. We determined the structure of this O-chain antigen and found that its repeating unit also contains a free amino group and a second phosphate: This alternating charge character places the M. morganii O-chain polysaccharide into a small family of zwitterionic polysaccharides (ZPSs) known to induce T-cell-dependent immune responses via presentation by class II major histocompatibility complex (MHCII) molecules. In vitro binding assays demonstrate that this O-chain interacts with MHCII in a manner that competes with binding of the prototypical ZPS antigen PSA from Bacteroides fragilis, despite its lack of a helical structure. Cellular studies also showed that the M. morganii polysaccharide induces activation of CD4(+) T-cells. Antibody binding experiments using acid hydrolyzed fragments representing the monomer and higher oligomers of the repeating unit showed that the phosphocholine group was the dominant element of the epitope with an overall affinity (K(D)) of about 5 × 10(-5) M, a typical value for an IgM anti-carbohydrate antibody but much lower than the affinity for phosphocholine itself. These data show that the structure of the M. morganii polysaccharide contains a unique zwitterionic repeating unit which allows for immune recognition by T-cells, making it the first identified T-cell-dependent O-chain antigen.

1‐O‐alkenyl‐sn‐glyceryl‐3‐phosphorylcholine may be a novel posttranslational modification used by the placenta

Neurokinin B (NKB) secreted by the placenta was previously found to be raised in preeclampsia and most of the symptoms in this disease appeared to be through progressive activation of the three neurokinin receptors. Further characterization of placental NKB revealed that placental NKB eluted in the approximate position of a 13mer and this was confirmed by TOF mass spectrometry which gave a mass of 1580. Although this is consistent with dimethylated NKB-Gly-Lys-Arg, further characterization (immunological and mass spectrometric fragment analysis) suggested a novel posttranslational modification containing phosphocholine (PC) with some evidence for glycerol and a coordinated alkene. The structure that fits all the data is that a form of platelet activating factor is attached to the aspartyl side chain at position 4 of NKB and thus now implicates placental NKB in the platelet pathology seen in preeclampsia. As it has been reported that it is the PC group per se attached to certain proteins secreted by filarial nematodes imparts them with immune inhibitory properties and thus survival in the host over long periods, attaching PC to placental secretory peptide hormones (also be found on the placental precursors of CRF, ACTH, and activin) may result in a similar situation.

Mechanisms in the serotype-independent pneumococcal immunity induced in mice by intranasal vaccination with the cell wall polysaccharide

We previously reported that cell wall polysaccharide (CWPS) given to mice intranasally with adjuvant induces serotype-independent immunity to pneumococci. Some strains make CWPS with one phosphocholine group (CWPS/1), but most express two per tetrasaccharide repeat unit (CWPS/2). Here, CWPS/1 and CWPS/2 were equally protective against colonization by CWPS/2-type pneumococci, but the related Streptococcus mitis polymer lacking phosphocholine was non-protective. Previously the protection was shown to be CD4 + T cell-dependent, abrogated by antiserum to interleukin (IL)-17A, and demonstrable in antibody-defective mice. Here, CWPS failed to protect IL-17A receptor knockout mice, further indicating IL-17A-dependence. When commercial CWPS/1 was size-fractionated preparatively, the larger exceeded the smaller molecules in their capacity to prime for IL-17A responses, and only the larger protected against pneumococcal colonization. However, a CWPS-tetanus toxoid conjugate – despite raising high titers of phosphocholine antibody – was non-protective, confirming the irrelevance of humoral immunity in this model. The results strengthen the concept that IL-17A-mediated T cell immunity is inducible by zwitterionic polysaccharides with sufficient chain length to provide coiled secondary structure. Coupling CWPS to protein, which paradoxically prevents protection, may occlude this regular linear conformation. We suggest that mucosal immunization with CWPS primes T H17 cells, which – upon contact with the phosphocholine of colonizing pneumococci – elaborate IL-17A, enhancing phagocytosis.

Importance of the phosphocholine linkage on sphingomyelin molecular properties and interactions with cholesterol; a study with phosphate oxygen modified sphingomyelin-analogues

We have characterized the molecular properties and membrane behavior of synthetically modified sphingomyelin analogues, modified on the oxygen connecting the phosphocholine group to the ceramide backbone. The oxygen was replaced with an S-atom (S-PSM), an NH-group (NH-PSM) or a CH 2-group (CH 2-PSM). Diphenylhexatriene and Laurdan anisotropy experiments showed that an S-linkage increased and NH- and CH 2-linkages decreased the stability of PSM-analogue bilayer membranes as compared to PSM. When the polarity of the interface was probed using Laurdan, S-PSM appeared to have a lower polarity as compared to PSM whereas NH-PSM and CH 2-PSM had higher polarities of their respective interfaces. Fluorescence quenching-studies with cholestatrienol showed that all compounds formed SM/cholesterol-rich domains. The S-PSM/cholesterol and PSM/cholesterol domains displayed a similar thermostability, whereas NH-PSM/cholesterol and CH 2-PSM/cholesterol domains were less thermostable. DSC on vesicles containing the PSM-analogues showed a more complex melting behavior as compared to PSM, whereas equimolar mixtures of the PSM-analogues and PSM showed almost ideal mixing with PSM for NH- and S-PSM. Our data show that the properties of the bond linking the phosphocholine head group to the 1-hydroxyl on the ceramide molecule is important for the stability of SM/SM and SM/cholesterol interactions.

Identification of a novel mammalian post-translational modification, phosphocholine, on placental secretory polypeptides

Placental neurokinin B appears to be post-translationally modified by phosphocholine (PC) attached to the aspartyl side chain at residue 4 of the mature peptide. Corticotrophin releasing factor (CRF) was found to be expressed by the rat placenta with the main secreted forms being phosphocholinated proCRF+/− one or two polysaccharide moieties. A combination of high-pressure liquid chromatography (HPLC) and two-site immunometric analysis suggested that PC was also attached to the placental precursors of adrenocorticotrophin, hemokinin, activin and follistatin. However, the fully processed forms of rat placental activin and CRF were free of PC. Formerly, the parasitic filarial nematodes have used PC as a post-translational modification, attached via the polysaccharide moiety of certain secretory glycoproteins to attenuate the host immune system allowing parasite survival, but it is the PC group itself which endows the carrier with the biological activity. The fact that treatment of proCRF peptides with phospholipase C but not endoglycosidase destroyed PC immunoreactivity suggested a simpler mode of attachment of PC to placental peptides than that used by nematodes. Thus, it is possible that by analogy the placenta uses its secreted phosphocholinated hormones to modulate the mother's immune system and help protect the placenta from rejection.

A comparative study of the effect of the antineoplastic ether lipid 1- O-octadecyl-2- O-methyl-glycero-3-phosphocholine and some homologous compounds on PKCα and PKCɛ

The effects of the anti-neoplastic ether lipid ET-18- OCH 3 and some structural homologues on the activity of protein kinase Cα (PKCα) were studied and compared with the effects the same had on the activity of PKCɛ. ET-18- OCH 3 progressively inhibited the activity of PKCα as the concentration was increased up to 30 mol% of the total lipid, above which the effect was one of activation. The experiments carried out with the homologues showed that the methoxy group bound at the sn-2 position of the glycerol of ET-18- OCH 3 is essential for both the initial inhibitory effect and the subsequent activation effect. On the other hand, variations in the type of bond linking substitutions in the sn-1 position, ether or ester, do not seem to play an important role in determining the activity of the enzyme. The effects were different on PKCɛ since ET-18- OCH 3 had a triphasic effect, activating the enzyme at low concentrations, inhibiting it at slightly higher concentrations and then activating it again at higher concentrations. In this case, when the homologues were used, it was observed that the presence of the methoxy group linked to the sn-2 position of glycerol and the type of bond linking substitutions to the sn-1 position were important for activating the enzyme, so that only homologues with ester bonds as LPC and PAPC were able to induce the initial activation step in a way similar to ET-18- OCH 3. Substitution of the phosphocholine group of ET-18- OCH 3 by phosphoserine led to a greater activation of PKCα, an effect that comes from the Ca 2+-phospholipid binding site probably because of the specific interaction of this site with the phosphoserine group. The action of ET-18- OCH 3 and its homologues, as demonstrated in this paper, may permit the selective inhibition or activation of PKCα and PKCɛ by using the most suitable range of concentrations.

Conformational response of the phosphatidylcholine headgroup to bilayer surface charge: torsion angle constraints from dipolar and quadrupolar couplings in bicelles.

The effects of bilayer surface charge on the conformation of the phosphocholine group of phosphatidylcholine were investigated using a torsion angle analysis of quadrupolar and dipolar splittings in, respectively, (2)H and (13)C NMR spectra of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) labelled in the phosphocholine group with either deuterons (POPC-alpha-d(2), POPC-beta-d(2) and POPC-gamma-d(9)) or carbon-13 (POPC-alpha-(13)C and POPC-alphabeta-(13)C(2)) and incorporated into magnetically aligned bicelles containing various amounts of either the cationic amphiphile 1,2-dimyristoyl-3-trimethylammoniumpropane (DMTAP) or the anionic amphiphile 1,2-dimyristoyl-sn-glycero-3-phosphoglycerol (DMPG). Three sets of quadrupolar splittings, one from each of the three deuteron labelling positions, and three sets of dipolar splittings ((13)C(alpha)-(31)P, (13)C(alpha)-(13)C(beta), (13)C(beta)-(14)N), were measured at each surface charge, along with the (31)P residual chemical shift anisotropy. The torsion angle analysis assumed fast anisotropic rotation of POPC about its long molecular axis, thus projecting all NMR interactions onto that director axis of motion. Dipolar, quadrupolar and chemical shift anisotropies were calculated as a function of the phosphocholine internal torsion angles by first transforming into a common reference frame affixed to the phosphocholine group prior to motional averaging about the director axis. A comparison of experiment and calculation provided the two order parameters specifying the director orientation relative to the molecule, plus the torsion angles alpha(3), alpha(4) and alpha(5). Surface charge was found to have little effect on the torsion angle alpha(5) (rotations about C(alpha)-C(beta)), but to have large and inverse effects on torsion angles alpha(3) [rotations about P-O(11)] and alpha(4) [rotations about O(11)-C(alpha)], yielding a net upwards tilt of the P-N vector in the presence of cationic surface charge, and a downwards tilt in the presence of anionic surface charge, relative to neutrality.

Synthetic Studies on Glycosphingolipids from Protostomia Phyla: Synthesis of Amphoteric Glycolipid Analogues from the Porcine Nematode, Ascaris suum.

AbstractFor Abstract see ChemInform Abstract in Full Text.