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1.
Secretory phospholipase A2 (sPLA2) hydrolyzes phosphatidylcholines (PC) within lipid bilayers to produce lyso-PC and a fatty acid, which can act as signaling molecule in biological membranes. The activity of sPLA2 depends on the membrane structure. Bilayer defects, curvature, and gel-fluid micro-heterogeneity are known to activate sPLA2. Here, we investigate if liquid-liquid immiscibility within model membranes is sufficient for sPLA2 activation. The onset of the hydrolytic activity of cobra-venom sPLA2 towards mixed monolayers of dimyristoyl-PC (DMPC)/cholesterol 2:1 (mol/mol) has been determined using infrared reflection-absorption spectroscopy (IRRAS) and polarization-modulated (PM-) IRRAS. The lag phase of sPLA2 activity increases exponentially with rising surface pressures starting at 12 mN/m. This indicates that enzyme activation is hampered at higher surface pressures. Below 12 mN/m, no lag phase is observed, and sPLA2 is efficiently activated. The surface pressure that is critical for sPLA2 activation correlates with the critical miscibility pressure according to the phase diagram of DMPC and cholesterol. Thus, coexisting, liquid-phase domains provide sufficient boundaries to activate sPLA2. Moreover, liquid-liquid immiscibility is an activating mechanism for sPLA2 that also applies to biological membranes under physiological conditions because the corresponding bilayer structure is associated with that of membrane rafts. 相似文献
2.
The reaction progress curve for the action of pig-pancreatic phospholipase A2 on dimyristoylphosphatidylcholine vesicles is characterized under a variety of conditions. The factors that regulate the rate of hydrolysis during the presteady-state phase determine the latency period. The results demonstrate that the accelerated hydrolysis following the latency phase of the reaction progress curve is due to the product-assisted binding of the enzyme to the substrate bilayer by chaning the number of bindings sites and therefore the binding equilibrium. A critical mole fraction of products appears to be formed in the substrate bilayers before the steady-state phase of hydrolysis begins. The latency phase shows a minimum at the phase-transition temperature of the substrate vesicles; however, we did not observe a significant binding of the enzyme to pure substrate bilayers even at the phase-transition temperature. The rate of binding of the enzyme is found to be fast and the rate of desorption of the bound enzyme is very slow compared to the latency phase. The rate of redistribution of products between substrate bilayers is rather slow. These observations demonstrate that during the latency phase of the action of phospholipase A2, a critical mole fraction of products is formed in the substrate bilayer. 相似文献
3.
W. Richard Burack Andrew R. G. Dibble Rodney L. Biltonen 《Chemistry and physics of lipids》1997,90(1-2):87-95
The action of phospholipase A2 (PLA2) on bilayer substrates causes the accumulation of reaction products, lyso-phospholipid and fatty acid. These reaction products and the phospholipid substrate generate compositional heterogeneities and then apparently phase separate when a critical mole fraction of reaction product accumulates in the membrane. This putative phase separation drives an abrupt morphologic rearrangement of the vesicle, which may be in turn responsible for modulating the activity of PLA2. Here we examine the thermotropic properties of the phase-separated lipid system formed upon hydrating colyophilized reaction products (1:1 palmitic acid:1-palmitoyl-2-lyso-phosphatidylcholine) and substrate, dipalmitoylphosphatidylcholine. The mixture forms structures which are not canonical spherical vesicles and appear to be disks in the gel-state. The main gel-liquid transition of these structures is hysteretic. This hysteresis is apparent using several techniques, each selected for its sensitivity to different aspects of a lipid aggregate's structure. The thermotropic hysteresis reflects the coupling between phase separation and changes in vesicle morphology. 相似文献
4.
Katsuhiko Hamaguchi Hiroshi Kuwata Kumiko Yoshihara Seiko Masuda Satoko Shimbara Sachiko Oh-ishi Makoto Murakami Ichiro Kudo 《Biochimica et Biophysica Acta (BBA)/Molecular and Cell Biology of Lipids》2003,1635(1):37-47
Although the expression of the prototypic secretory phospholipase A2 (sPLA2), group IIA (sPLA2-IIA), is known to be up-regulated during inflammation, it remains uncertain if other sPLA2 enzymes display similar or distinct profiles of induction under pathological conditions. In this study, we investigated the expression of several sPLA2s in rodent inflammation models. In lipopolysaccharide (LPS)-treated mice, the expression of sPLA2-V, and to a lesser extent that of sPLA2-IID, -IIE, and -IIF, were increased, whereas that of sPLA2-X was rather constant, in distinct tissues. 12-O-Tetradecanoylphorbol-13-acetate (TPA)-induced mouse ear edema, in which the expression of sPLA2-IID, -IIF and -V was increased, was significantly reduced by YM-26734, a competitive sPLA2-IIA inhibitor that turned out to inhibit sPLA2-IID, -IIE, -V and -X as well. In contrast, sPLA2-IIA was dominant in carageenin-induced pleurisy in rats, where the accumulation of exudate fluids and leukocytes was significantly ameliorated by YM-26734. These results indicate that distinct sPLA2s can participate in inflammatory diseases according to tissues, animal species, and types of inflammation. 相似文献
5.
Roger Sundler 《Chemistry and physics of lipids》1984,34(2):153-161
A reversible, temperature-dependent change in phospholipid packing occurring between 0°C and 12°C has been identified in microsomal membranes by the use of phospholipase A2 from Crotalus atrox. It manifests itself as a drastic increase in susceptibility to the phospholipase and depends on non-lipid (presumably protein) membrane components. It is suggested that this change could underlie the change in transmembrane mobility of phospholipids which occurs in the same temperature range. 相似文献
6.
Freeze-fracture electron microscopy was used to follow morphological changes induced by Naja mossambica mossambica venom V4II cardiotoxin in rod outer segment membrane preparations. The extent of the morphological changes depended on the purity of the cardiotoxin. Pure cardiotoxin had no detectable effect upon the preparation, but, when contaminated with venom phospholipase A2, let to a rapid disintegration of the membrane vesicles. With trace amounts (up to about 0.5% of the cardiotoxin) of phospholipase A2, the membrane vesicles disintegrated into smooth lamellae and particles in solution. These two components were separated by centrifugation. The pellet, which showed the presence of smooth lamellae and aggregated particles, was composed of unbleached rhodopsin, initial membrane lipids, lysolipids and cardiotoxin. The supernatant, which showed only the presence of dispersed particles, was composed of unbleached rhodopsin, lysolipids and cardiotoxin. With cardiotoxin containing larger amounts of phospholipase A2 (more than 0.5% of the cardiotoxin), membrane vesicles were disintegrated into large aggregates of amorphous material, composed of bleached rhodopsin, initial membrane lipids, lysolipids and cardiotoxin. These results confirm our previous observation on the release of integral membrane proteins from membrane vesicles by the action of cardiotoxin containing traces of phospholipase A2 (Gulik-Krzywicki, T., Balerna, M., Vincent, J.P. and Lazdunski, M. (1981) Biochim. Biophys. Acta 643, 101–114) and suggest its possible use for isolation and purification of integral membrane proteins. 相似文献
7.
Akihiko Saiga Yasuhide Morioka Takashi Ono Kazumi Nakano Yoshikazu Ishimoto Hitoshi Arita Kohji Hanasaki 《Biochimica et Biophysica Acta (BBA)/Molecular and Cell Biology of Lipids》2001,1530(1):67-76
We have previously shown the expression of group X secretory phospholipase A2 (sPLA2-X) in mouse splenic macrophages and its powerful potency for releasing fatty acids from various intact cell membranes. Here, we examined the potency of sPLA2-X in the production of lipid mediators in murine peritoneal macrophages. Mouse sPLA2-X was found to induce a marked release of fatty acids including arachidonic acid and linoleic acid, which contrasted with little, if any, release by the action of group IB and IIA sPLA2s. In resting macrophages, sPLA2-X elicited a modest production of prostaglandin E2 and thromboxane A2. After the induction of cyclooxygenase-2 (COX-2) by pretreatment with lipopolysaccharide, a dramatic increase in the production of these eicosanoids was observed in sPLA2-X-treated macrophages, which was completely blocked by the addition of either the specific sPLA2 inhibitor indoxam or the COX inhibitor indomethacin. In accordance with its higher hydrolyzing activity toward phosphatidylcholine, mouse sPLA2-X induced a potent production of lysophosphatidylcholine. These findings strongly suggest that sPLA2-X plays a critical role in the production of various lipid mediators from macrophages. These events might be relevant to the progression of various pathological states, including chronic inflammation and atherosclerosis. 相似文献
8.
Andrej Grkovich Aaron Armando Oswald Quehenberger Edward A. Dennis 《Biochimica et Biophysica Acta (BBA)/Molecular and Cell Biology of Lipids》2009,1791(10):975-982
Group IVA phospholipase A2 (GIVA PLA2) catalyzes the release of arachidonic acid (AA) from the sn-2 position of glycerophospholipids. AA is then further metabolized into terminal signaling molecules including numerous prostaglandins. We have now demonstrated the involvement of phosphatidic acid phosphohydrolase 1 (PAP-1) and protein kinase C (PKC) in the Toll-like receptor-4 (TLR-4) activation of GIVA PLA2. We also studied the effect of PAP-1 and PKC on Ca+ 2 induced and synergy enhanced GIVA PLA2 activation. We observed that the AA release induced by exposure of RAW 264.7 macrophages to the TLR-4 specific agonist Kdo2-Lipid A is blocked by the PAP-1 inhibitors bromoenol lactone (BEL) and propranolol as well as the PKC inhibitor Ro 31-8220; however these inhibitors did not reduce AA release stimulated by Ca+ 2 influx induced by the P2X7 purinergic receptor agonist ATP. Additionally, stimulation of cells with diacylglycerol (DAG), the product of PAP-1 mediated hydrolysis, initiated AA release from unstimulated cells as well as restored normal AA release from cells treated with PAP-1 inhibitors. Finally, neither PAP-1 nor PKC inhibition reduced GIVA PLA2 synergistic activation by stimulation with Kdo2-Lipid A and ATP. 相似文献
9.
P.I Castagnet M.E Roque S.J Pasquaré N.M Giusto 《Comparative biochemistry and physiology. Part B, Biochemistry & molecular biology》1998,120(4):683-691
The activities of enzymes involved in lipid metabolism—phospholipase A2 (PLA2) and phosphatidylethanolamine N-methyltransferase (PE N-MTase)—were found to be differently affected by pre-incubation of rod outer segments (ROS) under protein phosphorylating or dephosphorylating conditions. Exposure to cAMP-dependent protein kinase (PKA), under dark or light conditions, produced a significant increase in PE N-MTase activity, whereas PLA2 activity decreased. Under standard protein kinase C (PKC) phosphorylating conditions in light, PE N-MTase activity was stimulated and PLA2 activity was not affected. When the assays were performed in the dark, both enzymatic activities were unaffected when compared to the corresponding controls. Incubation of ROS membranes in light in the presence of PKC activators phorbol 12,13-dibutyrate (PDBu) and dioctanoylglycerol (DOG) resulted in the same pattern of changes in enzyme activities as described for standard PKC phosphorylating condition. Pre-incubation of membranes with the PKC inhibitor H-7 reduced the stimulation of PDBu on PE N-MTase activity, and had no effect on PLA2 activity in ROS membranes incubated with the phorbol ester. Pre-treatment of isolated ROS with alkaline phosphatase resulted in decreased PE N-MTase activity and produced a significant stimulation of PLA2 activity under dark as well as under light conditions when compared to the corresponding controls. These findings suggest that ROS protein phosphorylation and dephosphorylation modulates PE N-MTase and PLA2 activities in isolated ROS, and that these activities are independently and specifically modulated by particular kinases. Furthermore, dephosphorylation of ROS proteins has the opposite effect to that produced by protein phosphorylation on the enzymes studied. 相似文献
10.
Role of phospholipase A2 and prostaglandin E in growth and differentiation of myeloid leukemic cells
Rabi Simantov Leo Sachs 《Biochimica et Biophysica Acta (BBA)/Molecular Cell Research》1982,720(2):111-119
Phospholipase A2 activity and prostaglandin E synthesis have been studied in different clones of myeloid leukemic cells, which differ in their competence to be induced to differentiate by the macrophage and granulocyte differentiation-inducing protein or the tumor promoter 12-O-tetradecanoyl phorbol-13-acetate (TPA). Clones that could be induced to differentiate by this protein showed a higher basal phospholipase A2 activity than clones that could not be induced to differentiate by this protein inducer. Cell competence to be induced to differentiate by TPA did not show this correlation, and the clone with the least ability to respond to TPA showed the lowest number of binding sites for [20-3H]phorbol 12,13-dibutyrate. Differentiation induced by the protein was accompanied by a 7–14-fold increase in prostaglandin E synthesis, whereas differentiation induced by TPA did not show this increase. Externally added prostaglandin E1 did not induce differentiation but inhibited cell proliferation and the degree of inhibition in the different clones was related to the basal phospholipase A2 activity. The results indicate that increase of prostaglandin E synthesis was not an essential pre-requisite for differentiation, that prostaglandin E seems to be involved in the inhibition of cell proliferation in association with phospholipase A2, and that the differentiation-inducing protein and TPA can induce differentiation by different pathways. The amount of basal phospholipase A2 activity was also related to previously found differences in the ability of the clones to develop desensitization to β-adrenergic hormones or prostaglandin E1. 相似文献
11.
Najeh Krayem Goetz Parsiegla Hélène Gaussier Hanen Louati Raida Jallouli Pascal Mansuelle Frédéric Carrière Youssef Gargouri 《Biochimica et Biophysica Acta (BBA)/General Subjects》2018,1862(5):1247-1261
Background
Heterodimeric phospholipase A2 from venom glands of Tunisian scorpion Scorpio maurus (Sm-PLGV) had been purified. It contains long and short chains linked by a disulfide bridge. Sm-PLGV exhibits hemolytic activity towards human erythrocytes and interacts with phospholipid monolayers at high surface pressure. The investigation of structure-function relationships should provide new clues to understand its activity.Methods
Molecular cloning of Sm-PLGV and heterologous expression in Escherichia coli of three recombinant forms was used to determine the role of the short chain on enzymatic activity. Infrared spectroscopy assisted 3D model building of the three recombinant constructs (phospholipases with and without the penta-peptide and Long chain only) allowed us to propose an explanation of the differences in specific activities and their interaction with various phospholipids.Results
Nucleotide sequence of Sm-PLGV encodes 129 residues corresponding to the Long chain, the penta-peptide and the short chain. Although recombinant phospholipases without and with the penta-peptide have different specific activities, they display a similar substrate specificity on various phospholipid monolayers and similar bell-shaped activity profiles with maxima at high surface pressure. The absence of the short chain reduces significantly enzymatic and hemolytic activities. The 3D models pointed to an interaction of the short chain with the catalytic residues, what might explain the difference in activities of our constructs.Conclusion
Infrared spectroscopy data and 3D modeling confirm the experimental findings that highlight the importance of the short chain for the Sm-PLGV activity.General significance
New informations are given to further establish the structure-function relationships of the Sm-PLGV. 相似文献12.
Comparative studies were carried out on the Ca2+-transport systems of mitochondria and sarcoplasmic reticulum from longissimus dorsi muscle of genetically selected malignant hyperthermia-prone and normal pigs in order to identify the biochemical lesion responsible for the enhanced release of Ca2+ in the sarcoplasm occurring in porcine malignant hyperthermia. Mitochondria isolated from longissimus dorsi muscle of malignant hyperthermia-prone pigs contained a significantly (P < 0.001) higher amount of endogenous long-chain fatty acids. Similar amounts of endogenous mitochondrial phospholipase A2 were observed in both types of pigs, but the total activity in malignant hyperthermia-prone pigs was at least twice that of normal. Spermine, a phospholipase A2 inhibitor, lowered the activity in both types of mitochondria to a similar final level. Mitochondria of malignant hyperthermia-prone pigs showed a significantly (P < 0.001) higher oligomycin-insensitive (Ca2+ + Mg2+)-ATPase activity, but the Mg2+-ATPase and the (Ca2+ + Mg2+)-ATPase activities were similar in both types of pigs. Sarcoplasmic reticulum isolated from longissimus dorsi muscle of malignant hyperthermia-prone pigs showed a significantly higher (Ca2+ + Mg2+)-ATPase activity and a lower rate of Ca2+ uptake; the maximal amount and the rate of Ca2+ uptake by sarcoplasmic reticulum of malignant hyperthermia-prone pigs were half that of normal. Mitochondria from longissimus dorsi muscle of malignant hyperthermia-prone pigs inhibited the Ca2+-transport system of the sarcoplasmic reticulum of longissimus dorsi from both normal and malignant hyperthermia-prone pigs, but mitochondria from normal pigs had no influence on the sarcoplasmic reticulum from either type. Experimental evidence favours the concept that long-chain fatty acids released from skeletal muscle mitochondria by endogenous mitochondrial phospholipase A2 are responsible for the enhanced release of Ca2+ from mitochondria (Cheah, K.S. and Cheah, A.M. (1981) Biochim. Biophys. Acta 634, 70–84), and also additional release of Ca2+ from sarcoplasmic reticulum into the sarcoplasm during porcine malignant hyperthermia syndrome. 相似文献
13.
The interactions of phospholipids with four different cholesterol derivatives substituted with one OH or one keto group at position C20 or C22 of the side-chain were studied. The derivatives were the 22,R-hydroxy; 22,S-hydroxy; 22-keto- and 20,S-hydroxycholesterol. Two aspects of the interactions were investigated: (1) the effect of the cholesterol derivatives on the gel → liquid crystalline phase transition of dipalmitoylphosphatidylcholine (DPPC) and of dielaidoylphosphatidylethanolamine (DEPE) monitored by differential scanning calorimetry and (2) The effect on the lamellar → hexagonal HII phase transition of DEPE monitored by DSC and by 31P-NMR to determine structural changes. The gel → liquid crystalline phase transition was affected by the cholesterol derivatives to a much larger extent in the case of DPPC than of DEPE. In both cases, there was a differential effect of the four derivatives, the 22,R-hydroxycholesterol being the less effective. In DPPC-sterol 1:1 systems, 22,R-hydroxycholesterol does not suppress the melting transition, the ΔH values becomes 7.1 kcal · mol?1 as compared to 8.2 kcal · mol?1 for the pure lipid. 22,S-OH cholesterol has a much stronger effect (ΔH = 3.1 kcal · mol?1) and 22-ketocholesterol suppresses the transition completely. In DEPE mixtures of all these compounds, the melting transition of the phospholipid is still observable. The transition temperature was shifted to lower values (?13.5°C in the presence of 20,S-OH cholesterol). The ΔH of the transition was lowered by these compounds except in DEPE-22,R-OH cholesterol mixtures and the cooperativity of the transition (reflected by the width at half peak height) was reduced. The lamellar → hexagonal HII phase transition was also affected by the presence of these cholesterol derivatives. The transition temperature value was depressed with all these compounds. 20,S-OH cholesterol was the most effective followed by 22,R-OH cholesterol. The ΔH of the transition was not strongly affected. The molecular interfacial properties of these derivatives were studied by the monomolecular film technique. It is most likely that 22,R-OH cholesterol due to the hydroxyl groups at the 3β- and 22,R-positions orients with the sterol nucleus lying flat at the air/water interface, since the compression isotherm of either the pure sterol or the DOPC-sterol mixture (molar ration, 1:1) monomolecular film exhibits a transition at approx. 103 Å2, corresponding to the area of revolution of the sterol nucleus. This remarkable property, due probably to the existence of a kink between the side-chain and the long axis of the steroid nucleus, might explain the smaller effect of this sterol on the melting transition of either PC or PE systems. 相似文献
14.
Phospholipases A2 have been shown to be activated in a concentration dependent manner by a number of antimicrobial peptides, including melittin, magainin 2, indolicidin, and temporins B and L. Here we used fluorescently labelled bee venom PLA2 (PLA2D) and the saturated phospholipid substrate 1,2-dipalmitoyl-glycero-sn-3-phosphocholine (L-DPPC), exhibiting a lag-burst behaviour upon the initiation of the hydrolytic reaction by PLA2. Increasing concentrations of Cys-temporin B and its fluorescent Texas red derivative (TRC-temB) caused progressive shortening of the lag period. TRC-temB/PLA2D interaction was observed by Förster resonance energy transfer (FRET), with maximum efficiency coinciding with the burst in hydrolysis. Subsequently, supramolecular structures became visible by microscopy, revealing amyloid-like fibrils composed of both the activating peptide and PLA2. Reaction products, palmitic acid and 1-palmitoyl-2-lyso-glycero-sn-3-phosphocholine (lysoPC, both at > 8 mol%) were required for FRET when using the non-hydrolysable substrate enantiomer 2,3-dipalmitoyl-glycero-sn-1-phosphocholine (D-DPPC). A novel mechanism of PLA2 activation by co-fibril formation and associated conformational changes is suggested. 相似文献
15.
J.D. Líbano-Soares 《生物化学与生物物理学报:生物膜》2008,1778(5):1316-1323
In a previous paper we showed that bradykinin (BK), interacting with its B2 receptor, inhibits proximal tubule Na+-ATPase activity but does not change (Na+ + K+)ATPase activity. The aim of this paper was to investigate the molecular mechanisms involved in B2-mediated modulation of proximal tubule Na+-ATPase by BK. To abolish B1 receptor-mediated effects, all experiments were carried out in the presence of (Arg-Pro-Pro-Gly-Phe-Ser-Pro-Leu), des-Arg9-[Leu8]-BK (DALBK), a specific antagonist of B1 receptor. A dual effect on the Na+-ATPase activity through the B2 receptor was found: short incubation times (1-10 min) stimulate the enzyme activity; long incubation times (10-60 min) inhibit it. The stimulatory effect of BK is mediated by activation of phosphoinositide-specific phospholipase C β (PI-PLCβ)/protein kinase C (PKC); its inhibitory action is mediated by Ca2+-independent phospholipase A2 (iPLA2). Prior activation of the PI-PLCβ/PKC pathway is required to activate the iPLA2-mediated inhibitory phase. These results reveal a new mechanism by which BK can modulate renal sodium excretion: coupling between B2 receptor and activation of membrane-associated iPLA2. 相似文献
16.
Thylakoid membranes were treated with either pancreatic or snake venom phospholipase A2, and the residual phospholipid content of these membranes was determined and compared to the rates of Photosystem II and/or Photosystem I electron transports. The hydrolysis curves of both phosphatidylglycerol and phosphatidylcholine displayed a first, rapid phase which was almost temperature-insensitive, followed by a second, slower phase which depended strongly on the temperature. When pancreatic phospholipase A2 had access either to the outer face or to both faces of the thylakoid membrane, either only part of or all the phospholipids, respectively, could be hydrolysed. These results were interpreted as indicating an asymmetric distribution of phospholipids across the thylakoid membrane, phosphatidylglycerol and phosphatidylcholine being preferentially located in the outer and the inner layer, respectively. When acting on uncoupled thylakoid membranes, phospholipase A2 exerted an inhibitory effect on Photosystem II activity and a stimulatory effect on Photosystem I activity. The involvement of phosphatidylcholine and of phosphatidylglycerol in electron transport activities of Photosystem II and of Photosystem I are discussed with special reference to the role of the external and internal pools of these phospholipids. 相似文献
17.
H.A van der Helm P Buijtenhuijs H van den Bosch 《Biochimica et Biophysica Acta (BBA)/Molecular and Cell Biology of Lipids》2001,1530(1):86-96
Mesangial cells can be induced to express group IIA and group V secretory phospholipase A2 (sPLA2) at the mRNA level and at the protein level. In this report we quantitatively analyze the expression of both proteins in stimulated cells by Western blot techniques. We found that 75–80% of the total amount of synthesized group IIA sPLA2 was secreted. The synthesized group V sPLA2, however, was present almost exclusively intracellularly. The amount of group V present in the cell was comparable to the intracellular amount of group IIA sPLA2. We furthermore studied the localization and routing of both proteins. Using fusion proteins of the group IIA or group V pre-sPLA2 with green fluorescent protein it was established that both presequences are able to direct the proteins to the Golgi system. In immunofluorescence studies group V sPLA2 expressed by rat mesangial cells was located in a punctate pattern in the cytosol with an enrichment near the nucleus. Immunofluorescent confocal laser scanning microscopy revealed that the group V and IIA sPLA2 show partial colocalization in a Golgi-like structure in the inner part in the cell, but no colocalization was seen in the vesicles in the cytoplasm. The images also showed that group IIA sPLA2 was located throughout the cell while group V was mainly present in the inner part of the cell. After treatment of the cells with brefeldin A or monensin the group IIA enzyme could no longer be detected, while group V sPLA2 was still present although its localization was somewhat dependent on the treatment. Collectively, these results indicate that the two enzymes differ in both localization and routing in the cell, which underscores the hypothesis that the enzymes might have different functions. 相似文献
18.
Encapsulation of [Mn(bpy)3] cations in 2D [Mn(dca)3] sheet: Synthesis, X-ray structure and EPR study
The reaction of Mn(NO3)2 · 4H2O, 2,2′-bipyridine (bpy) and sodium dicyanamide (dca) in aqueous medium yielded the {[Mn(bpy)3][Mn(dca)3]2}n (1). The single-crystal X-ray analysis of 1 revealed that the anionic part of the complex, [Mn(dca)3]−, features infinite 2D sheets with a honeycomb-like porous structure having a void space of ca. 12 Å in which [Mn(bpy)3]2+ cations are encapsulated to yield a fascinating molecular assembly. MnII ions possess an octahedral geometry both in the anionic and cationic components of complex 1. In the anionic component, each MnII ion is bridged by three pairs of dicyanamide anions in an end-to-end fashion with two other MnII ions from adjacent [Mn(dca)3]− moieties. This type of linking propagates parallel to the bc crystallographic plane to form 2D sheets. [Mn(bpy)3]2+ is found to have somewhat “squeezed” upon encapsulation. No measurable magnetic interaction was evidenced through variable temperature magnetic susceptibility measurements. However, in addition to the broad g ≈ 2 resonance typical of magnetically diluted [Mn(bpy)3]2+ cations, EPR spectroscopy evidenced exchange narrowing of the [Mn(dca)3]− resonance at g ≈ 2 thus indicating operation of weak magnetic interactions extended over the whole 2D network through the dca− bridges. 相似文献
19.
Reaction of O,O′-diisopropylphosphoric acid isothiocyanate (iPrO)2P(O)NCS with 2- or 3-aminopyridine leads to the new N-phosphorylated thioureas RNHC(S)NHP(O)(OiPr)2 (R = 2-Py, HLI; 3-Py, HLII). Reaction of the potassium salt KLI with Ni(II) in aqueous EtOH leads to the new complex Ni[2-PyNHC(S)NP(O)(OiPr)2-N(Py),N(P),O]2, where the metal cation is coordinated by two deprotonated ligands LI through the pyridine and phosphorylamide nitrogen atoms, and the PO oxygen atoms. Using KLII leads to an oligomeric (or polymeric) structure, where the Ni(II) cation is coordinated by two anionic ligands LII through the CS sulfur atoms and the P-N nitrogen atoms, and the pyridine nitrogen atoms of neighboring molecules. The new compounds were investigated by 1H and 31P{1H} NMR spectroscopy, and microanalysis. Single crystal X-ray diffraction studies showed HLI forms both intra- and intermolecular hydrogen bonds, which in turn lead to the formation of a polymeric chain. Moreover, π?π stacking interactions were observed between molecules of two neighboring chains. 相似文献
20.
Mono- and biphasic kinetic effects of bile salts on the pancreatic IB phospholipase A2 (PLA2) catalyzed interfacial hydrolysis are characterized. This novel phenomenon is modeled as allosteric action of bile salts with PLA2 at the interface. The results and controls also show that these kinetic effects are not due to surface dilution or solubilization or disruption of the bilayer interface where in the mixed-micelles substrate replenishment becomes the rate-limiting step. The PLA2-catalyzed rate of hydrolysis of zwitterionic dimyristoylphosphatidylcholine (DMPC) vesicles depends on the concentration and structure of the bile salt. The sigmoidal rate increase with cholate saturates at 0.06 mole fraction and changes little at the higher mole fractions. Also, with the rate-lowering bile salts (B), such as taurochenodeoxycholate (TCDOC), the initial sigmoidal rate increase at lower mole fraction is followed by nearly complete reversal to the rate at the pre-activation level at higher mole fractions. The rate-lowering effect of TCDOC is not observed with the (62-66)-loop deleted ΔPLA2, or with the Naja venom PLA2 that is evolutionarily devoid of the loop. The rate increase is modeled with the assumption that the binding of PLA2 to DMPC interface is cooperatively promoted by bile salt followed by allosteric kcat?-activation of the bound enzyme by the anionic interface. The rate-lowering effect of bile salts is attributed to the formation of a specific catalytically inert E?B complex in the interface, which is noticeably different than the 1:1 EB complex in the aqueous phase. The cholate-activated rate of hydrolysis is lowered by hypolidemic ezetimibe and guggul extract which are not interfacial competitive inhibitors of PLA2. We propose that the biphasic modulation of the pancreatic PLA2 activity by bile salts regulates gastrointestinal fat metabolism and cholesterol homeostasis. 相似文献