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At least 757 records · Page 42Linked to original sources
PubMed1987

Properties of miniature postsynaptic currents during depolarization-induced release at a cholinergic neuroneuronal synapse.

1. Miniature postsynaptic currents were analyzed at an inhibitory cholinergic neuroneuronal synapse in the buccal ganglion of Aplysia. Under double voltage-clamp, it was possible to induce postsynaptic currents by long-duration depolarizations of the presynaptic neuron and to analyze these as the linear summation of individual miniature postsynaptic currents (MPSCs). The amplitude of these miniature currents (imin) was calculated from the ratio of the variance of the noise (E2) to the mean of the postsynaptic current (Im), according to Campbell's theorem, with imin = 2E2/Im. Their decay time (tau min) was obtained from the cutoff frequencies of the power spectra obtained from the noise. 2. Neither the conductance nor the decay time of MPSCs was voltage dependent. However, imin appeared to decrease when the quantal content of the response increased. Meanwhile, tau min increased slightly with Imin. 3. Carbamylcholine was injected into the neuropile and this led to a decrease in imin and a slight increase in tau min. 4. Power spectra obtained after the application of inhibitors of acetylcholinesterase (AChE), with or without curare, suggested that acetylcholine (ACh) does not accumulate during large depolarizations. 5. The possible origin of the nonlinear relationship between the variance and the mean of the postsynaptic currents is discussed.

Acetylcholine↗
PubMed1988

Acid-base chemistry of vitamin B6 compounds in methanol.

The enzymatic reactions involving pyridoxal 5'-phosphate (PLP) can be simulated in solutions; thus, this system forms a favorable model for understanding the requirements of the enzymatic catalysis. We have studied in methanol protonic equilibria of the imines formed between PLP or salicylaldehyde (SA) and various amino acids, using UV and NMR spectroscopy. A glass electrode and an operational pH* scale were used to control acidity. The first protonation of the phosphate of PLP imines can be detected by UV spectroscopy with pK* at 10.8, proved by [31P]-NMR. The second protonation of phosphate (pK* at 4.8) is accompanied by increased hydrolysis of the imines. The imines of aspartate deviate from the imines of nondicarboxylic amino acids indicating that the beta-carboxyl of aspartate is internally hydrogen-bonded. PLP-2-aminobutanol Schiff base does not show with [1H]-NMR at pH* 7 separate peaks for ketoenamine-enolimine tautomers even at -90 degrees C, SA-phenylalanine shows an unidentified absorption at 350-380 nm. This was tentatively assigned a trans structure.

Acid-Base Equilibrium↗
PubMed2002

Metal binding by melanins: studies of colloidal dihydroxyindole-melanin, and its complexation by Cu(II) and Zn(II) ions.

Melanins are colloidal pigments known to have a high affinity for metal ions. In this work, the nature of the metal-binding sites are determined and the binding affinities are quantified. Initial potentiometric titrations have been performed on synthetic dihydroxyindole (DHI) melanin solutions to determine the chemical speciation of quinole/quinone subunits. Two types of acidic functionalities are assignable: catechol groups, with pK(a) between 9 and 13, and quinone imines (QI), with pK(a) of 6.3. The presence of the quinone-imine tautomer has, to our knowledge, never been assessed in polymeric melanins. Melanin solutions obtained from N-methylated DHI lack the pK(a) 6.3 buffer, consistent with its inability to form the quinone-imine tautomer. EPR spectroscopy of the DHI-melanin samples demonstrates that the semiquinone radical is in too low a concentration to contribute to the bulk binding of metals. Changes in the titration curves after addition of Cu(II) and Zn(II) ions were analyzed to obtain the binding constants and stoichiometry of the metal-melanin complexes, using the BEST7 program. UV-Vis spectra at neutral and high pH are used to identify absorbances due to Cu-bound quinone imine and catechol groups. The derived binding constants were used to determine speciation of the Cu(II) and Zn(II) ions coordinated to the quinone imine and catechol groups at various pH. The mixed complexes, Zn(QI)(Cat)(-) and Cu(QI)(Cat)(-) are shown to dominate at physiological pH.

Colloids↗
PubMed2002

Reversible polymerization driven by folding.

Bisfunctionalized m-phenylene ethynylene imine oligomers were polymerized in the polar solvent acetonitrile, resulting in high-molecular weight poly(m-phenylene ethynylene imine)s. It is hypothesized that this polymerization, which proceeds through the reversible metathesis of imine bonds, is driven by the folding of the long m-phenylene ethynylene imine chains. Upon conducting the polymerization in a series of solvents in which the m-phenylene ethynylene oligomers exhibit different folding stabilities, it was possible to correlate the molecular weight of the resulting poly(m-phenylene ethynylene imine)s with the helical stability of the corresponding oligomers. The polymerization was also demonstrated to be reversible and responsive to solvent and temperature changes.

Journal Article↗
PubMed1997

X-ray Crystal Structures and ab Initio Calculations on the Photochemically Formed Dewar Isomers of the 4(3H)-Pyrimidinone Derivatives.

The structures of the 5-oxo-2,6-diazabicyclo[2.2.0]hex-2-enes (imine Dewar pyrimidinones) 2a-h have been studied by X-ray diffraction analysis and ab initio calculations at the HF/6-31G(d,p) and MP2/6-31G(d,p) levels of theory. The crystal structures of 1-alkyl-3-tert-butyl-6-methyl imine Dewar pyrimidinones 2a-c have been determined at low temperature. The X-ray diffraction studies revealed that both the 2-azetidinone and dihydroazete rings are almost planar, and their eight bond angles are nearly 90 degrees (81-100 degrees ). The C1-N2(=C3) bond distance in the dihydroazete ring is longer due to the bond angle strain by ca. 0.06 Å than that of a typical straight-chain imine molecule. Full geometry optimizations on the imine Dewar isomers (2a and 2d-h) show deviations between the HF and MP2 geometries. The MP2 structure of the 3-tert-butyl-1,6-dimethyl imine Dewar pyrimidinone 2a is compared with those of the X-ray diffraction. The results of the calculations are found to be in good agreement with the X-ray diffraction data. The full geometry optimizations are necessary with the inclusion of electron correlation for the highly strained molecules. The ab initio calculations of the 2-azetidinone 9, 3,4-dihydroazete 11, and N-ethylidenemethylamine 12 have been carried out at the HF and MP2/6-31G(d,p) levels of theory to compare with the structures, orbital hybridization, bond orders, and charge distributions of the Dewar pyrimidinones 2. The theoretical results reveal that the MP2 C1-N2 bond distance of the Dewar pyrimidinone 2a is consistent with the abnormally elongated X-ray C1-N2 bond distance. The electronegative N2 and N6 atoms in the Dewar pyrimidinones 2 give the great positive charge on the C1 atom. The smaller bond angles (ca. 90 degrees ) in the Dewars 2 than typical sp(2) and sp(3) bond angles (ca. 120 and 109 degrees ) increase p character in the endocyclic bonds and decrease p character in the exocyclic bonds.

Journal Article↗
PubMed2005

Functional properties of endogenous receptor- and store-operated calcium influx channels in HEK293 cells.

Activation of phospholipase C (PLC)-mediated signaling pathways in non-excitable cells causes the release of calcium (Ca2+) from inositol 1,4,5-trisphosphate (InsP3)-sensitive intracellular Ca2+ stores and activation of Ca2+ influx via plasma membrane Ca2+ channels. The properties and molecular identity of plasma membrane Ca2+ influx channels in non-excitable cells is a focus of intense investigation. In the previous studies we used patch clamp electrophysiology to describe the properties of Ca2+ influx channels in human carcinoma A431 cell lines. Now we extend our studies to human embryonic kidney HEK293 cells. By using a combination of Ca2+ imaging and whole cell and single channel patch clamp recordings we discovered that: 1) HEK293 cells contain four types of plasma membrane Ca2+ influx channels: I(CRAC), Imin, Imax, and I(NS); 2) I(CRAC) channels are highly Ca2+-selective (P(Ca/Cs)>1000) and I(CRAC) single channel conductance is too small for single channel analysis; 3) Imin channels in HEK293 cells display functional properties identical to Imin channels in A431 cells, with single channel conductance of 1.2 pS for divalent cations, 10 pS for monovalent cations, and divalent cation selectivity P(Ba/K)=20; 4) Imin channels in HEK293 cells are activated by InsP3 and inhibited by phosphatidylinositol 4,5-bisphosphate, but store-independent; 5) when compared with Imin, Imax channels have higher conductance for divalent (17 pS) and monovalent (33 pS) cations, but less selective for divalent cations (P(Ba/K)=4), 6) Imax channels in HEK293 cells can be activated by InsP3 or by Ca2+ store depletion; 7) I(NS) channels are non-selective (P(Ba/K)=0.4) and display a single channel conductance of 5 pS; and 8) I(NS) channels are not gated by InsP3 but activated by depletion of intracellular Ca2+ stores. Our findings provide novel information about endogenous Ca2+ channels supporting receptor-operated and store-operated Ca2+ influx pathways in HEK293 cells.

Calcium↗
PubMed2004

The biosynthesis of the thiazole phosphate moiety of thiamin (vitamin B1): the early steps catalyzed by thiazole synthase.

Thiazole synthase (ThiG) catalyzes an Amadori-type rearrangement of 1-deoxy-d-xylulose-5-phosphate (DXP) via an imine intermediate. In support of this, we have demonstrated enzyme-catalyzed exchange of the C2 carbonyl of DXP. Borohydride reduction of the enzyme DXP imine followed by top-down mass spectrometric analysis localized the imine to lysine 96. On the basis of these observations, a new mechanism for the biosynthesis of the thiazole phosphate moiety of thiamin pyrophosphate in Bacillus subtilis is proposed. This mechanism involves the generation of a ketone at C3 of DXP by an Amadori-type rearrangement of the imine followed by nucleophillic addition of the sulfur carrier protein (ThiS-thiocarboxylate) to this carbonyl group.

Amino Acid Sequence↗
PubMed2005

Diastereoselective and enantioselective Rh(I)-catalyzed additions of arylboronic acids to N-tert-butanesulfinyl and N-diphenylphosphinoyl aldimines.

Two new Rh(I)-catalyzed methods for the synthesis of chiral alpha-branched amines via addition of arylboronic acids to N-tert-butanesulfinyl and N-diphenylphosphinoyl imines have been developed. The syntheses are more functional group tolerant than alternative methods utilizing Grignard or organolithium reagents, and the imine activating groups used are easily removed. These methods are both high-yielding (70-97% yield) and very selective (>93:7 dr and 88-94% ee). Significantly, the N-tert-butanesulfinyl imine method works for aliphatic imines with enolizable protons. In addition, a one-pot procedure for the synthesis of N-tert-butanesulfinyl protected alpha-branched amines from aldehydes has been developed.

Aldehydes↗
PubMed2006

A direct catalytic asymmetric Mannich-type reaction via a dinuclear zinc catalyst: synthesis of either anti- or syn-alpha-hydroxy-beta-amino ketones.

The use of imines bearing a hydrolyzable nitrogen substituent in direct asymmetric Mannich reactions with alpha-hydroxyketones is developed. Previous work focused on the use of N-arylimines or nonenolizable imines, and the latter with only methoxy-substituted alpha-hydroxyacetophenones. Using a dinuclear catalyst devised from 2,6-di-(S)-2'-diphenylhydroxymethylpyrrolidino-N-methyl)-4-methylphenol and diethylzinc, a broad array of hydroxyacetylated aromatics, including phenyl, 2-furyl, 1-naphthyl, and 2-naphthyl, react well. In addition, the reactions focused on the use of enolizable imines. With the N-diphenylphosphinoyl, the reactions are anti selective with enantiomeric excesses ranging from 83 to 99%, except for the reaction of the 2-methoxy-2'-hydroxyacetylbenzene. With the N-Boc-imines, the reactions were syn selective with enantiomeric excesses from 90 to 94%. The dependence of the diastereoselectivity on the nature of the N-substituent presumably arises from the steric demands of the diphenylphosphinoyl group.

Amino Alcohols↗
PubMed2006

Do reaction conditions affect the stereoselectivity in the Staudinger reaction?

The stereochemistry is one of the critical issues in the Staudinger reaction. We have proposed the origin of the stereoselectivity recently. The effects of solvents, additives, and pathways of ketene generation on the stereoselectivity were investigated by using a clean Staudinger reaction, which is a sensitive reaction system to the stereoselectivity. The results indicate that the additives, usually existed and generated in the Staudinger reaction, and the pathways of the ketene generation do not generally affect the stereoselectivity. The solvent affects the stereoselectivity. The polar solvent is favorable to the formation of trans-beta-lactams. The addition orders of the reagents affect the stereoselectivity in the Staudinger reaction between acyl chlorides and imines. The addition of a tertiary amine into a solution of the acyl chloride and the imine generally decreases the stereoselectivity, which is affected by the interval between additions of the acyl chloride and the tertiary amine, and the imine substituents. Our current results provide further understanding on the stereochemistry of the Staudinger reaction between acyl chlorides and imines and on the factors affecting the stereochemistry and also provide a method to prepare beta-lactams with the desired relative configuration via rationally tuning the stereoselectivity-controlling factors in the Staudinger reaction.

Amines↗
PubMed2006

pH-Responsive dendritic core-shell architectures as amphiphilic nanocarriers for polar drugs.

In this paper a simple general synthetic concept was used to generate dendritic core-shell architectures based on hyperbranched poly(ethylene imine) cores and different shells which contain aliphatic chains and poly(ethylene glycol) chains, respectively. Using these dendritic core-shell architectures as nanocarriers we studied the encapsulation and transport of polar drugs, such as congo red. The results showed that the acid labile nanocarriers exhibited much higher transport capacities for congo red than the bare poly(ethylene imine). This gives an opportunity for the controlled release of encapsulated dyes and drugs by a pH-triggered cleavage of the imine bond. In two cases the pH-sensitivity was studied and a relatively high stability of the imine bond was observed at pH 8, while fast cleavage occurred at pH 5-7. This pH difference correspond to the pH-shift in malignant tissues (tumor, infection) compared to normal tissue, which could trigger the release of the encapsulated drugs.

Aldehydes↗
PubMed1991

Geometric and energy parameters in lysine-retinal chromophores.

The parameters used in the computer program ECEPP (Empirical Conformational Energy Program for Peptides) have been expanded to cover some key elements in retinal-containing proteins. These elements are 'all-trans retinal lysine with unprotonated imine', 'all-trans retinal lysine with protonated imine', '13-cis retinal lysine with unprotonated imine' and '13-cis retinal lysine with protonated imine' respectively. The geometric parameters of these four new 'amino acid residues' were derived by optimizing their molecular structures with the AM1 Hamiltonian included in MOPAC (Molecular Orbital PACkage), and their partial atomic charges were determined with a CNDO/2 (Complete Neglect of Differential Overlap) calculation. The parameters for nonbonded interactions and torsional potentials were obtained from the existing ECEPP parameters through a logical extension. The augmented ECEPP system thus obtained can be employed to investigate the conformation of bacteriorhodopsin and its proton-pumping mechanism from an energetic point of view. The computer modeling study on bacteriorhodopsin and other seven-helix membrane proteins, e.g. serotonin receptor and dopamine receptor, is under way in the Upjohn Laboratories.

Bacteriorhodopsins↗
PubMed1993

Secondary metabolites resulting from degradation of PR toxin by Penicillium roqueforti.

PR toxin is a secondary metabolite of the fungus Penicillium roqueforti. It is lethal to rats, mice, and cats. Usually, the amount of PR toxin in the culture medium decreases from its maximum on day 15 to zero within 3 to 4 days. We found that two were secondary metabolites produced in the culture medium of this fungus while the production of PR toxin was decreasing. We isolated and purified the two compounds in pure and colorless crystalline form. On the basis of elemental analysis and mass, 1H and 13C nuclear magnetic resonance, infrared, and UV spectroscopies, the two compounds were identified as PR-imine (C17H21O5N) and PR-amide (C17H21O6N). The structures of both compounds and of PR toxin (C17H20O6) were closely related, and the peak production of PR toxin appeared earlier than those of PR-imine and PR-amide. Moreover, PR toxin was transformed to PR-imine when PR toxin was incubated with the culture medium on a given culture day. Thus, we propose that PR toxin is degraded into PR-imine and PR-amide in the culture medium of P. roqueforti.

Amides↗
PubMed1986

Free radicals of acetaminophen: their subsequent reactions and toxicological significance.

The oxidation of acetaminophen to the corresponding phenoxyl free radical and N-acetyl-p-benzoquinone imine by mammalian peroxidases is discussed. The acetaminophen free radical is very reactive--forming dimers, and, ultimately, melanin-like polymeric products. A model compound, leading to more stable metabolites, can be obtained by introduction of methyl groups next to the oxygen, to produce 3,5-dimethylacetaminophen. The electron spin resonance spectrum of this free radical could be completely analyzed. The phenoxyl radical of the dimethyl analog does not form polymers or bind with nucleophiles. N-Acetyl-p-benzoquinone imine, a hepatic metabolite of acetaminophen, and its analog N-acetyl-3,5-dimethyl-p-benzoquinone imine are metabolized by rat liver microsomes and NADPH to their corresponding p-aminophenoxyl free radicals. The p-aminophenoxyl free radical formation could be suppressed by the deacetylase inhibitors sodium fluoride and paraoxon. Substitution of NADPH-cytochrome P-450 reductase for rat liver microsomes eliminates the deacetylase activity and results in the direct reduction of N-acetyl-3,5-dimethyl-p-benzoquinone imine to the 3,5-dimethylacetaminophen phenoxyl free radical. Neither the acetaminophen nor the 3,5-dimethylacetaminophen phenoxyl radical reduces oxygen to form superoxide or reacts with oxygen in any other detectable way.

Acetaminophen↗
PubMed1986

Horseradish peroxidase-catalyzed oxidation of acetaminophen to intermediates that form polymers or conjugate with glutathione.

Horseradish peroxidase catalyzed the polymerization of acetaminophen. Addition of reduced glutathione (GSH) to reaction mixtures resulted in decreased polymerization and formation of minor amounts of GSH-acetaminophen conjugates. The conjugates were identified as 3-(glutathion-S-yl)acetaminophen and 3-(glutathion-S-yl)diacetaminophen. Horseradish peroxidase also catalyzed polymerization of synthetic 3-(glutathion-S-yl)acetaminophen to a dimer conjugate. In contrast to acetaminophen, 3-(glutathion-S-yl)acetaminophen oxidation was slowly catalyzed by horseradish peroxidase. However, in reaction mixtures containing equimolar concentrations of acetaminophen and synthetic 3-(glutathion-S-yl)acetaminophen, the formation of 3-(glutathion-S-yl)diacetaminophen and 3-(diglutathion-S-yl)diacetaminophen was rapid and accounted for approximately 95% of the products, whereas acetaminophen polymers accounted for only 5% of the products. These findings suggest that horseradish peroxidase catalyzed the one-electron oxidation of acetaminophen to N-acetyl-p-benzosemiquinone imine which preferentially polymerized rather than reacted with GSH. N-Acetyl-p-benzosemiquinone imine may also oxidize 3-(glutathion-S-yl)acetaminophen to form acetaminophen and 3-(glutathion-S-yl)-N-acetyl-p-benzosemiquinone imine. The data indicate that once this conjugate radical is formed it reacts with either N-acetyl-p-benzosemiquinone minine or 3-(glutathion-S-yl)-N-acetyl-p-benzosemiquinone imine via a radical termination mechanism.

Acetaminophen↗
PubMed2000

Soluble-polymer-supported synthesis of beta-lactams on a modified poly(ethylene glycol)

A modified poly(ethylene glycol) (PEG) has been developed for the soluble-polymer-supported synthesis of beta-lactams. The monomethylether of PEG (MeOPEG) with an average M(W) of 5000 was used as the support, a 4-(3-propyl)phenyl residue as the spacer, and a 4-oxyphenylamino group as the moiety with the reactive functionality. From this modified PEG representative aromatic, heteroaromatic, unsaturated, and aliphatic imines were obtained in high yields by different procedures. The polymer-supported imines were then employed to prepare several beta-lactams by enolate/imine condensation and ketene/imine cycloaddition. Examples of the control of the absolute stereochemistry during the azetidinone ring formation are also reported. The reactions carried out on the polymer-bound imines showed a remarkable similarity to those performed on nonimmobilized imines, both in terms of yields and stereoselectivities. Removal of the beta-lactams from the polymer has also been accomplished to directly deliver the N-unsubstituted azetidinones.

Journal Article↗
PubMed1987

Decreased pneumotoxicity of deuterated 3-methylindole: bioactivation requires methyl C-H bond breakage.

The bioactivation of the pulmonary toxin 3-methylindole has been postulated to proceed via the formation of an imine methide. To test this hypothesis, the toxicity in mice of 3-methylindole has been compared to the toxicity of its perdeuteromethyl analog. Deuteration of the methyl group should slow the rate of production of the corresponding imine methide and diminish the toxicity of deutero-3-methylindole, if C-H bond breakage occurs prior to or during the rate-determining step. In agreement with this hypothesis, deutero-3-methylindole was synthesized and was shown to be significantly less toxic (LD50 735 mg/kg) than 3-methylindole (LD50 578 mg/kg). Both compounds produced the same lesion at the LD50 dose, bronchiolar damage and mild alveolar edema, indicating that deuteration of 3-methylindole did not change the pathologic process. However, at a much lower dose (25 mg/kg), 3-methylindole produced a mild bronchiolar lesion whereas deutero-3-methylindole did not damage lung tissue. Additionally, administration of deutero-3-methylindole caused less pulmonary edema compared to 3-methylindole, as assessed by increased wet lung weights. Finally, the depletion of pulmonary glutathione by deutero-3-methylindole was considerably slower than depletion by 3-methylindole. The electrophilic imine methide has been postulated to be the intermediate which binds with and depletes glutathione. Therefore, the evidence presented here supports the involvement of an imine methide as the primary reactive intermediate in 3-methylindole-mediated pneumotoxicity.

Animals↗
PubMed1992

Comparison of three tetramic acids and their ability to alter membrane function in cultured skeletal muscle cells and sarcoplasmic reticulum vesicles.

Cyclopiazonic acid is a potent inhibitor of calcium uptake and Ca(2+)-ATPase activity in sarcoplasmic and endoplasmic reticulum. In L6 muscle myoblasts, cyclopiazonic acid stimulates the uptake of tetraphenylphosphonium, a lipophilic membrane potential probe, and has antioxidant properties. The purpose of the present study was to investigate the structural requirements necessary for causing the surface charge alterations, and the antioxidant activity in L6 skeletal muscle myoblasts, and for inhibition of calcium transport by rat skeletal muscle sarcoplasmic reticulum vesicles. This was accomplished by comparing the effects of two structurally related tetramic acids, cyclopiazonic acid imine and tenuazonic acid, with cyclopiazonic acid. Cyclopiazonic acid imine inhibited oxalate-assisted 45Ca2+ uptake and ATPase activity in sarcoplasmic reticulum vesicles and stimulated tetraphenylphosphonium accumulation by L6 muscle myoblasts. However, these effects required an approximately fourfold higher concentration than that of cyclopiazonic acid. Tenuazonic acid, up to 1 mM, had no effect on oxalate-assisted 45Ca2+ uptake or Ca(2+)-ATPase activity in sarcoplasmic reticulum vesicles and did not stimulate tetraphenylphosphonium accumulation by L6 muscle myoblasts. Cyclopiazonic acid was only slightly more effective than cyclopiazonic acid imine at preventing the patulin-induced increase in thiobarbituric acid positive substance (used to estimate lipid peroxidation); tenuazonic acid was totally ineffective. Previously, it was shown that cyclopiazonic acid was twice as effective as cyclopiazonic acid imine at preventing increases in thiobarbituric acid positive substance in cultured renal cells, LLC-PK1. Thus, the indole nucleus of cyclopiazonic acid is essential for the membrane-associated biological activity; however, modification of the acetyl group reduces the potency of the activity.

Animals↗