UMLS:C0847097 - FACTA Search

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Query: UMLS:C0847097 (acidity)
15,165 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The ligand-binding surface of the T-lymphocyte glycoprotein CD2 has an unusually high proportion of charged residues, and ionic interactions are thought to play a significant role in defining the ligand specificity and binding affinity of CD2 with the structurally homologous ligands CD48 (in rodents) and CD58 (in humans). The determination of the electrostatic properties of these proteins can therefore contribute to our understanding of structure-activity relationships for these adhesion complexes that underpin T-cell adhesion to antigen-presenting cells. In this study, we investigated the pH titration behavior of the carboxyl groups of the N-terminal domain of rat CD2 (CD2d1) using the chemical shifts of backbone amide nitrogen-15 ((15)N) and proton NMR resonances, and carboxyl carbon-13 ((13)C) signals. The analysis revealed the presence of a glutamate (Glu41) on the binding surface of rat CD2 with an unusually elevated acidity constant (pK(a) = 6.73) for CD2d1 samples at 1.2 mM concentration. pH titration of CD2d1 at low protein concentration (0.1 mM) resulted in a slight decrease of the measured pK(a) of Glu41 to 6.36. The ionization of Glu41 exhibited reciprocal interactions with a second glutamate (Glu29) in a neighboring location, with both residues demonstrating characteristic biphasic titration behavior of the carboxyl (13)C resonances. Measurements at pH 5.5 of the two-bond deuterium isotope shift for the (13)C carboxyl resonances for Glu41 and Glu29 [(2)DeltaC(delta)(O(epsilon)D) = 0.2 and 0.1 ppm, respectively] were consistent with the assignment of the anomalous pK(a) to Glu41, under the strong influence of Glu29. The characterization of single site mutations of CD2d1 residues Glu41 and Glu29 to glutamine confirmed the anomalous pK(a) for Glu41, and indicated that electrostatic interaction with the Glu29 side chain is a significant contributing influence for this behavior in the wild-type protein. The implications of these observations are discussed with respect to recent structural and functional analyses of the interaction of rat CD2 with CD48. In particular, CD2 Glu41 must be a candidate residue to explain the previously reported strong pH dependence of binding of these two proteins in vitro.
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PMID:Determination of pK(a) values of carboxyl groups in the N-terminal domain of rat CD2: anomalous pK(a) of a glutamate on the ligand-binding surface. 1084 61

The hydrogen bonding properties of 1-methylcytosine (1-MeC) with the following guanine base derivatives have been studied in DMSO-d6, applying concentration-dependent 1H NMR spectroscopy: 9-ethylguanine, 7,9-dimethylguanine (7,9-DimeGH+), and 7,8-dihydro-8-oxo-9-methylguanine (8-O-9-MeGH), as well as three 9-ethylguanine complexes carrying different Pt(II) moieties at the N7 position. The association constants K for the Watson-Crick pairing schemes are by a factor 2-3 higher in the cases of platinated guanine complexes compared to the Watson-Crick pair between 9-ethylguanine and 1-methylcytosine (K = 6.9 +/- 1.3 M(-1)). Similar enhanced stabilities are observed for the pairs formed between 1-MeC and 7,9-DimeGH+ or 8-O-9-MeGH. The increase in N1H acidity of the guanine derivative upon modification at the N7 or C8 positions can be correlated with the association constants K; the result is a bell-shaped curve meaning that acidification initially stabilizes hydrogen bond formation up to a certain maximum; further acidification then leads to a destabilization. For two of the examples studied in solution, hydrogen bonding according to Watson-Crick between N7-platinated 9-ethylguanine and 1-methylcytosine has also been established by X-ray crystallography.
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PMID:Effects of N7-methylation, N7-platination, and C8-hydroxylation of guanine on H-bond formation with cytosine: platinum coordination strengthens the Watson-Crick pair. 1090 39

Bilirubin, the yellow-orange tetrapyrrole pigment of jaundice, is essentially insoluble in pure water, but is much more soluble in solutions of bile salts such as sodium taurocholate. The biophysical chemistry of bilirubin in bile salt solutions is affected by changes in the pH of the solution in the range 5-9, suggesting that interactions with bile salt molecules and micelles may alter the acidity of the pigment. We have examined this possibility by determining the apparent pKa values for a series of carboxyl 13C-enriched model compounds, including the bilirubin analog mesobilirubin XIIIalpha, in solutions of sodium taurocholate and sodium taurodeoxycholate. Apparent pKa values were determined by 13C NMR titrations in dimethyl sulfoxide-water mixtures. The results show that the acidity of all compounds is decreased, or pKa increased, in micellar bile salt solution relative to pure water and that the effect is greatest for the larger, less water-soluble compounds. We have proposed a model to explain these results and discussed the implications of these findings for the biophysical chemistry of bilirubin in bile.
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PMID:Acid dissociation constants of bilirubin and related carboxylic acid compounds in bile salt solutions. 1101 23

The metabolism of benzo[a]pyrene (BP) is known to lead to a large number of oxygenated compounds, some of which can bind covalently to DNA. We have studied the integrated metabolism of BP in vivo in germ-free rats given (14)C-labeled BP. Urinary metabolites were separated into groups according to acidity using lipophilic ion exchangers. The groups were analyzed by mass spectrometry and were further fractionated by high-performance liquid chromatography. The fraction of urinary metabolites previously shown to contain N-acetylcysteine and glucuronic acid conjugates was found to contain derivatives of 7-oxo-benz[d]anthracene-3,4-dicarboxylic acid as major components. These compounds, which were identified by mass spectrometry and NMR, accounted for about 30% of the total metabolites in urine, demonstrating that, surprisingly, ring opening is a major pathway for metabolism of BP in the germ-free rat. The dicarboxylic acid may be excreted in urine as an ester glucuronide. By using the single cell gel electrophoresis or COMET assay, we were able to demonstrate that the anhydride of 7-oxo-benz[d]anthracene-3, 4-dicarboxylic acid was an efficient inducer of DNA damage. Taken together, these results indicate that the novel ring opening metabolic pathway may provide alternative mechanisms for the toxicity of BP.
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PMID:Ring opening of benzo[a]pyrene in the germ-free rat is a novel pathway for formation of potentially genotoxic metabolites. 1111 46

Two-electron reduction occurs when the Re(V) precursors ReOX3(PPh3)2 and ReO(OEt)X2(PPh3)2 are reacted with biimidazole (biimH2) in boiling chloroform, affording rhenium(III) cationic complexes of the type cis,trans-[ReX2(PPh3)2(biimH2)]X with X = Cl, Br, and I. Crystal structures are determined for the compounds with the three halogens, as well as for the [ReCl2(PPh3)2(biimH2)](benzoate) salt. In all cases, the counterion is attached to the complex cation via hydrogen bonding with the N-H groups of coordinated biimidazole. Variable-temperature 1H NMR spectroscopy shows that a mixture of [ReCl2(PPh3)2(biimH2)](benzoate) and [ReCl2(PPh3)2(biimH2)]Cl is in slow exchange below -50 degrees C in CD2Cl2, indicating that ion pairing is retained in solution. Both N-H groups can be deprotonated with sodium methoxide, and their acidities are evaluated from UV-visible spectra. Competition between monodeprotonated [ReCl2(PPh3)2(biimH)] and various carboxylic acids reveals that the acidity of the first N-H proton corresponds to that of acetic acid (pKa(aq) approximately 4.8). By a similar competitive reaction between bis-deprotonated [ReCl2(PPh3)2(biim)]- and phenols, the second acidity is estimated to be close to that of phenol (pKa(aq) approximately 9.8).
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PMID:Preparations, characterizations, and structures of (biimidazole)dihalobis(triphenylphosphine)rhenium(III) salts: strong ion-pairing and acid-base properties. 1119 66

Dissolved organic matter (DOM) was concentrated from natural waters and the effluent of a wastewater treatment plant using a portable reverse osmosis (RO) system. The humic acid (HA), fulvic acid (FA) and hydrophilic (HyI) fractions were isolated and purified by the XAD-8 resin combined with the cation exchange resin method. The FA fractions predominated in natural waters and accounted for 54-68% of the total amount of dissolved organic carbon (DOC), whereas the HA and HyI fractions constituted, respectively, 13-29 and 9-30% of the total DOC. The effluent of wastewater was almost devoid of HA and the HyI fraction exceeded FA. The elemental compositions of HA and FA were in the ranges typical for natural humic materials, but the HyI fractions did not exhibit humic character. 1H NMR spectra revealed that the HyI fractions were almost devoid of aromatic protons and the aliphatic region featured more sharp signals than HA and FA fractions, indicating that HyI fractions were consisted of more simple compounds and less complex mixtures. The aliphatic functional groups in these fractions of DOM samples followed the order HA < FA <HyI. Proton titrations indicated that HA. FA and HyI fractions of DOM samples from different sources had similar total acidity whose range was 9.0-11.6 meq/g C. This similarity may be due to the sample fractionation method. The copper titration results showed that the number of total Cu binding sites for the three fractions of different DOM samples were quite similar (1.46-1.60 mmol/g C), which was consistent with the similarity in total acidity from proton titrations. The affinity of copper with each isolated fraction followed the order HA approximately = FA > HyI. The rate of Cu complexation with the HyI fraction was faster than the rate with the HA or FA fraction of the Suwannee River DOM, implying that copper reacted with relatively weak ligands faster than with strong ligands.
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PMID:Characterization of isolated fractions of dissolved organic matter from natural waters and a wastewater effluent. 1123 94

The preparation is described of two modified derivatives of the tripodal tetraamine tren, 2-hydroxy-N,N-bis(2-aminoethyl)ethylamine, NN(2)O222, and 2-amino-N,N-bis(2-hydroxyethyl)ethylamine, NNO(2)222, in which one and two primary amines, respectively, have been replaced with hydroxyl groups. The aqueous acid-base and metal ion (Ni2+, Cu2+, Zn2+) coordination properties of these two compounds were studied by potentiometric, spectrophotometric, and NMR titrations. Two and three acidity constants, respectively, were determined for NNO(2)222 and NN(2)O222 by potentiometry. NMR titrations proved that deprotonation of the two OH residues in NNO(2)222, and of the one in NN(2)O222, corresponded to pK(a) > 14. Acidity constants related to deprotonation of the terminal primary amine functions were similar in both NNO(2)222 and NN(2)O222 (and to those in the parent compound tren), whereas deprotonation of the tertiary ammonium N atom had a very different acidity constant in each of these three compounds. Charge repulsion, polar effects, and intramolecular hydrogen bond formation are responsible for the discrepancy. Chelated diamine metal complexes for each ligand studied depended only on the basicity of the corresponding two amines, suggesting that the hydroxyl group interacted with the metal ion very weakly in acidic or neutral solutions. The ML2+ species further deprotonated to form M(L - H)+ and M(L - 2H) complexes, in which the protons are released from the coordinated OH group. A pM vs pH correlation showed that replacing an NH2 group with a OH group in tren or NN(2)O222 makes the resulting metal complex less stable. Electronic spectra showed that the Cu(II) complexes of both NNO(2)222 and NN(2)O222 adopted a square pyramidal geometry rather than a trigonal bipyramidal geometry. The X-ray crystal structure analysis of the zinc complex [Zn(OH)(mu-NNO(2)222 - H)Zn(NNO(2)222)]2+, as its [BF4]- salt, shows a dinuclear molecule containing two zinc ions, each coordinated in a distorted trigonal bipyramid. The coordination environment at one zinc atom is composed of the four donor groups of a mono-O-deprotonated ligand NNO(2)222 and a hydroxyl ion with the central nitrogen atom of the ligand and the hydroxyl ion in equatorial positions. The oxygen atom of the deprotonated alkoxo group bridges to the second zinc atom, which is coordinated by this atom and one undeprotonated ligand NNO(2)222.
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PMID:Effects of sequential replacement of -NH2 by -OH in the tripodal tetraamine tren on its acidity and metal ion coordinating properties. 1126 60

[Tl(OCH2Me)]4 (1) was reacted with excess HOR to prepare a series of [Tl(OR)]n, where OR = OCHMe2 (2, n = 4), OCMe3 (3, n = 4), OCH2CMe3 (4, n = 4), OC6H3(Me)2-2,6 (5, n = infinity), and OC6H3(CHMe2)2-2,6 (6, n = infinity). Single-crystal X-ray diffraction experiments revealed that in the solid state the alkoxide-ligated compound 4 adopts a cubane structure, whereas the aryloxide derivatives, 5 and 6, formed polymeric chains. Compounds 1-6 were also characterized by 203,205Tl solution and 205Tl solid-state NMR spectroscopy. In solution it was determined that 1-4 retained the [Tl-O]4 cube structure, whereas the polymeric species 5 and 6 appeared to be fluxional. Variations in the solution and solid-state structures for the [Tl(OR)]4 cubes and polymeric [Tl(OAr)]infinity are influenced by the steric hindrance of the ligand. The acidity of the parent alcohol influences the degree of covalency at the Tl metal center, which is reflected in the 203,205Tl chemical shifts for 1-6.
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PMID:203,205Tl NMR studies of crystallographically characterized thallium alkoxides. X-ray Structures of [TI(OCH2CMe3)]4 and [TI(OAr)]infinity, where OAr = OC6H3(Me)2-2,6 and OC6H3(CHMe2)2-2,6. 1130 64

The role of polar solvents (particularly HMPA) in controlling the ratio of 1,2 to 1,4 addition of sulfur-substituted organolithium reagents to cyclohexenones and hexenal was studied. Low-temperature, multinuclear NMR studies provided quantitative information about the ratio of contact (CIP) and solvent-separated (SIP) ion pairs in solutions of dithianyllithiums and phenylthiobenzyllithiums in THF-HMPA solutions. The ratio of contact and separated ion pairs was manipulated by changes in the strength of solvation (generally through the addition of HMPA). Although the results are consistent with the CIP/SIP distribution being an important factor in determining the regioselectivity of these additions (Curtin-Hammett limitations prevent a direct correlation), it cannot be the only one. Changes in diastereomeric product ratios upon addition of HMPA suggest that complexation of HMPA to lithium has two effects. First, it causes ion pair separation, which enhances 1,4 addition. Second, it lowers the Lewis acidity and catalytic effectiveness of the lithium cation, which also favors 1,4 addition. For most sulfur-stabilized lithium reagents, 2 equiv of HMPA suffice to achieve >95% 1,4 addition, whereas 4 equiv of DMPU are required to achieve identical regiochemical and stereochemical results.
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PMID:The regioselectivity of addition of organolithium reagents to enones and enals: the role of HMPA. 1143 39

Highly ordered hexagonal mesoporous aluminosilicates (MAS-5) with uniform pore sizes have been successfully synthesized from assembly of preformed aluminosilcate precursors with cetyltrimethylammonium bromide (CTAB) surfactant. The aluminosilicate precursors were obtained by heating, at 100--140 degrees C for 2--10 h, aluminasilica gels at the Al(2)O(3)/SiO(2)/TEAOH/H(2)O molar ratios of 1.0/7.0--350/10.0--33.0/500--2000. Mesoporous MAS-5 shows extraordinary stability both in boiling water (over 300 h) and in steam (800 degrees C for 2 h). Temperature-programmed desorption of ammonia shows that the acidic strength of MAS-5 is much higher than that of MCM-41 and is comparable to that of microporous Beta zeolite. In catalytic cracking of 1,3,5-triisopropylbenzene and alkylation of isobutane with butene, MAS-5 exhibits greater catalytic activity and selectivity, as compared with MCM-41 and HZSM-5. The MAS-5 samples were characterized with infrared, UV--Raman, and NMR spectroscopy and numerous other techniques. The results suggest that MAS-5 consists of both mesopores and micropores and that the pore walls of MAS-5 contain primary and secondary structural building units, similar to those of microporous zeolites. Such unique structural features might be responsible for the observed strong acidity and high thermal stability of the mesoporous aluminosilicates with well-ordered hexagonal symmetry.
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PMID:Mesoporous aluminosilicates with ordered hexagonal structure, strong acidity, and extraordinary hydrothermal stability at high temperatures. 1145 29

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