Solution structure, mutagenesis, and NH exchange studies of the MutT enzyme–Mg-8-oxo-dGMP complex

The MutT pyrophosphohydrolase from E. coli (129 residues) catalyzes the hydrolysis of nucleoside triphosphates (NTP), including 8-oxo-dGTP, by substitution at Pβ, to yield NMP and pyrophosphate. The product, 8-oxo-dGMP is an unusually tight binding, slowly exchanging inhibitor with a K_D=52 nM, (ΔG°=−9.8 kcal/mol) which is 6.1 kcal/mol tighter than the binding of dGMP (ΔG°=−3.7 kcal/mol). The higher affinity for 8-oxo-dGMP results from a more favorable ΔH_binding (−32 kcal/mol) despite an unfavorable −TΔS°_binding (+22 kcal/mol). The solution structure of the MutT–Mg²⁺-8-oxo-dGMP complex shows a narrowed, hydrophobic nucleotide-binding cleft with Asn-119 and Arg-78 among the few polar residues. The N119A, N119D, R78K and R78A single mutations, and the R78K+N119A double mutant all showed largely intact active sites, on the basis of small changes in the kinetic parameters of dGTP hydrolysis and in ¹H–¹⁵N HSQC spectra. However, the N119A mutation profoundly weakened the active site binding of 8-oxo-dGMP by 4.3 kcal/mol (1650-fold). The N119D mutation also weakened 8-oxo-dGMP binding but only by 2.1 kcal/mol (37-fold), suggesting that Asn-119 functioned both as a hydrogen bond donor to C8=O, and a hydrogen bond acceptor from N7H of 8-oxo-dGMP, while aspartate at position −119 functioned as an acceptor of a single hydrogen bond. Much smaller weakening effects (0.3–0.4 kcal/mol) on the binding of dGMP and dAMP were found, indicating specific hydrogen bonding of Asn-119 to 8-oxo-dGMP. While formation of the wild type MutT–Mg²⁺-8-oxo-dGMP complex slowed the backbone NH exchange rates of 45 residues distributed throughout the protein, the same complex of the N119A mutant slowed the exchange rates of only 11 residues at or near the active site, indicating an increase in conformational flexibility of the N119A mutant. The R78K and R78A mutations weakened the binding of 8-oxo-dGMP by 1.7 and 1.1 kcal/mol, respectively, indicating a lesser role of Arg-78 than of Asn-119 in the selective binding of 8-oxo-dGMP, likely donating a single hydrogen bond to its C6=O. The R78K+N119A double mutant weakened the binding of 8-oxo-dGMP (K_I^slope=3.1 mM) by 6.5±0.2 kcal/mol which overlaps, within error with the sum of the effects of the two single mutants (6.0±0.3 kcal/mol). Such additive effects of the two single mutants in the double mutant are most simply explained by the independent functioning of Asn-119 and Arg-78 in the binding of 8-oxo-dGMP. Independent functioning of these two residues in nucleotide binding is consistent with their locations in the MutT–Mg²⁺-8-oxo-dGMP complex, on opposite sides of the active site cleft, with a distance of 8.4±0.5 Å between their side chain nitrogens.     

Kinetics and mechanism of the oxidation of acetaldehyde by chromic acid in perchloric acid

Summary The oxidation of MeCHO by chromium(VI) has been studied in HClO₄ medium over a wide range of experimental conditions and has been found to obey the rate law;v=k[MeCHO][HCrO ₄ ^– ][H⁺]. The calculated H and-S values for the reaction are 30±2kJ mol^–1 and 171±7J mol^–1deg^–1, respectively. The mechanism is discussed in terms of carbon-hydrogen bond cleavage.     

Differential scanning calorimetry studies on aged iron(III)-chromium(III)-hydroxide coprecipitates

DSC measurements in air from 20 to 800° are reported on aged iron(III)-chromium(III) coprecipitated hydroxides (with Fe³⁺Cr³⁺ wt% ratios of 91, 73, 11, 37 and 19). A comparison of the DSC profiles of aged coprecipitates with the thermal behaviour of freshly precipitated and well characterized samples provided information on the modification occurring in the structure and reactivity of the coprecipitates on prolonged aging.

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Zusammenfassung Es werden DSC-Messungen an gealterten kogefällten Eisen(III)Chrom(III)-hydroxiden (mit Fe³⁺Cr³⁺ Gewichtsanteilen von 91, 73, 11, 37 und 19) im Temperaturbereich von 20 bis 800° beschrieben. Ein Vergleich der DSC-Profile von gealterten Kofällungen mit dem thermischen Verhalten frisch gefällter und gut definierbarer Proben lieferte Informationen über die Veränderungen in Struktur und Reaktivität der Koniederschläge bei längerem Altern.

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Dedicated to Professor Wahid U. Malik, Founder-President, Indian Council of Chemists, India, with profound regards and fond memories of his youthful enthusiasm and ceaseless and pioneering efforts towards the development of chemical education and research activities at the University of Roorkee, Roorkee, India, on the occasion of his 70th birthday.

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Paper presented at the World Conference on Thermal Analysis, Capri (Italy), 1989.

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We thank Professor J. Pescia, Dr.G. Ablart and N. Favillier of Laboratoire de Magnétisme et d'Electronique Quantique for their interest in this work. A. C. Vajpei expresses his gratefulness to C.N.R.S. and C.N.O.U.S., Paris, for financial support of this work.     

Kinetics and mechanism of the oxidation of thiosulphate by hexachloroplatinate(IV)

Summary Rate constants for the oxidation of thiosulphate by hexachloroplatinate(IV) have been measured. The kinetics of the oxidation of thiosulphate follow a second-order rate law, first order with respect to thiosulphate and first order with respect to platinum(IV). The influence of pH is small. The rates are found to depend on the nature and concentration of the cations and follow the order: Cs⁺>Rb⁺>K⁺>Na⁺>Li⁺. The activation parameters calculated from the temperature studies are: H=42.9 k J mol^–1 and S=–102 JK^–1 mol^–1. A mechanism of the reaction in terms of intermediate formation of free radicals followed by the formation of tetrathionate is postulated to explain the kinetic behaviour.     

Synthesis and photophysics of leadsulphide nanocrystallites

Nanoparticles of lead sulphide have been stabilized in the presence of excess Pb²⁺ in aqueous basic medium by a simple chemical route of synthesis. These PbS nanoparticles were synthesized very conveniently at room temperature using hexametaphosphate as stabilizer. These nanoparticles have an absorption extending into the NIR region. A significant quantum confinement effect made the bandgap of lead sulphide nanoparticles produce a blue shift from 0.41 eV to about 1.5 eV. The size and morphology of the particles were studied by TEM. Particles were relatively small sized (about 6 nm) having narrow size distribution. XRD data analysis indicate that the product is a mixture of PbS, PbO and Pb(OH)₂. Both XRD pattern and HRTEM images confirm the crystalline structure of lead sulphide crystals. IR spectroscopy indicates the formation of PbS. PbS nanoparticles were fairly stable and could be stored for about three weeks at room temperature and for about two months at 5°C without agglomeration. These particles were photoactive and sensitized the reaction of aniline by light leading to the formation of azobenzene.     

Atomic Layer Deposited Ti2O3 Thin Films

Ti₂O₃ thin films have been prepared through atomic layer deposition and subjected to electrical resistivity measurements as a function of temperature. The as-prepared films were stable for up to three weeks. In Ti₂O₃ thin films, the insulator-metal transition is observed at ∼80 K, with nearly 3–4 orders of magnitude change in resistivity. The anomalous increase in electrical resistivity in the films is in accordance with the two-band model. However, the energy interval between the bands depending on the crystallographic c/a ratio leads to a change in electrical resistivity as a function of temperature.     

Kinetics of oxidation of hypophosphite ion by Au(III) in hydrochloric acid medium

Hypophosphite ion is oxidised by Au(III) in aqueous hydrochloric acid to give phosphorus acid and Au(I). The kinetics of the reaction has been studied spectrophotometrically in the UV region at different temperatures. The oxidation of hypophosphorous acid is first order with respect to both Au(III) and substrate. Hydrogen ion has no effect on the rate in acid media (0.15–1.0)M. The energy and entropy of activations are 128 ± 3.0kJ mol^?1 and 135.8 ± 6.5 JK^?1 mol^?1 respectively. The results are interpreted in terms of the probable formation of intermediate Au(lI).     

Interfacial regions governing internal cavities of dendrimers. Studies of poly(alkyl aryl ether) dendrimers constituted with linkers of varying alkyl chain length

This report deals with a study of the properties of internal cavities of dendritic macromolecules that are capable of encapsulating and mediating photoreactions of guest molecules. The internal cavity structures of dendrimers are determined by the interfacial regions between the aqueous exterior and hydrocarbon like interior constituted by the linkers that connect symmetrically sited branch points constituting the dendrimer and head groups that cap the dendrimers. Phloroglucinol-based poly(alkyl aryl ether) dendrimers constituted with a homologous series of alkyl linkers were undertaken for the current study. Twelve dendrimers within first, second, and third generations, having ethyl, n-propyl, n-butyl, and n-pentyl groups as the linkers and hydroxyl groups at peripheries in each generation, were synthesized. Encapsulation of pyrene and coumarins by aqueous basic solutions of dendrimers were monitored by UV-vis and fluorescence spectroscopies, which showed that a lower generation dendrimer with an optimal alkyl linker presented better encapsulation abilities than a higher generation dendrimer. Norrish type I photoreaction of dibenzyl ketone was carried out within the above series of dendrimers to probe their abilities to hold guests and reactive intermediate radical pairs within themselves. The extent of cage effect from the series of third generation dendrimers was observed to be higher with dendrimers having an n-pentyl group as the linker.     

Silyl modification of biologically active compounds. 12. Silyl group as true incentive to antitumour and antibacterial action of choline and colamine analogues

A series of triorganylsilyl(β‐dialkylaminoethoxy)silanes was prepared and characterized by elemental analysis, ¹H, ¹³C, ²⁹Si NMR and mass spectroscopy. Comparative study of ²⁹Si resonance of newly synthesized compounds showed correlation between its value and substituent nature at the silicon atom, and is shifted upfield for β‐triorganyl(N,N‐dialkylaminoethoxy)silanes in comparison with corresponding methiodides, revealing weak N^…Si interaction for proper silanes. In vitro antitumour and antimicrobial properties were investigated. The biological activity data exhibited a marked enhancement of inhibitory activity on trialkylsilylation against tumour cell lines and all the test bacterial/fungal strains. Copyright © 2006 John Wiley & Sons, Ltd.     

Diorganotin(IV) bis(O,O-alkylenedithiophosphates)

O,O-Alkylenedithiophosphates of diorganotin(IV) of the type R₂Sn[SP(S)O₂G]₂ (R = Me, Et, n-Bu, Ph; G = CH₂CMe₂CH₂, CMe₂CMe₂, CMe₂CH₂CHMe) have been synthesized by the reactions of diorganotin(IV) dichlorides with ammonium O,O-alkylenedithiophosphates or that of diorganotin(IV) oxides with O,O-alkylenedithiophosphoric acids in 1:2 molar ratio in benzene. These new complexes are white solids which are soluble in common organic solvents and are monomeric in refluxing benzene; and they have been characterized by elemental analysis and by different spectroscopic (IR, ¹H, ¹³C, ³¹P and ¹¹⁹Sn NMR) studies, on the basis of which a six coordinated octahedral structure has been suggested in solution.