A SPIN TRAPPING STUDY OF THE PHOTOCHEMISTRY OF 5,5-DIMETHYL-1-PYRROLINE N-OXIDE (DMPO)

The photochemistry of 5,5-dimethyl-l-pyrroline N -oxide (DMPO) has been studied in benzene, cyclohexane and aqueous buffer solutions (pH 7.4) by means of electron paramagnetic resonance (EPR) and the spin trapping technique. Ultraviolet irradiation of DMPO in aqueous buffer with unfiltered UV radiation from a Xe arc lamp results in photoionization of the spin trap and the generation of the DMPO cation radical, DMPO⁺. The aqueous electron, e_aq⁻, was trapped by DMPO and detected as the DMPO/H adduct. The DMPO⁺- reacted with the water to yield the DMPO/OH adduct. Ultraviolet irradiation of DMPO in nitrogen-saturated benzene gave an unidentified carbon-centered DMPO adduct that was replaced by hydroperoxyl and alkoxyl adducts of DMPO when oxygen was present. Experiments employing ¹⁷O₂ gas indicated that the oxygen in the DMPO alkoxyl adduct was derived from molecular oxygen. However, UV irradiation of DMPO in cyclohexane yielded the cyclohexyl and cyclohexyloxyl adducts of DMPO in nitrogen-saturated and air-saturated solutions, respectively. These observations suggest that in aprotic solvents UV irradiation of DMPO generates a carbon-centered radical (R⁻), derived from the trap itself, which in benzene reacts with oxygen to yield an alkoxyl radical (RO⁻), possibly via a peroxyl radical (ROO⁻) intermediate. In cyclohexane R⁻ abstracts a hydrogen atom from the solvent to yield the cyclohexyl radical in the absence of oxygen and the cyclohexyloxyl radical in the presence of oxygen. These findings indicate that when DMPO is used as a spin trap in studies employing short-wavelength UV radiation (λ < 300 nm) the photochemistry of DMPO cannot be ignored.     

Iron chelation properties of an extracellular siderophore exochelin MN

The coordination chemistry of an extracellular siderophore produced by Mycobacterium neoaurum, exochelin MN (ExoMN), is reported along with its pK(a) values, Fe(III) and Fe(II) chelation constants, and aqueous solution speciation as determined by spectrophotometric and potentiometric titration techniques. Exochelin MN is of particular interest as it can efficiently transport iron into pathogenic M. leprae, which is responsible for leprosy, in addition to its own parent cells. The Fe(III) coordination properties of ExoMN are important with respect to understanding the Fe(III) acquisition and uptake mechanism in pathogenic M. leprae, as the siderophores from this organism are very difficult to isolate. Exochelin MN has two hydroxamic acid groups and an unusual threo-beta-hydroxy-l-histidine available for Fe(III) chelation. The presence of threo-beta-hydroxy-l-histidine gives rise to a unique mode of Fe(III) coordination. The pK(a) values for the two hydroxamic acid moieties, the histidine imidazole ring and the alkylammonium groups on ExoMN, correspond well with the literature values for these moieties. Proton-dependent Fe(III)- and Fe(II)-ExoMN equilibrium constants were determined using a model involving sequential protonation of the Fe(III)- and Fe(II)-ExoMN complexes. These data were used to develop a model whereby deprotonation reactions on the surface of the complex in the second coordination shell result in first coordination shell isomerization. The overall formation constants were calculated: log beta(110) = 39.12 for Fe(III)-ExoMN and 16.7 for Fe(II)-ExoMN. The calculated pFe value of 31.1 is one of the highest among all siderophores and their synthetic analogues and indicates that ExoMN is thermodynamically capable of removing Fe(III) from transferrin. The E(1/2) for the Fe(III)ExoMN/Fe(II)ExoMN(-) couple was determined to be -595 mV from quasi-reversible cyclic voltammograms at pH = 10.8, and the pH-dependent E(1/2) profile was used to determine the Fe(II)-ExoMN protonation constants.     

Non-empirical LCAO-MO-SCF calculations of the electronic structure of SiF2

Non-empirical LCAO-MO-SCF calculations on SiF₂ using two Gaussian basis sets are reported. The larger basis set gives a calculated geometry in good agreement with experiment. The effect on the energy and population analysis of optimization of the Si 3d exponent was investigated. 3d orbitals are found to be much less important in the bonding than in the isoelectronic molecule SO₂.     

Organosilicon compounds : LI. Further studies of the base-catalysed solvolysis of N-(triorganosilyl)anilines

Relative rates of solvolysis of some N-triorganosilylanilines in mixtures of ethanol and aqueous potassium hydroxide have been determined, with results as follows. (i) For XC₆H₄NHSiEt₃ compounds in MeOH (5 vol) + aq. alkali (2 vol) at 50°: (X =) H, 1.0;p-Me, 0.80;p-OMe, 0.83;m-Me, 0.90; o-Me, 0.87; p-SMe, 1.90; p-F, 1.7; p-Cl, 2.8; o-Cl, 14; m-Cl, 4.2; m-NO₂, 18; p-CN, ca. 43; p-NO₂, ca. 120. (ii) For PhNHSi(C₆H₄Y)₃ compounds in MeOH (10 vol) + aq. alkali (1 vol) at 50°: (Y =) H, 1.0; p-OMe, 0.12; p-Cl, ca. 32; m-Cl, ca. 84. (iii) For PhNHSiR₃ compounds in MeOH (5 vol) + aq. alkali (2 vol) : (R₃ =) Et₃, 1.0; Et₂Me, 18 (at 30°); Me₂-i-Pr, 8 (at 30°);Me₂-t-Bu, 0.012 (at 50°);i-Pr₃, 0.006 (at 50°). In series (i) the relative rates correlate with σ, or where appropriate σ^?-constants, with a ? value of 1.6. It is suggested that in the transition state of the rate-determining step the OSi bond is fully formed, or almost so, the SiN bond approximately 20—50% broken, and the bond between the nitrogen atom and a proton from the solvent ca. 10—30% formed.     

Aquation of K2 TcBr6 in dilute acid solutions

The photochemical aquation of K₂ TcBr₆ in 2M HRr, 1M HClO₄ and 1M H₂SO₄ has been studied. The absorption spectra of the various Tc(IV) species were measured after the electrophoretic separation. The spectrophotometric changes and the yield of each species as a function of the UV irradiation time were determined. After 25 hours of irradiation of HBr and HClO₄ solutions the main species were the cationic ones (80%) but in H₂SO₄ solution the neutral species reached a yield of 90%. The oxidation of Tc(IV) species to TcO ₄ ^– proceeded more rapidly and extensively in HClO₄ than in HBr and H₂SO₄.     

The protection of uracil and guanine residues in oligonucleotide synthesis

The protection of uracil and 2-$\text{N}$-acyl guanine residues with 4-$\text{O}$-phenyl [or 4-$\text{O}$-(2,4-dimethylphenyl)] and 6-$\text{O}$-(2-nitrophenyl) groups as in $\text{7a}$ [or $\text{7b}$] and $\text{9}$, respectively, is described. These $\text{O}$-aryl protecting groups, which appear to withstand the usual conditions of oligonucleotide synthesis, may readily be removed by treatment with 2-nitrobenzaldoximate ions.     

A simplified synthesis of the hexamethylplatinate(IV) ion

Tetra-n-butylammonium hexachloroplatinate (IV) reacts with lithium methyl/lithium iodide in ether to give a solution containing lithium hexamethylplatinate (IV). With lithium methyl/lithium bromide in ether however, tetrabutylammonium hexamethylplatinate (IV) is precipitated together with lithium halides. Solid [Bu₄N)₂[Pt(Ch₃)₆] is stable under nitrogen at room temperature, but ether solutions of [Pt(Ch₃)₆]^2- decompose in a few minutes at room temperature in the absence of excess lithium methyl.     

A traceless solid-phase synthesis of pteridines

The linking of pyrimidines to polystyrene supports via either a 2- or 4-thioether provides access to pteridines through solid-phase synthesis. Oxidative cleavage (dimethyldioxirane) followed by nucleophilic substitution by amines, azide, or water completes a traceless synthesis of pteridines.     

Investigation of the metabolic fate of 2-, 3- and 4-bromobenzoic acids in bile-duct-cannulated rats by inductively coupled plasma mass spectrometry and high-performance liquid chromatography/inductively coupled plasma mass spectrometry/electrospray mass spectrometry

Inductively coupled plasma mass spectrometry (ICPMS) has been used to determine the rate and routes of excretion of bromine following the intraperitoneal administration (50 mg kg(-1)) of 2-, 3- and 4-bromobenzoic acids to male bile-duct-cannulated rats. Analysis of urine and bile for (79/81)Br using ICPMS showed that all three bromobenzoic acids were rapidly excreted (82-98%) within 48 h of dosing, primarily via the urine. High-performance liquid chromatography/inductively coupled plasma mass spectrometry (HPLC/ICPMS) was then used to obtain metabolite profiles for bile and urine. These profiles revealed that extensive metabolism had taken place, with the unchanged bromobenzoic acids forming a minor part of the total of compound-related material detected. Concomitant MS studies, supplemented by alkaline hydrolysis, enabled the identification of the major metabolite of all three of the bromobenzoic acids as a glycine conjugate. Ester glucuronide conjugates were also identified, but formed only a small proportion of total.     

Chemical Research Faculties: An International Directory : Edited by G.L. Pollock. The American Chemical Society, Washington, 1984, xxxv + ca. 530 pages. US $129.95 (U.S.A. and Canada); US $155.95 (elsewhere). ISBN 0-8412-0817-4.

Heterobinuclear complexes of formula [LMCl₂(pz)M′(tfb)] (M = Ru, L = p-cymene, M′ = Rh; M = Ir, L = C₅Me₅, M′ = Rh; M = Rh, L = C₅Me₅, M′ = Ir) and [(C₅Me₅)IrCl(pz)₂Rh(tfb)] (tfb = tetrafluorobenzo[5.6]bicyclo[2.2.2]octan-2,5,7-triene) have been prepared. The molecular structure of [(p-cymene)Ru(μ-Cl)2(μ-pz)Rh(tfb)] has been determined by X-ray diffraction. It consists of two moieties, (p-cymene)Ru and (tfb)Rh, triply-bridged by a pyrazolate group and two chlorine atoms.