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91.
Potentiometric analyses indicate that previous investigations have overestimated the stability of ferric borate complexes.
The FeB(OH)
4
2+
formation constant result obtained in the present work isBβ
1
*
= [FeB(OH)
4
2+
][H+][Fe3+]-1[B(OH)3]-1 = (5.4±0.3) x 10-3 at 25.0°C and 0.7 molal ionic strength. Our result indicates that solution concentrations of FeOH2+ and FeB(OH)
4
2+
are approximately equal in aqueous solution for boric acid concentrations on the order of 0.3 molal. Fe(B(OH)4)
2
+
is a minor species in solution compared to FeB(OH)4
2+ for conditions such that [B(OH)3][H+]-1≤ 350, and ferric borate complexation is insignificant in solutions such as seawater where [B(OH)3] ≤ 4× 10-4 molal. 相似文献
92.
Complexes of W(CO)(5) with neutral diatomic pnictogen ligands N(2), P(2), As(2), Sb(2), and Bi(2) and anionic Group 14 ligands Si(2) (2-), Ge(2) (2-), Sn(2) (2-), and Pb(2) (2-) coordinated in both side-on and end-on fashion have been optimized by using density functional theory at the BP86 level with valence sets of TZP quality. The calculated bond energies have been used to compare the preferential binding modes of each respective ligand. The results were interpreted by analyzing the nature of the interaction between the ligands and the metal fragment using an energy partitioning method. This yields quantitative information regarding the strength of covalent and electrostatic interactions between the metal and ligand, as well as the contributions by orbitals of different symmetry to the covalent bonding. Results show that all the ligands studied bind preferentially in a side-on coordination mode, with the exception of N(2), which prefers to coordinate in an end-on mode. The preference of the heavier homologues P(2)-Bi(2) for binding in a side-on mode over the end-on mode in the neutral complexes [(CO)(5)WE(2)] comes mainly from the much stronger electrostatic attraction in the former species. The energy difference between the side-on and end-on isomers of the negatively charged complexes with the ligands Si(2) (2-), Ge(2) (2-), Sn(2) (2-), and Pb(2) (2-) is much less and it cannot be ascribed to a particular bonding component. 相似文献
93.
94.
Certain antibiotics such as amphotericin B and nystatin are known to interact selectively with cholesterol in bilayer lipid membranes, resulting in changes in the transmembrane electrical properties. The possibilities for use of this effect in selective trace organic analysis are demonstrated by experiments performed with a simple electrical circuit incorporating a conventional research pH meter. Transmembrane resistance-based responses correlate with aqueous antibiotic concentration and are rapid and reversible. The limit of detection of the technique is approximately 10-9 M of stimulant. 相似文献
95.
Thompson H Wasse JC Skipper NT Hayama S Bowron DT Soper AK 《Journal of the American Chemical Society》2003,125(9):2572-2581
The technique of hydrogen/deuterium isotopic substitution has been used to extract detailed information concerning the solvent structure in pure ammonia and metallic lithium-ammonia solutions. In pure ammonia we find evidence for approximately 2.0 hydrogen bonds around each central nitrogen atom, with an average N-H distance of 2.4 A. On addition of alkali metal, we observe directly significant disruption of this hydrogen bonding. At 8 mol % metal there remains only around 0.7 hydrogen bond per nitrogen atom. This value decreases to 0.0 for the saturated solution of 21 mol % metal, as all ammonia molecules have then become incorporated into the tetrahedral first solvation spheres of the lithium cations. In conjunction with a classical three-dimensional computer modeling technique, we are now able to identify a well-defined second cationic solvation shell. In this secondary shell the nitrogen atoms tend to reside above the faces and edges of the primary tetrahedral shell. Furthermore, the computer-generated models reveal that on addition of alkali metal the solvent molecules form voids of approximate radius 2.5-3.0 A. Our data therefore provide new insight into the structure of the polaronic cavities and tunnels, which have been theoretically predicted for lithium-ammonia solutions. 相似文献
96.
E. J. Land A. Thompson T. G. Truscott K. V. Subbarao M. R. Chedekel 《Photochemistry and photobiology》1986,44(6):697-702
The photochemistries of the melanin precursors dopa, 5-S-cysteinyldopa (5-SCD) and 2.5-S,S'-dicysteinyldopa (2.5-SCD) were investigated by 265-nm laser flash photolysis. The quantum yield of hydrated electron following flash photolysis of dopa (9.1%) was half the yield of dopasemiquinone (19.6%), implying that dopasemiquinone is formed via two primary photochemical mechanisms: photionisation (giving e) or photohomolysis (giving H˙ ). Dopasemiquinone rapidly disproportionates to form dopaquinone and re-form dopa. Dopaquinone in turn decays via a base-catalysed unimolecular cyclisation eventually to form dopachrome. Assignment of the transient species was confirmed by previous pulse radiolysis studies of the one-electron oxidation of dopa. In contrast, flash photolysis of the cysteinyldopas, 5-SCD and 2,5-SCD results in lower photoionisation quantum yields and the production of initial transient species whose absorption spectra were markedly different from their semiquinone absorption spectra previously determined pulse radiolytically. These observations indicate that the primary cysteinyldopa photochemical species is not such a semiquinone, but rather results from S-CH2 bond photohomolysis. Absorption spectra and rate constants for the formation and decay of various transient species are reported. 相似文献
97.
Indium and palladium are determined polarographically with a dropping mercury electrode in a toluene phase, following extraction as the acetylacetonates from aqueous solutions containing 2-10 ppm of the metals (or 0.1-0.5 ppm with a greater ratio of sample volume to solvent volume). Of 31 elements examined, only Co(III), Cr(III), Cu(II), Fe(III), In, Mo(VI), Pd and Ti(IV) gave extractable complexes with reduction waves in the available potential range. Specificity for palladium is obtained by extraction at pH 0.5, and for indium at pH 9 m the presence of cyanide and ascorbic acid as masking agents. 相似文献
98.
Corley M. Thompson Sue G. Taylor William W. McGee 《Journal of polymer science. Part A, Polymer chemistry》1994,32(1):113-120
The kinetics of formation of polyurethane prepolymers is studied by an analytical technique which involves reactive quenching of the isocyanate, separation of the oligomeric species by GPC, and measurement by UV absorbance of the quenched moieties. The precision of the kinetic parameters and the ratios of the oligomers are determined. The effect of changes in reactant stoichiometry on the ratio of oligomers is measured and compared to the value predicted by an equation from Flory. Toluene diisocyanate is shown to give fewer high oligomers than predicted, while methylene diphenylene diisocyanate gives nearly the predicted values. Tetramethylxylene diisocyanate gives more high oligomers than predicted, an unexpected but possibly important result. Catalyst is shown to increase the reaction rate of the last by more than 200 times. © 1994 John Wiley & Sons, Inc. 相似文献
99.
A 90% () mixture of acetophenone-chloroform is an effective extractant for reduced molybdoantimonylphosphoric acid. The extraction is quantitative over the acidity range of 0.04 to 4.8 N sulfuric acid. The mixed solvent extractant is virtually immiscible with water and can be used at aqueous/organic volume ratios from 1:1 to 40:1 with no loss of solvent or decrease in % extraction. Phosphate is easily determined from 5 to 1000 ppb. No interference from silicate was experienced. 相似文献
100.
In vitro high throughput screening of compounds for favorable metabolic properties in drug discovery
Masimirembwa CM Thompson R Andersson TB 《Combinatorial chemistry & high throughput screening》2001,4(3):245-263
Drug metabolism can have profound effects on the pharmacological and toxicological profile of therapeutic agents. In the pharmaceutical industry, many in vitro techniques are in place or under development to screen and optimize compounds for favorable metabolic properties in the drug discovery phase. These in vitro technologies are meant to address important issues such as: (1) is the compound a potent inhibitor of drug metabolising enzymes (DMEs)? (2) does the compound induce the expression of DMEs? (3) how labile is the compound to metabolic degradation? (4) which specific enzyme(s) is responsible for the compound's biotransformation? and (5) to which metabolites is the compound metabolized? Answers to these questions provide a basis for judging whether a compound is likely to have acceptable pharmacokinetic properties in vivo. To address these issues on the increasing number of compounds inundating the drug discovery programs, high throughput assays are essential. A combination of biochemical advances in the understanding of the function and regulation of DMEs (in particular, cytochromes P450, CYPs) and automated analytical technologies are revolutionizing drug metabolism research. Automated LC-MS based metabolic stability, fluorescence, radiometric and LC-MS based CYP inhibition assays are now in routine use. Automatible models for studying CYP induction based on enzyme activity, quantitative RT-PCR and reporter gene systems are being developed. We will review the utility and limitations of these HTS approaches and highlight on-going developments and emerging technologies to answer metabolism questions at the different stages of the drug discovery process. 相似文献