The sensitized photodehydrochlorination of poly(vinyl chloride)—II. Model compound studies

p-Cresol, a model compound for an additive shown previously to be a potent photosensitizer of dehydrochlorination in poly(vinyl chloride) PVC, has been shown to sensitize dehydrochlorination in t-butyl chloride, a model for PVC. A quantitative study of the effect of chlorinated alkanes upon the fluorescence of p-cresol in solution reveals that quenching is due to charge transfer interactions, the halogenated compound acting as electron acceptor. Since the mono-halo compound t-butyl chloride does not quench p-cresol fluorescence, it was concluded that the photosensitization occurs via the triplet state of the p-cresol, and this view was confirmed by addition of triplet quenchers. A mechanism for the reaction is proposed, initial exciplex formation being followed by electron transfer.     

Identification of ligand binding sites on proteins using a multi-scale approach

Identification of a ligand binding site on a protein is pivotal to drug discovery. To date, no reliable and computationally feasible general approach to this problem has been published. Here we present an automated efficient method for determining binding sites on proteins for potential ligands without any a priori knowledge. Our method is based upon the multiscale concept where we deal with a hierarchy of models generated using a k-means clustering algorithm for the potential ligand. This is done in a simple approach whereby a potential ligand is represented by a growing number of feature points. At each increasing level of detail, a pruning of potential binding site is performed. A nonbonding energy function is used to score the interactions between molecules at each step. The technique was successfully employed to seven protein-ligand complexes. In the current paper we show that the algorithm considerably reduces the computational effort required to solve this problem. This approach offers real opportunities for exploiting the large number of structures that will evolve from structural genomics.     

Triorganotin aryl selenides

The SnSe bond in trimethyltin aryl selenides is cleaved on reaction with selenenyl halides, sulphenyl halides, alkyl halides and allyl halides. Thus Me₃SnSePh reacts with PhSeCl, 4-Me-2-NO₂C₆H₃SCl, RI (R = CH₃ or Ph₃SnCH₂) and CH₂CHCH₂Br to give PhSeSePh, 4-Me-2-NO₂C₆H₃SSePh, RSePh and CH₂CHCH₂SePh, respectively. The diselenide, PhSeSePh, is also obtained from Me₃SnSePh on reaction with either 4-MeC₆H₄SO₂Cl or NaIO₄. Exchange reactions also occur between Me₃SnSePh and Ph₃SnCl or PhHgCl.     

Determination of intrinsic bacterial surface acidity constants using a donnan shell model and a continuous pK(a) distribution method

Intrinsic acidity constants (pK(a)(int)) for Bacillus subtilis (Gram+) and Escherichia coli (Gram-) cells were calculated from potentiometric titration data at different salt concentrations. Master curves were generated by replotting charge excess data as a function of pH(S) (pH at the location of surface reactive sites) where pH(S) was determined as a function of Donnan potential, Psi(DON). This potential decreased in magnitude with increasing ionic strength, from -48.5+/-0.2 to -3.5+/-0.0 mV for B. subtilis and -47.9+/-0.3 to -3.5+/-0.0 mV for E. coli at 0.01 and 0.5 M K(+), respectively, indicating an efficient surface charge neutralization by counterions. A fully optimized continuous (FOCUS) pK(a) distribution method revealed four binding sites on B. subtilis and E. coli surfaces from the master curves with pK(a)(int) values of 3.59+/-0.38, 4.33+/-0.57, 5.94+/-0.66, and 8.64+/-0.57 for B. subtilis and 3.73+/-0.44, 4.85+/-0.71, 6.56+/-0.64, and 8.79+/-0.62 for E. coli. These were assigned to functional groups according to reported pK(a) ranges of 2.0-6.0 (carboxylic acid), 3.2-3.5 (phosphodiesters), 5.6-7.2 (phosphoric acid), and 9.0-11.0 (amine groups). Average points of zero salt effect (pH(pzse)) for B. subtilis experiments were 6.63+/-0.21 and 6.42+/-0.08 as a function of pH(bulk) and pH(S), respectively. Under the same criteria, E. coli calculations yielded 5.73+/-0.23 and 5.45+/-0.05. An understanding of metal and proton reactivity on bacterial cell surfaces can be addressed quantitatively through the use of electrostatic and chemical equilibrium modeling techniques proposed in this study. The results are consistent with those of electrical force microscopy studies used to document the intrinsic electrochemical heterogeneity of bacterial cell surfaces.     

Radiation-induced redox speciation of77Br in aqueous solution as determined by anion chromatography

⁷⁷Br is of potential use in diagnostic nuclear medicine as the parent of ultrashort-lived^77mSe in a radioisotopic generator system. The chemical form(s) of the radiobromine in aqueous solution, and their radiation-induced chemical transformations, are of importance for biomedical generator development. Approximately 150 mCi of radiobromine, initially present as Br^–, was studied over a period of three weeks and analyzed for BrO _x ^– (x=0–4) by anion exchange chromatography. Radiolysis of the solution gave rise to a small quantity of BrO ₃ ^– , but 99.5% of the activity remained as bromide.     

SCF theory of molecular interactions

Perturbation theory is used to analyze the interactions between two closed-shell systems in the SCF approximation. The effects of orbital overlap are included. The dominant terms through second order are identified, including electrostatic, penetration, polarization, charge transfer, and SCF dispersion effects. The basis set limit is studied so that basis set superposition contributions may be identified. Calculations are presented for He? He and Na⁺? H₂O.     

α,ω-Disubstituted polymethylpolyphosphonates and polyphenylpolyphosphonates from condensation polymerization

Condensation polymerization of phosphonates through formation of P? O? P linkages has been achieved by (1) volatilization of methyl chloride from mixtures of CH₃P(O)Cl₂ with CH₃P(O)(OCH₃)₂; (2) volatilization or chemical removal of water from CH₃P(O)(OH)₂; and (3) volatilization of HCl from mixtures of CH₃P(O)Cl₂ with CH₃P(O)(OH)₂ or C₆H₅P(O)Cl₂ with C₆H₅P(O)(OH)₂. Depending on the proportions of the reagents, the polymerization products consist of various mixtures of chain molecules of the type \documentclass{article}\pagestyle{empty}\begin{document}${\rm X \hbox{--} P}({\rm O})({\rm R})\rlap{--}[{\rm O \hbox{--} P}({\rm O})({\rm R})\rlap{--}]_n {\rm X}$\end{document} for R = CH₃ and X = OCH₃, Cl, or OH, or for R = C₆H₅, x = Cl or OH. ³¹P nuclear magnetic resonance (NMR) was used to investigate both the polymethylpolyphosphonates and the polyphenylpolyphosphonates; and ¹H NMR of the CH₃P and CH₃O moieties was also used to study the polymethylpolyphosphonates. In the methoxyl-terminated polymethylpolyphosphonates, which was the system studied most extensively, no detectable amounts of cyclic molecules were found at equilibrium, but a crystalline methylphosphonic anhydride, CH₃PO₂, exhibited some ring structures. The equilibrium size distributions gave evidence that the sorting of the mono- and difunctional phosphorus-based units making up the oligomeric chains is affected by neighboring units. Kinetic measurements demonstrated that the condensation polymerization is a complicated process involving considerable scrambling of terminal groups with bridging oxygen atoms.     

DNA binding and biological activity of some platinum(II) intercalating compounds containing methyl-substituted 1,10-phenanthrolines

This study documents the first detailed investigation into the relationship between molecular structure and biological activity of platinum(II) complexes containing methylated derivatives of 1,10-phenanthroline (phen). A series of square planar platinum(II) compounds incorporating methylated derivatives of phen, 4-methyl-1,10-phenanthroline (4-Mephen), 5-methyl-1,10-phenanthroline (5-Mephen), 4,7-dimethyl-1,10-phenanthroline (4,7-Me2phen), 5,6-dimethyl-1,10-phenanthroline (5,6-Me2phen) and 3,4,7,8-tetramethyl-1,10-phenanthroline (3,4,7,8-Me4phen) were synthesised and the relationship between their structure and biological activity investigated. The biological activity of these compounds was quantified using the in vitro cytotoxicity assay against the L1210 Murine leukaemia cell line. Large variation in cytotoxicities with different methylation was observed. The 5- and 5,6-methylated derivatives of phen displayed a greater biological activity, with IC50 values of 2.8 +/- 0.8 microM and 1.5 +/- 0.3 microM respectively, compared with the phen compound, with an IC50 value of 9.7 +/- 0.3 microM, while all the others were inactive with IC50 values over 50 microM. Binding constants were determined using circular dichroism spectroscopy (CD) and induced circular dichroism (ICD). ICD was used to highlight any differences in the spectra. Viscometry studies and linear dichroism (LD) experiments indicate that the platinum(II) complexes intercalate although for [Pt(en)(4-Mephen)]Cl2 and [Pt(en)(4,7-Me2phen)]Cl2 this mode of binding appears to be concentration dependent. The binding of the platinum(II) complexes to the oligonucleotide d(GTCGAC)2 was studied using two-dimensional 1H NMR spectroscopy. The addition of each metal complex to the hexamer d(GTCGAC)2 produced upfield shifts of the metal complex resonances, characteristic of intercalation. Through the observation of NOE cross-peaks, two-dimensional NMR studies provided insight into the site and groove preferences of these compounds when binding to DNA.