Application of the TORO technique of 1H NMR to the structural analysis of cyclic peptide isomers having a slightly distorted symmetry from C2

The extended TORO technique was applied to the structural analysis of endo-D-Tyr-gramicidin S, cyclo(-Val-Orn-Leu-D-Phe-D-Tyr-Pro-Val-Orn-Leu-D-Phe-Pro-), which has a slightly distorted symmetry from C2, by the insertion of D-Tyr and equivalent alpha-proton chemical shifts in the 1H NMR spectrum. All NMR signals of the two dominant isomers of this antibiotic with trans-trans prolines were determined by using the extended TORO technique with TOCSY and ROESY spectra. This technique is generally applicable for distinguishing overlapped signals of alpha- and amide protons from the main chains of peptides.     

Synthesis of symmetric HIJ-ring model of ciguatoxin

A tricyclic model ether compound comprising 6/8/6 ether rings was synthesized via a new route including conjugate addition to form a symmetric eight-membered ether-ketone with syn/trans stereochemistry in selective manner. The corresponding vinyl triflate of this ketone was allowed to convert to the vinyl methyl derivative via cross-coupling reaction. This endo-olefinic tetrahydro-2H-oxocin was selectively reduced to afford α-methyl product. The corresponding exo-olefinic oxocane derivative, on the other hand, provided the β-methyl isomer as the major diastereoisomer (2/1). Our previous report on the synthesis of this final product was revised due to some rearrangement and the mechanisms are discussed.     

Photosensitivity and photochemical reactions of poly(vinyl p-azidocinnamate)

The spectral sensitivity of poly(vinyl p-azidocinnamate)(PVACi) was discussed by comparison with poly(vinyl cinnamate)(PVCi) and poly(vinyl p-azidobenzoate). The photochemical reaction of PVACi was investigated with respect to the reactivity of two functional groups, namely, azido and cinnamoyl groups. It was found that cinnamoyl groups dimerized like PVCi and azido groups did not add to double bonds appreciably. The quantum yield of photolysis of azido groups was two times that of dimerization of cinnamoyl groups. A marked wavelength dependence of π–π^* and n–π^* absorption was observed in both the reactions of azido and cinnamoyl groups.     

Solvent extraction of microgram amounts of magnesium with 8-quinolinol and tetrabutyl-ammonium iodide followed by spectrophotometry with chlorophosphonazo-III

A rapid and sensitive solvent-extraction procedure for the separation of magnesium is reported. Microgram (0.1–10) amounts of magnesium are extracted with a chloroform solution of 8-quinolinol and tetrabutylammonium iodide in the presence of tartrate and phosphate. Magnesium is then back-extracted into an aqueous buffer solution (pH 7.3; tetrabutylammonium hydroxide—boric acid) and determined spectrophotometrically using chlorophosphonazo-III. Up to 500 mg of sulphate, phosphate or cyanide, 200 mg of chloride, 20 mg of aluminum, barium or silicate, and 2 mg of calcium can be tolerated.     

Peroxyoxalate chemiluminescence of N,N'-bistosyl-1H,4H-quinoxaline-2,3-dione and related compounds. Dependence on electronic nature of fluorophores

The title compound N,N'-bistosyl-1H,4H-quinoxaline-2,3-dione (TsQD) provides peroxyoxalate chemiluminescence (PO-CL) when reacted with hydrogen peroxide in the presence of fluorophores. The chemiluminescence (CL) efficiency of TsQD was superior to that of other related compounds such as bis(2,4,6-trichlorophenyl) oxalate (TCPO), a typical oxalate for the peroxyoxalate PO-CL, under an aqueous condition. Factors affecting the PO-CL efficiency are discussed from the viewpoint of the structures of the substrates and the electronic nature of the fluorophores. A linear correlation of the logarithmic values evaluated from the CL quantum yields with the oxidation potentials of the aromatic fluorophores supports the involvement of the chemically initiated electron exchange luminescence (CIEEL) mechanism in both TsQD- and TCPO-CL systems. Also, an excellent Hammett relationship was derived from the correlation between the sigma(+) values and the relative singlet excitation yields in TsQD-CL enhanced by a series of fluorescent para,para'-disubstituted distyrylbenzenes.     

One-milliliter wet-digestion for inductively coupled plasma mass spectrometry (ICP-MS): determination of platinum-DNA adducts in cells treated with platinum(II) complexes

Platinum (Pt)–DNA adducts formed by the anti-tumor agent cisplatin are recognized by the DNA mismatch repair (MMR) system. To investigate the involvement of MMR proteins including hMLH1 in the removal of these adducts, we developed a mL-scale wet-digestion method for inductively coupled plasma mass spectrometry (ICP-MS). The detection limit was 0.01 ng mL^–1 Pt, which corresponded to 2 pg Pt/g DNA when 10 g of DNA was used. The mean relative errors were 5.4% or better for a dynamic range of 0.01–10 ng mL^–1 Pt. DNA (~500 g) had no matrix effect. To improve the accuracy, DNA preparations were treated with ribonuclease and the apparent reduction in the concentration of Pt was corrected using cellular DNA levels, which were determined with Hoechst 33258. No significant differences were observed, in terms of the formation of Pt–DNA adducts or their removal over 6 h, between hMLH1-deficient HCT116 cells, a human colorectal cancer cell line, and hMLH1-complemented HCT116+ch3 cells (n=5; P>0.05), indicating that the hMLH1-dependent DNA repair systems contribute to neither the formation nor the removal of the adducts at detectable levels. In addition, approximately 19% of the adducts were removed within 6 h in both cell lines. A time course analysis (~24 h) suggested that the removal of cisplatin-generated Pt–DNA adducts follows first-order kinetics (t_1/2=32 h). The amount of Pt–DNA adduct formed by oxaliplatin in 1 h was 56% (ratio of means) of that generated by an equimolar concentration of cisplatin in HCT116. The proposed procedure could be useful for determining Pt–DNA adducts formed by Pt(II) complexes.     

Development of a chemical structure comparison method for integrated analysis of chemical and genomic information in the metabolic pathways

Cellular functions result from intricate networks of molecular interactions, which involve not only proteins and nucleic acids but also small chemical compounds. Here we present an efficient algorithm for comparing two chemical structures of compounds, where the chemical structure is treated as a graph consisting of atoms as nodes and covalent bonds as edges. On the basis of the concept of functional groups, 68 atom types (node types) are defined for carbon, nitrogen, oxygen, and other atomic species with different environments, which has enabled detection of biochemically meaningful features. Maximal common subgraphs of two graphs can be found by searching for maximal cliques in the association graph, and we have introduced heuristics to accelerate the clique finding and to detect optimal local matches (simply connected common subgraphs). Our procedure was applied to the comparison and clustering of 9383 compounds, mostly metabolic compounds, in the KEGG/LIGAND database. The largest clusters of similar compounds were related to carbohydrates, and the clusters corresponded well to the categorization of pathways as represented by the KEGG pathway map numbers. When each pathway map was examined in more detail, finer clusters could be identified corresponding to subpathways or pathway modules containing continuous sets of reaction steps. Furthermore, it was found that the pathway modules identified by similar compound structures sometimes overlap with the pathway modules identified by genomic contexts, namely, by operon structures of enzyme genes.     

Kinetics of ring-opening radical polymerization of vinyloxiranes

Ring-opening radical polymerization has been followed by the flash photolysis method. As an initiator radical, the arylthiyl radical (ArS^˙) was used; the addition reaction rate constants of ArS^˙ to vinyl monomer containing epoxy-ring such as vinyloxiranes have been determined at first. The rate constant of ring opening reaction of the adduct radical changing to ArSCH₂CH ? CROCH₂^˙ were determined in the form of the relative values, which were converted to the absolute values. The ring-opening reaction rate constants were ca. 10⁶ s^?1, which indicates that the ring-opening rates are faster than usual radical propagation rates. The rate constants for vinyl monomer with 5-member ring (1,4-epoxy-1,4-dihydronaphthalene) were similarly evaluated.     

Liquid-phase microextraction of tributyltin and triphenyltin coupled with gas chromatography-tandem mass spectrometry. Comparison between 4-fluorophenyl and ethyl derivatizations

This paper describes the liquid-phase microextraction (LPME) of tributyltin (TBT) and triphenyltin coupled with gas chromatography-tandem mass spectrometry. The 4-fluorophenylation and ethylation reactions were used for the derivatization of the organotins. For the two derivatizations, the LPME parameters such as organic solvent, stirring rate, temperature, extraction time and the other additional conditions were examined. Using pure water, the calibration curves, method detection limits (MDLs) and reproducibilities (RSDs) of the two derivatizations were compared under the respective optimized procedures. The 4-fluorophenyl derivatization, which showed a lower MDL (0.36 ng/l) and better reproducibility (RSD = 11% at 10 ng/l) for TBT, was applied to the analysis of seawater. The TBT was detected in the range from 1.1 to 2.0 ng/l in the seawater samples collected in Osaka Bay.