排序方式: 共有38条查询结果,搜索用时 31 毫秒
1.
Abstract Photoreactions, such as base release and decomposition of the base moiety, induced by either 20 ns laser pulses at 193 nm or continuous 254 nm irradiation, were studied for a series of uracil and adenine derivatives in neutral aqueous solution. The quantum yield of chromophore loss (φ) depends significantly on the nature of the nucleic acid constituent and the saturating gas (Ar, N2O or O2). In the case of polynucleotides the destruction of nucleotides was measured by high-performance liquid chromatography after hydrolysis; the quantum yields (φ) are comparable to those of chromophore loss or larger. The φcl and aφdn of 0.04–0.1 for poly(U) and poly(dU), obtained for both wavelengths of irradiation, are due to processes originating from the lowest excited singlet state, i.e. formation of photohydrates and photodimers, and a second part from photoionization using λirr= 193 nm. Irradiation at 193 nm effectively splits pyrimidine dimers and thus reverts them into monomers. The quantum yield for release of undamaged bases (φbr) from nucleosides, nucleotides and polynucleotides upon irradiation at 254 nm is typically φbr= (0.1–1) × 10?4 Breakage of the N-glycosidic bond is significantly more efficient for λirr=193 nm, e.g. φbr= 1.1 × 10?3, 0.8 × 10?3, 4.3 × 10?3 and 0.5 × 10?3 for poly(A), poly(dA), poly(U) and poly(dU) in Ar-saturated solution, respectively. Enhanced φ values for λirr= 193 nm, essentially for adenine and its derivatives, are caused by photo-processes that are initiated by photoionization. 相似文献
2.
3.
The catalytic mechanisms of transition-metal compounds during the hydrogen sorption reaction of magnesium-based hydrides were investigated through relevant experiments. Catalytic activity was found to be influenced by four distinct physico-thermodynamic properties of the transition-metal compound: a high number of structural defects, a low stability of the compound, which however has to be high enough to avoid complete reduction of the transition metal under operating conditions, a high valence state of the transition-metal ion within the compound, and a high affinity of the transition-metal ion to hydrogen. On the basis of these results, further optimization of the selection of catalysts for improving sorption properties of magnesium-based hydrides is possible. In addition, utilization of transition-metal compounds as catalysts for other hydrogen storage materials is considered. 相似文献
4.
Vahan V. Mkrtchyan Samvel S. Petrosyan Gagik N. Vardanyan 《Discrete Mathematics》2010,310(10-11):1588-1613
For and a cubic graph let denote the maximum number of edges that can be covered by matchings. We show that and . Moreover, it turns out that . 相似文献
5.
Gagik G. Melikyan Boghos Mikailian Ruth Sepanian Pogban Toure 《Journal of organometallic chemistry》2009,694(5):785-3562
The novel high-temperature reaction of Co2(CO)6-complexed propargyl cations, occurring in the spontaneous and stereoselective (90-97% d,l-) manner and yielding the radical dimers, d,l-3,4-diaryl-1,5-hexadiynes (2, 11, 14), is described. Despite the alleged thermal lability and delicate nature of the requisite Co2(CO)6-complexed propargyl cations, the reaction temperature can be elevated from 20 °C to 147 °C, shortening the reaction time from 660 min to less than 1 min. Isotopic enrichment experiments detected (MS TOF/ESI/APCI) an incorporation of upto, eight 13CO ligands into the metal core, suggesting a single electron, cluster-to-cluster reduction along the reaction coordinate. The reaction kinetics is found to be sensitive toward the electronic nature of the substituents (H, OMe) and to the substitution pattern (0-, 4-, 3,4,5-) on the periphery of the aromatic ring. Calculation data suggest that the spontaneous transfer of a single electron from the metal cluster, onto a π-bonded propargyl moiety, is dependent upon the negative charge on aromatic C1′ carbon atom, located alpha to the cationic center. The spontaneous conversion of diamagnetic species (propargyl cation) to paramagnetic counterparts (propargyl radical) indicates that the π-bonded organometallic cations can act as the prototypes for transition metal-based thermal sensors. Their application in photochemical research, electronic devices, molecular electronics, and biomedical fields can also be envisioned. 相似文献
6.
Gagik G. Melikyan Erin Voorhees Christopher Wild Ryan Spencer Justin Molnar 《Tetrahedron letters》2010,51(17):2287-3468
The phenyl group incorporated into a carbon tether provides for the synthesis of 1,5-cyclodecadiynes with 95-100% d,l-diastereoselectivity due to intrinsic conformational constraints and preorganization of the Co2(CO)6-complexed propargyl intermediates. 相似文献
7.
Melikyan GG Villena F Sepanian S Pulido M Sarkissian H Florut A 《Organic letters》2003,5(19):3395-3397
[reaction: see text] The spontaneous generation and stereoselective coupling of Co(2)(CO)(6)-complexed propargyl radicals have been discovered. One- and two-step complementary methods (Method A: (1) HBF(4); (2) CH(2)Cl(2), 20 degrees C; Method B: Tf(2)O, CH(2)Cl(2), 20 degrees C) provided an easy access to synthetically useful d,l-3,4-diaryl-1,5-alkadiynes (de 74-98%). 相似文献
8.
Aqueous solutions of plasmid (pBR322 and pTZ18R) and calf thymus DNA were excited by 20 ns laser pulses at 193 nm. The quantum yields of single- and double-strand break formation, interstrand cross-links, locally denatured sites, (6–4)photoproducts and biological inactivation (Φssb , Φdsb , Φicl , Φids , Φ6–4 and Φina , respectively) were measured. The quantum yields are virtually independent of intensity, demonstrating a one-quantum process. The obtained values in aerated neutral solution in the absence of additives are Φssb = 1.5 × 10- -3 , Φdsb , = 0.06 × 10- -3 (dose: 10–200 J m-2 ), Φicl =ΦIds = 0.1 × 103 and Φ6–4 = 0.5 × 10- -3 Both Φssb and Φdsb decrease strongly with increasing concentrations of TE buffer (0.01–10 m M ). Biological inactivation of the pTZ18R plasmid was determined from the transformation efficiency of Escherichia coli bacteria strains AB1157, AB1886 uvr and A82480 uvr rec; the Φina values are 1.4 × 103 , 2.1 × 103 and 3 × 10- -3 , respectively. The monoexponential survival curves in all cases show that a single damage site leads to inactivation (one single hit). The biological consequences of different photoproducts are discussed. 相似文献
9.
Gagik G. Gurzadyan Helmut Gorner Dietrich Schulte-Frohlinde 《Photochemistry and photobiology》1993,58(4):477-485
Plasmid pTZ18R and calf thymus DNA in aerated neutral aqueous solution were irradiated by continuous 254 nm light. The quantum yields are φssb= 4.0 × 10-5 and φdsb= 1.4 × 10-6 for single- and double-strand break formation, respectively, φbr= 2.3 × 10-5 for base release, φdn= 2.1 × 10-3 for destruction of nucleotides, and φicl×φlds× 1 × 10-6 for interstrand cross-links and locally denatured sites, respectively. The presence of Tris-HCI/ ethylenediaminetetraacetic acid (10:1, pH 7.5) buffer strongly reduces φssb, The corresponding φ values, obtained on employing pulsed 193 nm laser irradiation, are much larger than those using λirr, = 254 nm. This is ascribed to a contribution of chemical reactions induced by photoionization, which is absent for 254 nm irradiation. The quantum yields of inactivation of plasmid DNA (λirr= 254 nm) were measured by transformation of the Escherichia coli strains AB1157 (wild type), φina(1157) = 1.6 × 10-4, AB1886 (uvr-), φina(1886) = 4.2 × 10-4, AB2463 (rec-), φina(2463) = 4.1 × 10-4 and AB2480 (uvr- rec-), φina(2480) = 3.1 × 10-3. The quantum yields of inactivation of plasmid DNA are compared with those of the four E. coli strains (denoted as chromosomal DNA inactivation) obtained from the literature. The results for E. coli strain AB2480 show that the chromosomal DNA and the plasmid DNA are both inactivated by a single pyrimidine photodimer per genome. With the E. coli strain AB2463 inactivation of plasmid and chromosomal DNA is the same for the same total damage per genome and is ~ 10 times smaller than for AB2480. This is explained by photodimer repair in chromosomal and plasmid DNA and by the absence of dsb repair in both cases. In the repair wild-type strain AB1157, inactivation of the plasmid DNA is roughly 100 times higher than that of the chromosomal DNA. We postulate that a portion of this difference is due to repair of dsb by the recA system in chromosomal DNA and that such repair does not take place in the plasmid DNA. The biological results from 254 nm irradiation are compared with those from 193 nm laser irradiation. 相似文献
10.
The TiCl4-mediated formal [3+3] cyclocondensation of 1,3-bis(trimethylsilyloxy)-1,3-butadienes with 1,1-dimethoxy-4,4-dichlorobut-1-en-3-one afforded a variety of functionalized 6-dichloromethyl-4-methoxysalicylates with very good regioselectivity. Some of the products were transformed into 6-formyl-4-methoxysalicylates. The employment of Me3SiOTf instead of TiCl4 resulted in a change of the regioselectivity and in the formation of functionalized 2-(dichloromethyl)pyran-4-ones. 相似文献