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51.
T. Melvin M. A. Plumb S. W. Botchway P. O'Neill A. W. Parker 《Photochemistry and photobiology》1995,61(6):584-591
Irradiation of DNA with 193 nm light results in monophotonic photoionization, with the formation of a base radical cation and a hydrated electron (φP1 = 0.048–0.065). Although >50% of the photoionization events initially occur at guanine in DNA, migration of the “hole” from the other bases to guanine occurs to yield predominantly its radical cation or its deprotonated form. From sequence analysis, the data reveal that 193 nm light induces single strand breaks (ssb) in double-stranded DNA preferential 3’ to a guanine residue. However, it has previously been reported that 193 nm light yields very low yields of ssb (<2% of the yield of eaq). The distribution of these ssb at guanine is nonrandom, showing a dependence on the neighboring base moiety. The efficiency of ssb formation at nonguanine sites is estimated to be at least one order of magnitude lower. The preferred cleavage at guanine is consistent with migration and localization of the electron loss center at guanine. It is argued that singlet oxygen and the photoionized phosphate group of the sugar moiety are not major precursors to ssb. At present, the mechanisms of strand breakage are not known although a guanine radical or one of its products remain potential precursors. 相似文献
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This paper proves a strong convergence theorem for sequences of pseudo-holomorphic maps from a Riemann surface to a symplectic
manifoldN with tamed almost complex structure. (These are the objects used by Gromov to define his symplectic invariants.) The paper
begins by developing some analytic facts about such maps, including a simple new isoperimetric inequality and a new removable
singularity theorem.
The main technique is a general procedure for renormalizing sequences of maps to obtain “bubbles on bubbles.” This is a significant
step beyond the standard renormalization procedure of Sacks and Uhlenbeck. The renormalized maps give rise to a sequence of
maps from a “bubble tree”—a map from a wedge Σ V S2 V S2 V ... →N. The main result is that the images of these renormalized maps converge in L1,2 ∪C° to the image of a limiting pseudo-holomorphic map from the bubble tree. This implies several important properties of the
bubble tree. In particular, the images of consecutive bubbles in the bubble tree intersect, and if a sequence of maps represents
a homology class then the limiting map represents this class. 相似文献
56.
Non-steady-state kinetic studies reveal that the elimination of HBr from 2-(p-nitrophenyl)ethyl bromide in alcohol/alkoxide media, the classical concerted E2 reaction, actually takes place by a two-step mechanism involving the intermediate formation of the carbanion. 相似文献
57.
Bruckbauer A Ying L Rothery AM Zhou D Shevchuk AI Abell C Korchev YE Klenerman D 《Journal of the American Chemical Society》2002,124(30):8810-8811
We present a new, general method for the controlled deposition of biological molecules on surfaces, based on a nanopipet operating in ionic solution. The potential applied to the pipet tip controls the flux of biological molecules from the pipet, allowing fine control of the delivery rate. We used the ion current to control the distance of the pipet from the surface of a glass slide and deposited the fluorescently labeled DNA or protein G at a defined location onto the surface. Features of 830 nm size were obtained by depositing the biotinylated DNA onto a streptavidin surface; 1.3 mum size spots were obtained by depositing protein G onto a positively charged glass surface. 相似文献
58.
Naphthoquinone , a fully functionalized model for intermediates in ansamycin synthesis, was prepared by the intramolecular Claisen condensation of benzofuran followed by oxidation of the novel tricyclic compound . 相似文献
59.
Chatterjee B Noveron JC Resendiz MJ Liu J Yamamoto T Parker D Cinke M Nguyen CV Arif AM Stang PJ 《Journal of the American Chemical Society》2004,126(34):10645-10656
The syntheses, structures, and N2 adsorption properties of six new supramolecular metallacycles are reported. Flexible ditopic linkers, 1-4, with systematically varied lengths and conformational degrees of freedom were synthesized utilizing ester linkages. They were used in combination with (dppp)M(OTf)2, where M = Pt(II) and Pd(II), and cis-(Me3P)2Pt(OTf)2 to form flexible supramolecular metallacycles 5-10 in 88-98% isolated yields. Their structures were characterized via multinuclear NMR and X-ray crystallography. The metallacycles stack to form porous structures in the crystalline state. The pore dimensions depend on both the phosphorus ligands attached to the metals and the flexible linkers. Adsorption studies on the porous materials show that 5a, 6, 8, and 9 held 11.7, 16.5, 5.7, and 6.8 cm3/g STP of N2 at 77 K, respectively. A guest-exchange study with nitromethane and toluene reveals that the nanopore in 5 is flexible, a property which was transferred from the linker to the supramolecular structure in the solid state. 相似文献
60.
K.J.L. Paciorek J. Kaufman J.H. Nakahara T.I. Ito R.H. Kratzer R.W. Rosser J.A. Parker 《Journal of fluorine chemistry》1977,10(4):277-288
2,5-Bis(perfluoro-n-heptyl)-, 2-perfluoroalkylether-5-perfluoro-n-heptyl-, and 2,5-bisperfluoroalkylether-1,3,4-oxadiazoles were synthesized and characterized. 2,5-Bis(perfluoro-n-heptyl)-1,3,4-oxadiazole was thermally and hydrolytically stable at 325°C; however, in the presence of air, degradation took place at 235°C. The perfluoroalkylether analogue exhibited thermal and hydrolytic stability at 325°C; it was found to be unaffected by Jet-A fuel and air at 235°C. At 325°C in air some degradation occured as evidenced by volatiles production, oxygen consumption, and 96% starting material recovery. 相似文献