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991.
The synthesis of the alkylating subunit of the DNA cross-linking agent, isochrysohermidin (2), and its subsequent incorporation into conjugates with distamycin A (1) are described. The DNA binding properties of these agents were compared to that of distamycin A, using a fluorescence intercalator displacement (FID) assay. 相似文献
992.
Gamble BM Gallagher PA Shoemaker JA Parks AN Freeman DM Schwegel CA Creed JT 《The Analyst》2003,128(12):1458-1461
This paper evaluates the chemical stability of four arsenosugars using tetramethylammonium hydroxide (TMAOH) as an extraction solvent. This solvent was chosen because of the near quantitative removal of these arsenicals from difficult to extract seafood (oysters and shellfish). Four arsenosugars (3-[5'-deoxy-5'-(dimethylarsinoyl)-beta-ribofuranosyloxy]-2-hydroxypropylene glycol--As(328), 3-5'-deoxy-5'-(dimethylarsinoyl)-beta-ribofuranosyloxy]-2-hydroxypropanesulfonic acid--As(392), 3-[5'-deoxy-5'-(dimethylarsinoyl)-beta-ribofuranosyloxy]-2-hydroxypropyl hydrogen sulfate--As(408), and 3-[5'-deoxy-5'-(dimethylarsinoyl)-beta-ribofuranosyloxy]-2-hydroxypropyl-2,3-hydroxypropyl phosphate--As(482)) were evaluated. The stability of these four arsenosugars were studied independently in a solution of 2.5% TMAOH at 60 degrees C over a period of up to 8 h. Two arsenosugars, As(328) and As(392), were found to be relatively stable in this solution for up to 8 h. However, As(408) and As(482) formed detectable quantities of dimethylarsinic acid (DMAA) and As(328) within 0.5 and 2 h, respectively. It was found that 97% of As(408) degrades after 8 h of treatment producing 3.4 times as much DMAA as As(328). This is contrary to As(482), which produces 13 times as much As(328) as DMAA and only 37% of the As(482) was converted by the 8 h treatment at 60 degrees C. These degradation products led to the investigation of weaker TMAOH extraction solvents. Three different concentrations (2.5%, 0.83% and 0.25%) were used to determine the effect of TMAOH concentration on the degradation rate of As(408). By reducing the TMAOH concentration to 0.83%, the conversion of the arsenosugar to As(328) and DMAA is nearly eliminated (less than 5% loss). Arsenosugars, As(408) and As(482), were also studied in 253 mM NaOH to verify the degradation products. The NaOH experiments were conducted to investigate a possible hydroxide based reaction mechanism. Similar degradation plots were found for each arsenosugar when compared to the 2.5% TMAOH data. A mechanism has been proposed for the formation of As(328) from As(408) and As(482) in base via an SN2 reaction (hydroxide attack) at the side chain carbon adjacent to the inorganic ester. The formation of DMAA is observed in all arsenosugars after prolonged exposure. This probably occurs via an SN2 attack at the arsenic atom. 相似文献
993.
Facile synthesis of cis and trans olefinic analogues of oxytocin 1 that have carbon in place of sulfur is achieved via ring-closing metathesis (RCM) on a resin-bound linear precursor peptide. Hydrogenation of the cis olefin, 3, proceeds selectively to generate the previously reported saturated derivative 5. Biological testing on rat uterus strips shows that cis compound 3 has an EC50 value of 38 ng/mL (EC50 for oxytocin is 2.7 ng/mL) whereas 5 and trans olefin 4 are less active. [reaction--see text] 相似文献
994.
Pepper SE Bunker DJ Bryan ND Livens FR Charnock JM Pattrick RA Collison D 《Journal of colloid and interface science》2003,268(2):408-412
Technetium is a long-lived product of nuclear fission that readily forms the soluble pertechnetate anion [TcO(4)](-). Green rusts (layered hydrous oxides containing both Fe(II) and Fe(III) and with interlayer sulfate or carbonate anions) concentrate >99.8% of 99Tc, present as [TcO(4)](-), from aqueous solution, even in the presence of high concentrations of NaNO(3), a common constituent of radioactive waste streams. The mechanism of removal from solution is apparently reduction and formation of strong Tc(IV) surface complexes. X-ray absorption spectroscopy shows that [TcO(4)](-) is indeed reduced by reaction with both sulfate- and carbonate-form green rusts and is found in a TcO(2)-like environment. On contact with air, the green rusts oxidize to poorly crystalline goethite but the Tc environment is unchanged. There is no increase in Tc solubility associated with oxidation of the host green rust. This behavior suggests that green rusts may be useful in the treatment of Tc-containing waste streams, in groundwater cleanup, and in restricting Tc migration from repositories. 相似文献
995.
一类非线性m-点边值问题正解的存在性 总被引:26,自引:4,他引:22
设α∈C[0,1],b∈C([0,1],(-∞,0)).设φ(t)为线性边值问题 u″+a(t)u′+b(t)u=0, u′(0)=0,u(1)=1的唯一正解.本文研究非线性二阶常微分方程m-点边值问题 u″+a(t)u′+b(t)u+h(t)f(u)=0, u′(0)=0,u(1)-sum from i=1 to(m-2)((a_i)u(ξ_i))=0正解的存在性.其中ξ_i∈(0,1),a_i∈(0,∞)为满足∑_(i=1)~(m-2)a_iφ_1(ξ_i)<1的常数,i∈{1,…,m-2}.通过运用锥上的不动点定理,在f超线性增长或次线性增长的前提下证明了正解的存在性结果. 相似文献
996.
We propose a picture of Wigner function scars as a sequence of concentric rings along a two-dimensional surface inside a periodic orbit. This is verified for a two-dimensional plane that contains a classical hyperbolic orbit of a Hamiltonian system with 2 degrees of freedom. The stationary wave functions are the familiar mixture of scarred and random waves, but the spectral average of the Wigner functions in part of the plane is nearly that of a harmonic oscillator and individual states are also remarkably regular. These results are interpreted in terms of the semiclassical picture of chords and centers. 相似文献
997.
Let be a surface, and let be a holomorphic curve in representing a primitive homology class. We count the number of curves of geometric genus with nodes passing through generic points in in the linear system for any and satisfying .
When , this coincides with the enumerative problem studied by Yau and Zaslow who obtained a conjectural generating function for the numbers. Recently, Göttsche has generalized their conjecture to arbitrary in terms of quasi-modular forms. We prove these formulas using Gromov-Witten invariants for families, a degeneration argument, and an obstruction bundle computation. Our methods also apply to blown up at 9 points where we show that the ordinary Gromov-Witten invariants of genus constrained to points are also given in terms of quasi-modular forms. 相似文献
998.
论述了天光一号电子束泵浦KrF准分子激光预放大器电子束产生和传输系统的研制。针对低电压(400kV)和高电流密度(143Acm-2),重点考虑了二极管真空绝缘界面、电极结构、漂移区长度和主膜支撑等问题并介绍了实验结果:二极管电压在450 kV以上仍能稳定工作,电流上升前沿约20ns,电子束总束能4.2kJ以上。只要开关导通时间选在PFL电压峰值附近,Marx 建立时间抖动小于20 ns,对总束能的影响将小于5%。 相似文献
999.
1000.