CHLORANIL-PHOTOSENSITIZED MONOMERIZATION OF DIMETHYLTHYMINE CYCLOBUTANE DIMERS and EFFECT OF MAGNESIUM PERCHLORATE

The photosensitized monomerization of the cyclobutane dimers of 1,3-dimethylthymine by p-chloranil was investigated by means of steady-state irradiation and laser-flash photolysis. Quantum yields for the monomerization are 0.34 for the cis,syn dimer, 0.39 for the trans,syn dimer, and much less than 10(-2) for the cis,anti isomer. Formation of the chloranil anion radical associated with quenching of triplet chloranil by the dimers demonstrates that electron transfer from dimers to triplet chloranil occurs to initiate the monomerization. Kinetic analysis suggested that the syn-dimer cation radicals undergo the ring cleavage at greater than or equal to 10(9) s-1 before escaping from the solvent cage, while the reactivity of the anti-dimer cation radical is very low. The different reactivities of the syn and anti dimer cation radicals are discussed in terms of through-bond coupling between the n orbitals of N(1) and N(1') involving the cyclobutane-ring sigma orbitals. In the cases of the syn-dimers, the sensitizer-dimer ion-radical pairs undergo the rapid geminate recombination that works as a major energy dissipating channel responsible for the lower-than-unity quantum yields. It has been found that the presence of Mg(ClO4)2 at 0.1 M enhances approximately 1.5 times either the monomerization of the syn dimers or the formation of the chloranil anion radical. A laser-flash photolysis study shows that Mg2+ forms a complex with either the triplet or the anion radical of chloranil. The net salt effects are attributed to the retardation of the rapid geminate recombination by the participation of Mg2+ in the sensitizer-dimer ion-radical pairs.     

DETECTION OF RADICAL ION INTERMEDIATES IN FLAVIN-PHOTOSENSITIZED PYRIMIDINE DIMER SPLITTING

Photosensitized splitting of cis-syn- and trans-syn-l,3-dimethyluracil dimers by 2′,3′,4′,5′-tetraacetylri-boflavin in acetonitrile containing a trace of perchloric acid was studied by laser flash photolysis. Protonation of the flavin prior to excitation resulted in excited singlet and triplet states that abstracted an electron from the dimers and yielded the protonated flavin radical (F1H₂′⁺), which was detected by absorption spectroscopy. Electron abstraction by the excited singlet state predominated over abstraction by the triplet state. Approximately one-third to one-half of the excited states quenched by the trans-syn dimer yielded F1H₂⁺, the balance presumably undergoing back electron transfer within the geminate radical ion pair generated by the initial electron transfer. A covalently linked dimer-flavin exhibited very inefficient flavin radical ion formation, consistent with the known low efficiency of dimer splitting in this system. These results constitute the first identification of a flavin radical ion intermediate in photosensitized pyrimidine dimer splitting.     

PHOTOOXIDATION OF MAGNESIUM TETRAPHENYLPORPHYRIN IN THE PRESENCE OF METHYL VIOLOGEN

Abstract— A study of the photooxidation of magnesium tetraphenylporphyrin by oxygen in methanol with methyl viologen present has been carried out. The reaction was normally followed by the decrease of Soret band absorbance with time. The influences of air, methyl viologen, light wavelength and intensity, presence of ß-carotene, pH, and MgTPP concentration were investigated. From bulk photolysis experiments, three products have been separated and partially characterized. A mechanism based on superoxide as the dominant oxidant is proposed.     

CIDNP DETECTED FLASH PHOTOLYSIS OF cis–syn 1,3 DIMETHYLTHYMINE DIMER

Abstract— Theanthraquinone–2-sulfonate photo-sensitized splitting of cis-syn 1,3-dimethylthymine dimer gives rise to large CIDNP effects in the reaction product 1,3-dimethylthymine. The polarization originates from a radical ion pair formed by electron transfer from the dimer to the triplet state sensitizer. In a deoxygenated solution the sign of the polarization of theC–6 proton is reversed compared to the predicted one on account of the CIDNP sign rules. In an aerated solution the correct sign is observed. This can be accounted for by assuming reduction of the lifetime of the radical pair in the presence of oxygen. The time-resolved photo-CIDNP technique was used to study the time dependence of the 1,3-dimethylthymine signal. To account for this time-dependence a cation radical disproportionation reaction is proposed.     

INTERACTIONS OF COBALT(II) TETRASULFOPHTHALOCYANINE WITH NITRITE IN THE PRESENCE OF NITRATE AND PERCHLORATE IONS

Abstract

Spectroscopic changes observed on addition of nitrite to solutions of cobalt(II) tetra-sulfophthalocyanine ([Co(II)TSPc]^4-) in the presence of N^? ₃ or ClO^? ₄ are reported. There is spectroscopic evidence for the oxidation of [Co(II)TSPc]^4- to a [Co(III)TSPc]^3- species in the presence of nitrite ions. Equilibrium and kinetic studies for the interaction between [Co(II)TSPc]^4- and NO^? ₂ are reported. The rate was found to be first order in both [Co(II)TSPc]^4- and NO^? ₂. The rate constant for the forward reaction, k _f=1.6 × 10^?4 dm³mol^?1s^?1 was determined at 20°C for the interaction between nitrite ions and [Co(II)TSPc]^4- in the presence of NO₃ ^? or ClO₄ ^? ions.     

在表面活性剂存在下, 二甲苯胺蓝-1与镁的显色反应研究

二甲苯胺蓝-1(XB-1)作为镁的显色剂^[1],由于试剂本身和它与镁生成的络合物的吸收峰相距太近,所以一直未能受到重视。但自从表面活性剂引入该显色反应后,这一问题才得解决^[2],本文报道溴化十六烷基三甲胺(CTMAB)等表面活性剂对镁与XB-1显色反应的影响。     

WAVELENGTH DEPENDENCE (150–290 nm) OF THE FORMATION OF THE CYCLOBUTANE DIMER AND THE (6–4) PHOTOPRODUCT OF THYMINE

The action cross sections for the formation of the cyclobutane dimer and the (6-4) photoproduct of thymine as well as the absorption cross sections of thymine were determined in the wavelength region between 150 and 290 nm. Thymine films sublimed on glass plates were irradiated by monochromatic photons in a vacuum; the induced photoproducts were quantitatively analyzed by high-performance liquid chromatography (HPLC). Under our conditions, two major peaks appeared on the HPLC chromatograms of irradiated samples. The two peaks were identified as being the cis-syn cyclobutane dimer and the (6-4) photoproduct, based on their HPLC retention times, absorption spectra in the effluent, and photochemical reactivity. The fractions of the two photoproducts increased linearly with the fluence at low fluences over the entire wavelength range. Their action cross sections were determined by the slopes of the linear fluence response curve at 10 nm intervals between 150 and 290 nm. The two action spectra showed a similar wavelength dependence and had a maximum at 270 nm as well as two minor peaks at 180 and 220 nm, at which wavelengths the peaks of the absorption spectrum of thymine sublimed on a CaF2 crystal plate appeared. The quantum yields had relatively constant values of around 0.008 for the dimer and 0.013 for the (6-4) photoproduct above 200 nm, decreasing to 0.003 and 0.006, respectively, at 150 nm as the wavelength became shorter.     

THE PHOTOLYSIS OF TRYPTOPHAN IN THE PRESENCE OF OXYGEN

Abstract— Quantum yields for the destruction of tryptophan by a single 500 J flash in aqueous solution have been determined over the pH range 1–13 in both air-equilibrated and nitrogen-saturated conditions. When these quantum yields are compared with the quantum yields for radical formation and photoejection of electrons, it is found that there is good agreement only for the nitrogen-saturated case. In air-equilibrated solutions of tryptophan, there is a large disparity between the measured degradation quantum yields and those for photoejection of electrons and radical formation. Oxygen, therefore, is playing a major role in the photochemical decomposition and it is proposed that the major reaction which occurs, under normal atmospheric conditions, is the reaction of the lowest triplet excited state of tryptophan with oxygen.
Preliminary photolysis-product distributions against pH are discussed, and indicate that a total of nine major products are formed in the presence of oxygen.     

三乙醇胺存在下火焰原子吸收光谱法测定钼

研究了在火焰原子吸收光谱法中三乙醇胺对钼的增感作用。在60g·L~(-1)三乙醇胺存在下,可使钼的吸光度增感25％,且能消除多种共存元素的干扰。建立了钼的质量分数大于0.4％的合金钢中钼的测定方法。     

金属镁中杂质元素的快速测定

以盐酸溶液(1 1)溶解金属铗样品,用电感耦合等离子体原子发射光谱(ICP—AES)法同时测定金属镁样品中的杂质元素铁、硅、锰、铝、铜和镍。采用标准溶液与样品溶液基体相一致的方法消除基体干扰。对标准样品进行测定,测定结果与标准值基本一致,相对标准偏差小于7％。t检验结果证明,该方法不存在系统误差。     

以双硫酯为链转移剂的活性自由基聚合

合成并研究了两种双硫酯链转移剂的纯化方法 ,进行了多种单体以双硫酯为链转移剂的活性自由基聚合及嵌段共聚 .发现以PhC(S)SC(CH3) 2 Ph为链转移剂的效果比PhC(S)SCH(CH3)Ph好 ,聚合产物的多分散性系数较小 .引发剂与链转移剂的摩尔数比为 1∶3 5～ 1∶4 2时 ,得到多分散性系数小 ,实测分子量与理论分子量相近的聚合产物 .聚合物的分子量随时间和转化率的增加而增加 ,加入第二单体形成嵌段共聚物 ,具有活性聚合特征 .聚甲基丙烯酸酯大分子引发剂引发丙烯酸酯单体聚合时 ,聚合速度最快 .     

卟吩复合体的电子结构

本文用EHMO法对卟吩及其复合体的电子结构进行了计算。由卟吩复合体的差示电荷分布的分析表明, 在卟吩复合体中卟吩分子属于两性电子施受体, 分子中有固定的施体中心和受体中心部位。体系在相对取向角φ为45°时能量最小。两个分子相对旋转一周要克服四重势垒, 势垒高度为～0.09 eV。本文把简并态微扰理论推广运用于两个分子组成的复合物体系, 并由此定量地解释了卟吩与其复合体之间存在的能级对应现象。     

THE KINETICS OF THE DECOMPOSITION OF CYCLOHEXYL HYDROPEROXIDE IN THE PRESENCE OF VANADYL DIBENZOYLMETHANE

The kinetics of the decomposition of cyclohexyl hydroperoxide(CHHP) in benzene catalyzed by vanadyl dibenzoylmethane[V0(DBM).,] has been studied.It was found that the products of decomposition of CHHP were cyclohexanol and cyclohexanone,which are produced in about equimolar amount,and the product cyclohexanol obviously inhibited the decomposition of CHHP.The kinetics data can be satisfactorily described by the following equation (with [CHHP]0>>[VO(DBM)2]0)R0=kK[CHHP]0[VO(DBM)2]0/(1+k[CHHP]0)This is the kinetic evidence for the formation of a catalyst-hydro-peroxide intermediate.In the equation K is the stability constant of the catalyst-hydroperoxide intermediate complex;k is the rate constant for the decomposition of the complex.The rate constant K at 500℃ may be expressed as follows:k=1.9×108exp(-53.7×103/RT)S-1 with the activation energy Ea=53.7kJ mol-1     

PHOTOCLEAVAGE OF DNA IN THE PRESENCE OF SYNTHETIC WATER-SOLUBLE PORPHYRINS

In the presence of oxygen and visible light, various synthetic water-soluble porphyrins cleave pBR 322 plasmid supercoiled DNA (form I) producing relaxed (form II) and linear (form III) DNA corresponding to single-strand and double-strand breaks respectively. Large variations are observed in the efficiency of the porphyrins containing a diamagnetic metal or no metal at all. Singlet oxygen (¹O₂) seems to be involved in the mechanism of cleavage consistent with the inhibitory effect of the azide anion, N^–₃. The higher efficiency of cationic porphyrins (as compared to anionic ones) is due to their greater affinity for DNA as shown by experiments carried out at either high ionic strength or in the presence of the surfactant, sodium dodecyl sulfate.     

PHOTOPHYSICAL PROPERTIES OF PORPHYRIN-CHLORIN SYSTEMS IN THE PRESENCE OF SURFACTANTS

Abstract The therapeutic efficacy of PDT is related to the capability of the photosensitizer to absorb light at a wavelength that can penetrate into tissues. We have synthesized two systems, a haematoporphyrin-chlorin (HPC) and a dihaematoporphyrin ether or ester (DHE) with the terminal ring converted to a chlorin (DHEC). The presence of the chlorin moiety provides an extra band at ˜ 660 nm with a relative amplitude from 5 to 10 times larger than that of the porphyrin at 630 nm. Since both HPC and DHEC strongly aggregate in buffer, we have studied their photophysical properties in the presence of cationic surfactants at different concentrations below and above the critical micelle concentrations. Absorption spectra were measured together with emission spectra and fluorescence decays at different observation wavelengths under excitation at 364 nm. The results were compared with those obtained for DHE in the same environmental conditions. As for DHE, the presence of micelles disaggregate both compounds, resulting in a large increase in the relative emission intensity at ˜ 670 nm due to the presence of the chlorin moiety. The fluorescence decays could be fitted by two or three exponential components indicating the presence of more than one molecular species and/ or conformations. On the basis of our measurements the chlorin molecule does not seem to modify appreciably the photophysical properties of the porphyrin molecules but does superimpose its absorption and emission spectrum onto that of the porphyrin. This result may be of relevance in the possible use of these compounds in PDT.     

THE MODIFICATION OF RIBONUCLEASE-A BY NEARULTRAVIOLET IRRADIATION IN THE PRESENCE OF PSORALEN

Abstract— Ribonuclease A is inactivated when irradiated under oxygen by UV-A light in the presence of psoralen. The rate of inactivation is greatly reduced by sodium azide. ascorbate or nitrogen, whereas the substrate gives only very limited protection. A ribonuclease sample modified to 40% remaining activity presented a significant modification of amino acid residues known to be sensitive to oxidation and 1.4 mol of bound psoralen per mol of protein. The secondary structure of the enzyme, as assessed by circular dicroism was not changed by irradiation; neither was aggregation of the enzyme to a higher mol wt evident. Studies on the tryptic peptides fractionated by high performance liquid chromatography showed that the photomodification occurs with very low selectivity. All the five peptides containing hystidine, tyrosine and methionine residues were greatly modified, although two, those containing histidine residues 12 and 119 in the sequence, amino acids known to be involved in the catalytic activity of ribonuclease. are modified to a greater extent. The protein bound psoralen. revealed by radioactivity in the HPLC eluate, was not found associated to only one or few peptide peaks but spread on a large zone of elution.