THE FLAVIN SENSITISED PHOTOOXIDATION OF ASCORBIC ACID: A CONTINUOUS AND FLASH PHOTOLYSIS STUDY

Abstract —The mechanism of the photooxidation of ascorbic acid by flavin mononucleotide has been studied by steady state and flash photolysis techniques. The primary reaction consists of an electron abstraction from ascorbic acid by the flavin singlet and triplet states by a diffusional process and in the latter case also by the formation of a triplet flavin-ascorbic acid complex. Under anaerobic conditions, the secondary reactions consist primarily of a radical recombination and a 'dark'chemical back reaction, leading to the reformation of unchanged flavin and ascorbic acid. In the presence of oxygen, the 'back reactions'are prevented, resulting in the efficient photooxidation of ascorbic acid.     

OXIDATION OF ASCORBIC ACID AS AN INDICATOR OF PHOTOOXIDATIVE STRESS IN THE EYE

When whole retinal pigmented epithelium (RPE) cells isolated from bovine eyes are incubated with 14C-labeled ascorbic acid and exposed to a visible laser, the ascorbic acid is oxidized to dehydro-L-ascorbic acid (DHA). The amount of ascorbic acid which is oxidized is proportional to the radiant exposure of the sample (i.e. the total amount of radiation per unit area delivered over the exposure time). Blue light is more effective than red light in driving the reaction. The amount of label appearing in the DHA fraction is increased if unlabeled DHA is present in the reaction mixture, indicating that some redox cycling of ascorbate is occurring in the RPE cells. The ascorbic acid oxidizing activity does not depend on intact cells, is not inactivated by heating the cells to 80 degrees C, and appears to reside mainly in the subcellular fraction which contains melanin pigment granules. The ascorbic acid oxidation may be caused by free radicals formed when melanin is illuminated with light. This reaction appears to be a useful method for quantifying the production of free radicals during photooxidative stress.     

抗坏血酸作基体改进剂石墨炉原子吸收光谱法测水中铷

提出了以抗坏血酸作基体改进剂石墨炉原子吸收光谱测定水中铷的新方法.本法特征量为4.6pg Rb,检出限(3s)为 0.27μg·L~(-1)样品加标回收率为95%～110%.方法简便、快速、结果准确可靠.     

STUDIES ON PLANT BILE PIGMENTS—9. PHOTOOXIDATION OF A-DIHYDROBILINDIONE AS PHYTOCHROME MODEL IN ACID MEDIUM

Abstract— The photooxidation of octaethyl-2, 3-dihydrobilindione ( 2 ) as a model for the P_r-chromophore has been studied in acidic methanol. When 2 is titrated with HCl to the cation and irradiated under N₂ with an excess of I₂, violin and rhodin type pigments are formed bearing methoxy-substituents at either C-4,5 or C-15,16, respectively. The reaction is slowed down as compared to neutral conditions, it is no longer regioselective at the C-5 methine bridge, and the formation of dimers is suppressed. From the data of this and earlier chemical model studies on the photooxidation of 2 , and the known properties of phytochrome, a reaction for the P_r→ P_fr phototransformation is suggested, in which the chromophore is attacked by the protein moiety in a sequence of oxidation and nucleophilic substitution, which may lead to a primary redox signal for the physiological response.     

抗坏血酸和亚硝酸反应的动力学研究

抗坏血酸和亚硝酸反应的动力学研究肖士民，李翠荣（华南理工大学应用化学系广州５１０６４１）关键词抗坏血酸，亚硝酸，动力学Ｎ－亚硝基胺是一种很强的致癌物质，在人胃的弱酸性环境中，可由来源于食物及防腐剂的亚硝酸盐与来源于药物的仲胺化合而成。近年来国内外都有...     

PHOTOREDUCTION OF ZINC PORPHIN BY ASCORBIC ACID*

Abstract— Zinc porphin is photoreduced to zinc chlorin through an intermediate dihydroporphin (PH₂) by ascorbic acid in ethanol containing 1% to 10% (v/v) piperidine. Under the same conditions zinc chlorin is more slowly photoreduced to zinc tetrahydroporphin. The reactions leading to chlorin are photosensitized by the product chlorin and so are autocatalytic in red light. Quantum yields for these reactions range up to 0.05.Other aliphatic amines catalyze these reactions, but at rates peculiar to the amine. The immediate product of reduction of zinc porphin, PH₂, is distinguished by an intense band at 437 nm; it reverts to porphin in the dark in the presence of oxygen or dehydroascorbic acid. Its conversion to chlorin is effected by light absorbed by porphin or chlorin, but not by light absorbed by PH₂ itself. A suggested structure for PH₂, compatible with the observed reactions, has added hydrogens on one bridge carbon and one β-pyrrole carbon. The possibility of an analogy between these reactions and the biochemical conversion of protochlorophyll to chlorophyll is discussed.     

一阶导数溶出伏安法测定抗坏血酸

利用抗坏血酸可定量还原Fe(Ⅲ)-邻二氮菲(phen)为Fe(Ⅱ)-phen,建立了在玻碳电极上进行抗坏血酸的一阶导数阴极溶出伏安测定方法。在pH为4.5的HOAc-NaOAc缓冲体系中,抗坏血酸的浓度在5.0×10-6~2.0×10-4mol.L-1范围内与一阶导数溶出伏安峰电流值的增加呈线性关系,相关系数为0.998。该方法应用于测定水果和蔬菜中的抗坏血酸,回收率在98%~104%之间,测得结果与光度法的测定结果一致。     

间接光度法测定药物中的抗坏血酸

研究了一种简便的选择性高的药物中抗坏血酸含量的测定方法。该方法基于在弱酸性介质中 ,有盐酸羟胺、碘酸存在下 ,抗坏血酸还原对氨基苯磺酸与N 1 萘乙二胺盐酸盐混合体系生成的有色物质 ,体系的最大吸收波长为 5 40nm ,工作曲线的线性范围为 0 .5～ 4 .0 μg·ml- 1。方法灵敏度高 ,选择性好 ,对药用维生素C中抗坏血酸含量进行测定 ,结果满意     

抗坏血酸在聚甲苯胺蓝膜修饰的玻碳电极上的电催化氧化及流动注射分析

在玻碳电极上用循环伏安法电聚合一层聚甲苯胺蓝膜,该膜修饰电极对抗坏血酸(VC)的氧化具有良好的电催化作用,能够降低其过电位约100mV。将该修饰电极应用于流动注射能大大提高VC的检测灵敏度,其线性范围为2.0×10-7～1.2×10-3mol·L-1,检出限为4.8×10-8mol·L-1,连续注射检测,其电流信号能够保持稳定24h。     

高效液相色谱电化学发光法检测抗坏血酸

基于在恒电流条件下电解产生不稳定化学发光试剂Mn(Ⅲ)能直接氧化抗坏血酸产生化学发光,将高效液相色谱法与电化学发光法相结合,建立了高效液相色谱电化学发光法测定抗坏血酸的检测方法。在优化的试验条件下,对啤酒、人体血清、尿液中抗坏血酸含量进行了分析,方法的检出限为3.0×10-5g.L-1,相对标准偏差为2.2%(n=11,5.0×10-2g.L-1VC),线性范围1×10-4~1×10-1g.L-1。     

脂质体体系中甲基紫精与抗坏血酸的光敏氧化还原

叶绿素a作为一种光敏色素嵌入脂质体的类脂双分子层液晶态膜中,光照时观察到脂质体外部水相中的甲基紫精被脂质体内部水相中的抗坏血酸还原。用来配制缓冲液的三羟甲基氨基甲烷同时可起质子载体作用。本文还初步讨论了pH值的改变对光敏反应的影响。     

FURTHER STUDIES ON THE CRYSTAL-VIOLET-SENSITIZED PHOTOOXIDATION OF CYSTEINE TO CYSTEIC ACID

Abstract —Crystal violet sensitizes the selective photooxidation of cysteine to cysteic acid; hydrogen peroxide is also formed as an end product. The participation of singlet oxygen in the photoreaction has been ruled out, since exposure of cysteine to this reagent, generated by chemical or photochemical processes, gives only cystine as a product. The photoreaction is inhibited by radical scavengers such as hydroquinone and allylic alcohol. A mechanism is proposed involving hydrogen abstraction by the triplet dye from the thiol group of cysteine.     

光化学反应在分析化学中的应用研究 光激活产生碘测定食品中的抗坏血酸

本文探讨了酸性介质中于紫外光的照射下,利用光激活产生碘的光化学动力学法测定抗坏血酸的可能性。结果表明,在0.25mol/L的硫酸介质中,在过量碘化钾的存在下,抗坏血酸的含量与到达滴定终点的时间在较宽的浓度范围内呈良好的线性关系。用于蜂蜜,黄瓜、蕃茄及Vc药片等样品分析,其相对标准偏差均在1％以下,准确度令人满意。     

抗坏血酸和草酸在金属氧化物微粒修饰电极上的电催化

不同金属氧化物微粒修饰于玻炭电极表面,对某些有机化合物,如抗坏血酸和草酸的氧化反应呈现一定的催化活性。在试验的37种金属氧化物微粒修饰电极中,电催化活性高的金属氧化物有CeO₂、ZrO₂、Co₂O₃、Cr₂O₃、SnO-2和α-Al₂O₃等。研究了金属氧化物性质对电催化活性影响的规律性。除循环伏安法外还以计时电位法,计时电流法以及旋转圆盘电极探讨了电催化机理。     

PHOTOOXIDATION OF XANTHOBILIRUBIC ACID IN AQUEOUS SOLUTION: PRODUCT AND MECHANISM STUDIES

Abstract— Xanthobilirubic acid, an oxodipyrrylmethene with a chromophore very similar to that of bilirubin, was aerobically irradiated as its sodium salt in borate buffer at pH 8. Two photooxidation products, methylethylmaleimide and 5-formyl-2,4-dimethyl-1 H -pyrrole-3-propanoic acid, were isolated. These products can be rationalized on the basis of either a Type I or a Type II (singlet oxygen) photooxygenation mechanism. The reaction is inhibited by azide, a singlet oxygen quencher, and sensitized by methylene blue. However, both the self-sensitized and the methylene blue-sensitized reactions are enhanced, rather than inhibited, by high concentrations of 1,4-diazabicyclo[2.2.2]octane, another known singlet oxygen quencher. It is therefore proposed that the diazabicyclooctane can also act as an electron donor.
The self-sensitized photooxidation of xanthobilirubic acid is an autocatalytic reaction. This is supported by the fact that addition of a previously irradiated solution to a freshly-prepared solution significantly increases the rate of photodegradation. A similar catalyst is formed, but much more slowly, from xanthobilirubic acid in the dark in the presence of oxygen.     

抗坏血酸及L-半胱氨酸的流动注射动力学同时测定法

pH2～9时,抗坏血酸与L-半胱氨酸均可与Fe3+-邻菲咯啉混合液反应,生成Fe2+-邻菲咯咻有色配合物,但两者反应速度相差极大。根据这个原理,采用流动注射分析停流技术,建立了抗坏血酸与L-半胱氨酸的动力学吸光光度同时测定法。两者测定的线性范围分别为0～24mg·L-1和0～280mg·L-1,相应的检出限为1mg·L-1和16mg·L-1,抗坏血酸与L-半胱氨酸浓度分别为12.0mg·L-1和160.0mg·L-1时,测定的相对标准偏差分别为0.9％和0.6％,采样频率为30样·h-1。该法灵敏、快速、准确,选择性较高,且流路及设备简单,实用性强,用于实际样品测定,抗坏血酸及L-半胱氨酸的加标回收率分别可达95.2％和92.8％。     

福林试剂光度法快速测定全血中抗坏血酸

对福林试剂光度法快速测定血中抗坏血酸的反应条件进行研究,结果表明,反应的λ