- Synergistic effect of 1-(2,5-dioxoimidazolidin-4-yl)urea and Tween-80 towards the corrosion mitigation of mild steel in HCl
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Herein, the synergistic effect of 1-(2,5-dioxoimidazolidin-4-yl)urea (DMU) and Tween-80 on the corrosion of mild steel in a 1 mol L-1 HCl solution was investigated by weight-loss tests, electrochemical methods and surface analysis (SEM/SECM). The experiments revealed that the combination of DMU and Tween-80 strongly inhibited the corrosion of mild steel as compared to individual inhibitors, and the adsorption mode belonged to the Langmuir isothermal type. Based on the electrochemical results, DMU and DMU-Tween-80 behaved as mixed-type inhibitors, and their best inhibition efficiencies were 77.31% and 96.35%, respectively. The calculated synergistic parameter value was larger than unity, indicating that the enhancement of inhibition efficiency was due to the synergistic effect of DMU and Tween-80. Surface analysis techniques confirmed the presence of inhibitors on the mild steel surface. Quantum chemical parameters based on density functional theory provided a further insight into the mechanism of inhibition. Moreover, molecular dynamics simulations were carried out to explore the configurational adsorption behaviour of DMU on an Fe(110) surface.
- Zhang, Weiwei,Li, Hui-Jing,Wang, Chen,Wang, Li-Juan,Li, Gen,Ma, Hengyu,Pan, Qianwen,Wu, Yan-Chao
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- Ultrasensitive detection of uric acid in serum of patients with gout by a new assay based on Pt&at;Ag nanoflowers
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A ultrasensitive assay for the determination of uric acid (UA) based on Pt&at;Ag nanoflowers (Pt&at;Ag NFs) was constructed. H2O2 was formed by the reaction of uricase and UA and produced the hydroxyl radical (OH). The system was catalyzed by Pt&at;Ag NFs to change the color of 3,3′,5,5′-tetramethylbenzidine (TMB) from colorless to blue, and the morphology and chemical properties of Pt&at;Ag NFs were characterized by transmission electron microscopy and X-ray photoelectron spectroscopy. Under the optimized conditions, a linear relationship between the absorbance and UA concentration was in the range of 0.5-150 μM (R2 = 0.995) with a limit of detection of 0.3 μM (S/N = 3). The method can be applied to detection of UA in actual samples with satisfactory results. The proposed assay was successfully applied to the detection of UA in human serum with recoveries over 96.8percent. Thus, these results imply that the UA assay provides an effective tool in fast clinical analysis of gout.
- Chen, Shujun,Lin, Daiqin,Qiu, Ping,Tang, Xiaomin,Wang, Xue
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- Kinetics and mechanism of allantoin racemization
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The kinetics and mechanism of racemization of allantoin have been examined; racemization proceeds via two independent pathways that can be separately monitored. One pathway involves proton exchange at C5 with solvent, The other pathway occurs via intramolecular attack of N8 on C4 to form a symmetrical bicyclic intermediate, which can decompose to form either enantiomer of allantoin. The intramolecular pathway proceeds more rapidly from the allantoin anion than from neutral allantoin. This result is explained by conformational analyses based on experimental NMR data and computational results, which suggest that the ureido arm of anionic allantoin adopts a cis-conformation, allowing intramolecular attack. Neutral allantoin adopts a trans-conformation. The proton exchange pathway is buffer-catalyzed and also proceeds more rapidly at basic pH, although it is suggested that the reaction occurs from neutral allantoin. The relatively slow rate of racemization, particularly at physiological pH, suggests that nonenzymatic racemization of allantoin is not a viable mechanism for the in vivo generation of (S)-allantoin. (C) 2000 Academic Press.
- Kahn, Kalju,Tipton, Peter A.
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- Correction: Absolute stereochemistry and preferred conformations of urate degradation intermediates from computed and experimental circular dichroism spectra (Org. Biomol. Chem. (2011) 9 (5149-5155))
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Correction for 'Absolute stereochemistry and preferred conformations of urate degradation intermediates from computed and experimental circular dichroism spectra' by Silvio Pipolo et al., Org. Biomol. Chem., 2011, 9, 5149-5155.
- Pipolo, Silvio,Percudani, Riccardo,Cammi, Roberto
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- Synthesis of highly pure 14C-labelled DL-allantoin and 13C NMR analysis of labelling integrity
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A number of synthetic approaches are assessed to prepare allantoin labelled with 14C given certain requirements and technical limitations. A method that fulfils these criteria is described to achieve the synthesis of highly pure 14C-labelled allantoin with the label introduced to the ureido carbonyl group in the final step by reaction of 5-chlorohydantoin with [14C]urea. The chosen method favours high purity at the expense of radiochemical yield, which is achieved at a level of 8%. The integrity of the label is then investigated by performing an NMR analysis of 13C- labelled allantoin synthesized by the same method. The 13C NMR spectrum confirms partial scrambling of the label to the C-2 position by equilibration of the product via a putative bicyclic intermediate, which had been suggested by other workers. The 14C-labelled allantoin synthesized by this method is therefore assigned as DL-[H2N 14CO/14C-2]allantoin. This study also includes the first full characterization of a side product, 5-hydroxy-5-methoxyhydantoin, obtained by the reaction of a 5-hydroxyhydantoin intermediate with the methanol solvent. Copyright
- Patching, Simon G.
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- Uricase-catalyzed oxidation of uric acid using an artificial electron acceptor and fabrication of amperometric uric acid sensors with use of a redox ladder polymer
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Electrochemical oxidation of uric acid catalyzed by uricase (uric acid oxidase, UOx; EC 1.7.3.3) was studied using several redox compounds including 5-methylphenazinium (MP) and 1-methoxy-5-methylphenazinium (MMP) as electron acceptors for UOx, which does not contain any redox cofactor. It was found that MP and MMP were useful to mediate electrons from UOx to an electrode in the enzymatic oxidation of uric acid. A novel redox polymer, poly(N-methyl-o-phenyIenediamine) (poly-MPD), containing the MP units was also found to possess the mediation ability for UOx, and poly-MPD was immobilized together with UOx onto an electrode substrate covered with a self-assembled monolayer of 2-aminoethanethiolate with use of glutaraldehyde as a binding agent The resulting electrode (pory-MPD/UOx/Au) exhibited amperometric responses to uric acid with very fast response of ～30 s, allowing reagentless amperometric determination in a concentration range covering that in the blood of a healthy human being. Kinetic parameters of the apparent Michaelis constant and the maximum current response obtained at the poly-MPD/UOx/Au suggested that electrochemical oxidation of uric acid was controlled by diffusion of uric acid into the enzyme film and that the redox polymer worked well in mediating between active sites of UOx molecules and the electrode substrate.
- Nakaminami, Takahiro
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- Identification of the true product of the urate oxidase reaction
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The O2-dependent oxidation of urate catalyzed by urate oxidase has been examined in order to identify the immediate product of the enzymatic reaction. Specifically labeled [13C]urates were utilized as substrates, and the time courses were monitored by 13C NMR. On the basis of chemical shift values and 18O labeling, the product of the reaction was identified as 5- hydroxyisourate. This identification was substantiated by calculation of the 13C NMR spectrum of 5-hydroxyisourate using ab initio density functional theory methods. The predominant tautomers of urate and allantoin in aqueous solution were identified from 13C NMR titration data; the ionization behavior of urate and 5-hydroxyisourate were also examined by computational methods. The nonenzymatic pathway for production of allantoin from 5- hydroxyisourate was delineated; the reaction proceeds by the hydrolysis of the N1-C6 bond, followed by an unusual 1,2-carboxylate shift and decarboxylation to form allantoin.
- Kahn, Kalju,Serfozo, Peter,Tipton, Peter A.
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- A Novel Cascade Nanoreactor Integrating Two-Dimensional Pd-Ru Nanozyme, Uricase and Red Blood Cell Membrane for Highly Efficient Hyperuricemia Treatment
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Nanozyme-based cascade reaction has emerged as an effective strategy for disease treatment because of its high efficiency and low side effects. Herein, a new and highly active two-dimensional Pd-Ru nanozyme is prepared and then integrated with uricase and red blood cell (RBC) membrane to fabricate a tandem nanoreactor, Pd-Ru/Uricase@RBC, for hyperuricemia treatment. The designed Pd-Ru/Uricase@RBC nanoreactor displayed not only good stability against extreme pH, temperature and proteolytic degradation, but also long circulation half-life and excellent safety. The nanoreactor can effectively degrade UA by uricase to allantoin and H2O2 and remove H2O2 by using Pd-Ru nanosheets (NSs) with the catalase (CAT)-like activity. More importantly, the finally produced O2 from H2O2 decomposition can in turn facilitate the catalytic oxidation of UA, as the degradation of UA is an O2 consumption process. By integrating the high-efficiency enzymatic activity, long circulation capability, and good biocompatibility, the designed Pd-Ru/Uricase@RBC can effectively and safely treat hyperuricemia without side effects. The study affords a new alternative for the exploration of clinical treatment of hyperuricemia.
- Ming, Jiang,Zhu, Tianbao,Li, Jingchao,Ye, Zichen,Shi, Changrong,Guo, Zhide,Wang, Jingjuan,Chen, Xiaolan,Zheng, Nanfeng
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- Method for producing allantoin for cosmetics
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The invention discloses a method for producing allantoin for cosmetics, which comprises the following steps: adding a glyoxylic acid solution, urea and a catalyst into a microwave reactor, filtering and separating the product after the reaction is finished, and recrystallizing to obtain the allantoin finished product. By preparing a mesoporous Nb2O5-ZrO2 composite oxide, preparing solid superacidMoO3/Nb2O5-ZrO2, preparing an ionic liquid precursor PyPS, preparing phosphomolybdic vanadic acid, and carrying out in-situ loading on Co-based solid ionic liquid, the catalyst prepared by in-situ compounding of the solid superacid MoO3/Nb2O5-ZrO2 and the Co-based solid ionic liquid is obtained. The prepared composite catalyst can solve the problem that the catalyst is difficult to separate from aproduct, can also overcome the defect that the catalyst is inactivated due to the fact that the acidity of the catalyst is easy to lose, can keep a solid state, and is non-corrosive to a reaction device, high in catalytic efficiency and good in selectivity.
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Paragraph 0016-0017; 0019-0020; 0022-0023; 0025-0026; 0029
(2020/11/10)
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- (2,5-Dioxoimidazolidin-4-ylidene)aminocarbonylcarbamic Acid as a Precursor of Parabanic Acid, the Singlet Oxygen-Specific Oxidation Product of Uric Acid
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Previously, we identified that parabanic acid (PA) and its hydrolysate, oxaluric acid (OUA), are the singlet oxygen-specific oxidation products of uric acid (UA). In this study, we investigated the PA formation mechanism by using HPLC and a time-of-flight mass spectrometry technique and identified unknown intermediates as (2,5-dioxoimidazolidin-4-ylidene)aminocarbonylcarbamic acid (DIAA), dehydroallantoin, and 4-hydroxyallantoin (4-HAL). DIAA is the key to PA production, and its formation pathway was characterized using 18O2 and H218O. Two oxygen atoms were confirmed to be incorporated into DIAA: the 5-oxo- oxygen from singlet oxygen and the carboxylic oxygen from water. Isolated DIAA and 4-HAL gave PA stoichiometrically. A plausible reaction scheme in which two pathways branch out from DIAA is presented, and the potential for PA as an endogenous probe for biological formation of singlet oxygen is discussed.
- Iida, Sayaka,Yamamoto, Yorihiro,Fujisawa, Akio
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- Method for preparing allantoin using titanium dioxide-based composite solid catalyst
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The invention provides a method for preparing allantoin using a titanium dioxide-based composite solid catalyst. The preparation method comprises the following steps of (1) mixing tetrabutyl titanate,graphene, dimethyl formamide and ethanol to form a solution A; (2) mixing water, ethanol and acetic acid to form a solution B; (3) mixing copper salt, zinc salt, silver salt, water and ethanol to form a solution C; (4) simultaneously adding the solution B and the solution C into the solution A, standing, drying and grinding, so as to obtain a powdery material; (5) calcining the powdery material,so as to obtain the solid catalyst; (6) soaking the solid catalyst, zinc chloride and niobium pentachloride into hydrochloric acid, and then drying to obtain a titanium dioxide-based complex/lewis acid composition; (7) performing oxidizing reaction on glyoxal and nitric acid to produce glyoxalic acid, and performing condensation reaction on the glyoxalic acid, urea and composition, so as to obtainthe allantoin. The method can reach excellent yield.
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Paragraph 0037-0039
(2018/12/14)
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- Synthetic method of allantoin
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The invention provides a synthetic method of allantoin. The synthetic method of the allantoin comprises the following steps of slowly adding hydrogen peroxide into a homogeneous reactor filled with an oxalaldehyde solution, adding a catalyst, and reacting for 90 min to obtain an intermediate product a glyoxylic acid solution, wherein the molar ratio of the hydrogen peroxide and the oxalaldehyde is 0.5 to 0.8:1, and the dosage of the catalyst is 1.6 to 2.8 percent; then adding urea and the catalyst into the glyoxylic acid solution, reacting for 90min at the temperature of 60 DEG C, and after completing reaction, cooling crystallizing, standing, filtering, washing, drying and smashing the reaction product to obtain the target product allantoin. According to the synthetic method of the allantoin provided by the invention, phosphotungstic acid is adopted as the catalyst so as to directly synthesize the allantoin by reaction under a moderate condition, so that the method is simple and efficient, and solves the problem of blasting boiling caused by jump of temperature during an oxidization operation process, the reaction time is shortened, the enterprise production cost is reduced, the yield of the glyoxylic acid and the allantoin is improved, the catalyst used in reaction can be recycled and recovered, and the synthetic method of the allantoin has a very high popularization value.
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Paragraph 0011; 0012; 0013; 014; 0015; 0016-0019
(2017/07/04)
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- Synthesis technology of allantoin
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The invention discloses a synthesis technology of allantoin. The synthesis technology comprises the following steps: adding a glyoxylic acid water solution, urea, and a catalyst into a reactor, wherein the catalyst is a mixture composed of sulfamic acid and modified macroporous strongly-acidic styrene type cation resin according to a weight ratio of 1:5; carrying out reactions for 6-8 hours at a temperature of 60-63 DEG C, cooling the reaction mixture to the room temperature, filtering, carrying out recrystallization, washing, drying to obtain a finished product of allantoin; merging the filtrate and washing liquid, and processing the merged liquid to obtain the catalyst. The catalyst can be repeatedly used. According to the synthesis method, glyoxylic acid and urea are taken as the raw materials, sulfamic acid and modified macroporous strongly-acidic styrene type cation resin are taken as a composite catalyst, the allantoin is synthesized in one step; the macroporous cation resin is modified by metals and then mixed with sulfamic acid, the catalytic efficiency and repeated utilization rate of the catalyst are improved, the using amount of catalyst is reduced; the technical procedure is simple, no pollution is generated, and the synthesis technology is green and environment-friendly, has a good ecological effect, and generates good social benefits and economic benefits.
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Paragraph 0013; 0014; 0016; 0017; 0019; 0020
(2017/08/27)
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- Preparation method for high-purity allantoin
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The invention relates to a preparation method for high-purity allantoin. The preparation method comprises the following concrete steps: a) providing a solution formed by allantoin and hot water; b) adding an organic solvent into the solution; and c) carrying out recovery so as to obtain high-purity allantoin. The method provided by the invention can effectively remove impurities like allantoic, acid urea, glycoluril and biuret in allantoin and is suitable for industrial production.
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Paragraph 0030
(2017/01/02)
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- Conjunction technology of allantoin
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The invention discloses a synthesis technology of allantoin. The technology comprises the following steps: 1, adding a glyoxalic acid solution, urea and a catalyst to a reaction container according to a ratio of the catalyst to glyoxalic acid of 38-62g.mol, and heating the above obtained mixture under stirring to 40-75DEG C in order to start a reaction; and 2, stopping heating and stirring after the reaction is carried out for 2-5h, cooling the obtained reaction mixture to room temperature, carrying out pumping filtration, separating the catalyst, re-crystallizing crude allantoin, and drying the re-crystallized crude allantoin to obtain the finished allantoin product, wherein the catalyst is gel type strong-acid cation exchange resin or macro-porous type strong-acid cation exchange resin. The technology has the advantages of small dosage of the catalyst, short reaction time, recycling of the catalyst, easy separation of the product from the catalyst, and no generation of waste acids or sewage, and is a green allantoin synthesis technology suitable for industrial production.
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Paragraph 0011
(2016/12/01)
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- Rationally designed non-enzymatic fluorogenic 'turn-on' probe for uric acid
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Abstract The simple and cost-effective detection of uric acid in serum or urine is highly desirable as uric acid acts as a biomarker for various diseases. Herein, we report a rationally designed, non-enzymatic probe for the detection of uric acid in serum samples over a wide range of concentrations (lower: ～60 μM, higher: ～700 μM and normal: ～120-380 μM) with 'turn-on' fluorogenic behaviour.
- Pradhan, Tuhin,Maiti, Sukhendu,Kumar, Rajesh,Lee, Yun Hak,Kim, Jong Wan,Lee, Joung Hae,Kim, Jong Seung
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- Identification of ros produced by photodynamic activity of chlorophyll/cyclodextrin inclusion complexes
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Photodynamic therapy (PDT) is a way of treating malignant tumors and hyperproliferative diseases. It is based on the use of photosensitizer, herein the chlorophyll a (chl a), and a light of an appropriate wavelength. The interaction of the photosensitizer (PS) with the light produces reactive oxygen species (ROS), powerful oxidizing agents, which cause critical damage to the tissue. To solubilize chl a in aqueous solution and to obtain it as monomer, we have used cyclodextrins, carriers which are able to interact with the pigment and form the inclusion complex. The aim of this study is to examine which types of ROS are formed by Chl a/cyclodextrin complexes in phosphate buffered solution and cell culture medium, using specific molecules, called primary acceptors, which react selectively with the reactive species. In fact the changes of the absorption and the emission spectra of these molecules after the illumination of the PS provide information on the specific ROS formation. The 1O2 formation has been tested using chemical methods based on the use of Uric Acid (UA), 9,10-diphenilanthracene (DPA) and Singlet oxygen sensor green (SOSG) and by direct detection of Singlet Oxygen ( 1O2) luminescence decay at 1270 nm. Moreover, 2,7-dichlorofluorescin and ferricytochrome c (Cyt Fe3+) have been used to detect the formation of hydrogen peroxide and superoxide radical anion, which reduces Fe3+ of the ferricytochrome to Fe2+, respectively. For the first time, photodynamic activity in vitro of natural Chlorophyll a (Chl a) has been investigated evidencing which types of ROS are formed. Chl a has been solubilized in aqueous solution by means of various cyclodextrins forming inclusion complexes. The ROS production has been carried out in the system using specific molecules, called primary acceptors, which react selectively with the reactive species.
- Cellamare, Barbara M.,Fini, Paola,Agostiano, Angela,Sortino, Salvatore,Cosma, Pinalysa
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p. 432 - 441
(2013/07/19)
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- Synthesis of Allantoin catalyzed by SO4 2-/La 2O3-SiO2-ZrO2
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Allantoin was synthesized using glyoxalic acid and urea as raw materials and La2O3-promoted solid acid SO4 2-/SiO2-ZrO2 as catalyst. The present research focused on the effects of La2O3 doping amount, catalyst dosage, reactant ratio and reaction temperature and time on the yield of allantoin. The optimum condition for reactions is as follows: amount of catalyst, 2.5 g; doping amount of La2O3, 0.12 wt %; molar ratio of reactants n(urea): n(glyoxylic acid) = 4:1; reaction temperature, 75°C and reaction time, 5 h. The yield of allantoin at optimum condition was up to 46 %. The melting point determination, infrared spectroscopy and elemental analysis of the product confirmed that the obtained product is allantoin and the analysis indicated that the purity was 98.5 % or above.
- Liu, Lixiu,He, Aijiang,Li, Xiangbiao
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experimental part
p. 2298 - 2300
(2012/10/07)
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- Absolute stereochemistry and preferred conformations of urate degradation intermediates from computed and experimental circular dichroism spectra
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The enzymatic oxidation of urate leads to the sequential formation of optically active intermediates with unknown stereochemistry: (-)-5-hydroxyisourate (HIU) and (-)-2-oxo-4-hydroxy-4-carboxy-5- ureidoimidazoline (OHCU). In accordance with the observation that a defect in HIU hydrolase causes hepatocarcinoma in mouse, a detoxification role has been proposed for the enzymes accelerating the conversion of HIU and OHCU into optically active (+)-allantoin. The enzymatic products of urate oxidation are normally not present in humans, but are formed in patients treated with urate oxidase. We used time-dependent density functional theory (TDDFT) to compute the electronic circular dichroism (ECD) spectra of the chiral compounds of urate degradation (HIU, OHCU, allantoin) and we compared the results with experimentally measured ECD spectra. The calculated ECD spectra for (S)-HIU and (S)-OHCU reproduced well the experimental spectra obtained through the enzymatic degradation of urate. Less conclusive results were obtained with allantoin, although the computed optical rotations in the transparent region supported the original assignment of the (+)-S configuration. These absolute configuration assignments can facilitate the study of the enzymes involved in urate metabolism and help us to understand the mechanism leading to the toxicity of urate oxidation products. The Royal Society of Chemistry 2011.
- Pipolo, Silvio,Percudani, Riccardo,Cammi, Roberto
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experimental part
p. 5149 - 5155
(2011/09/13)
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- Oxidation of 9-β-d-ribofuranosyl uric acid by one-electron oxidants versus singlet oxygen and its implications for the oxidation of 8-oxo-7,8-dihydroguanosine
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Uric acid, a cellular antioxidant, undergoes oxidation in the presence of one-electron oxidants as well as singlet oxygen. In the present work, the oxidation pathways and products formed from oxidation of the uric acid nucleoside are compared to the more commonly studied uric acid free base. A wider distribution of products, including allantoin, urea, caffolide, and 5-carboxamido-5-hydroxyhydantoin nucleosides, are formed when the N9 position of uric acid is glycosylated. The oxidation pathways share some features in common with the oxidation of 8-oxo-7,8-dihydroguanosine, but the additional spectrum of products implies that the combination of oxidative and deaminative damage to guanosine may lead to a more complex set of DNA lesions than originally described.
- Nguyen, Khiem V.,Muller, James G.,Burrows, Cynthia J.
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supporting information; experimental part
p. 2176 - 2180
(2011/05/05)
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- Joint voltammetric determination of dopamine and uric acid
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Electrocatalytic activities of gold particles deposited onto an electrode unmodified and modified with a self-organized monolayer of cystamine in oxidation of dopamine and uric acid were compared. The possibility of joint determination of dopamine and uric acid on an electrode modified with self-organized monolayer of cystamine with gold nanoparticles was examined.
- Shaidarova,Chelnokova,Romanova,Gedmina,Budnikov
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experimental part
p. 218 - 224
(2011/06/10)
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- Characterization of the structure and function of Klebsiella pneumoniae allantoin racemase
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The oxidative catabolism of uric acid produces 5-hydroxyisourate (HIU), which is further degraded to (S)-allantoin by two enzymes, HIU hydrolase and 2-oxo-4-hydroxy-4-carboxy-5-ureidoimidazoline decarboxylase. The intermediates of the latter two reactions, HIU and 2-oxo-4-hydroxy-4-carboxy-5- ureidoimidazoline, are unstable in solution and decay nonstereospecifically to allantoin. In addition, nonenzymatic racemization of allantoin has been shown to occur at physiological pH. Since the further breakdown of allantoin is catalyzed by allantoinase, an enzyme that is specific for (S)-allantoin, an allantoin racemase is necessary for complete and efficient catabolism of uric acid. In this work, we characterize the structure and activity of allantoin racemase from Klebsiella pneumoniae (KpHpxA). In addition to an unliganded structure solved using selenomethionyl single-wavelength anomalous dispersion, structures of C79S/C184S KpHpxA in complex with allantoin and with 5-acetylhydantoin are presented. These structures reveal several important features of the active site including an oxyanion hole and a polar binding pocket that interacts with the ureido tail of allantoin and serves to control the orientation of the hydantoin ring. The ability of KpHpxA to interconvert the (R)- and (S)-enantiomers of allantoin is demonstrated, and analysis of the steady-state kinetics of KpHpxA yielded a kcat/Km of 6.0 × 105 M- 1 s- 1. Mutation of either of the active-site cysteines, Cys79 or Cys184, to serine inactivates this enzyme. The data presented provide new insights into the activity and substrate specificity of this enzyme and enable us to propose a mechanism for catalysis that is consistent with the two-base mechanism observed in other members of the aspartate/glutamate family.
- French, Jarrod B.,Neau, David B.,Ealick, Steven E.
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experimental part
p. 447 - 460
(2012/06/18)
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- Spirodihydantoin is a minor product of 5-hydroxyisourate in urate oxidation
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(Chemical Equation Presented) Spirodihydantoin is a minor product from oxidation of uric acid (～0.15% yield), while spiroiminodihydantoin is a major product from oxidation of 8-oxo-7,8-dihydroguanine (37% yield, pH 10.2). High pH and temperature favor the formation of both spiro compounds. 18O labeling experiments and in situ generation and decomposition of 5-hydroxy-N7-methylisouric acid indicate that spirodihydantoin and allantoin and spiroiminodihydantoin and guanidinohydantoin are products of 5-hydroxyisourate and 5-hydroxy-8-oxo-7,8-dihydroguanine intermediates, respectively.
- Yu, Hongbin,Niles, Jacquin C.,Wishnok, John S.,Tannenbaum, Steven R.
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p. 3417 - 3420
(2007/10/03)
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- Electrochemical oxidation of 6-thioxanthine at pyrolytic graphite electrode
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6-Thioxanthine has been studied in phosphate buffers of pH 1.2 - 11.0 using linear and cyclic sweep voltammetry, coulometry, controlled potential electrolysis and spectral studies. Two well-defined oxidation peaks, I a (1e, 1H+, pH range 1.2-6.0) and IIa (8e, 8H+, pH range 1.2-11.0) are observed. A reduction peak, III c, is noticed in the reverse sweep. Kinetic studies indicate that the decay of the UV- absorbing intermediate is a first order reaction. The studies show that in acidic medium, thio group of 6-thioxanthine exists as S-H, whereas in neutral and basic medium thio group is in the form of C=S. The products of electrooxidation have been characterized and a tentative EC mechanism has been suggested for the oxidation of 6-thioxanthine.
- Goyal, Rajendra N.,Singh, Udai P.,Abdullah, Adil A.
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p. 1044 - 1050
(2007/10/03)
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- Further insights into the electrooxidation of N-methyluric acids and correlation of oxidation potentials with frontier MO energies
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The electrochemical oxidation of various N-methylated uric acids has been studied at a pyrolytic graphite electrode at physiological pH 7.2. The observed behavior was compared with uric acid to evaluate the effect of a methyl group. The E(p) value was found to shift to less positive potentials when a methyl group is present at the N-1 position and to more positive potentials when substitution is at the N-3 position or at nitrogens of the imidazole ring. The values of ΔE(p) followed the additivity of the substituents effect only when two methyl groups are present in different rings. The methyl groups were also found to increase the electron density at N atoms of uric acids, and an excellent correlation between the oxidation potential and energy of the highest occupied molecular orbital was observed. On the basis of these studies it is concluded that the presence of a methyl group in the pyrimidine ring restricts the formation of allantoin. The peroxidase-catalysed oxidation of the N-methylated derivatives of uric acid was found to follow a pathway similar to that observed during electrochemical oxidation.
- Goyal, Rajendra N.,Thankachan,Jain, Neena
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p. 1515 - 1524
(2007/10/03)
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- Electrochemical and peroxidase catalysed oxidation of 9-β-D-ribofuranosyluric acid 5′-monophosphate
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The electrochemical oxidation of 9-β-D-ribofuranosyluric acid 5′-monophosphate (UA-9R-5′-P) in aqueous solution has been studied in the pH range 2.10-10.0. The evidence strongly indicates that UA-9R-5′-P is oxidized in a 2e-, 2H+ reaction to give an unstable diimine which subsequently decomposes. A UV absorbing intermediate is observed during electrooxidation which decays in a pseudo first-order reaction to give alloxan, urea and ribosyl phosphate at pH 3.0. Controlled potential electrolysis results in the transfer of 2.0 ± 0.2 electrons per molecule and three major products are obtained at pH 7.0; allantoin, 5-hydroxyhydantoin-5-carboxamide and D-ribose. Tentative reaction schemes are proposed to explain the formation of these products. Oxidation of UA-9R-5′-P in the presence of peroxidase and H2O2 also generates an intermediate which has spectral and kinetic properties identical to those of the intermediate generated electrochemically. Thus, it is believed that electrochemical and enzymic oxidation of UA-9R-5′-P proceed by identical reaction mechanisms.
- Goyal, Rajendra N.,Rastogi, Arshi
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p. 2423 - 2429
(2007/10/03)
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- Differential pulse voltammetric investigations of uric acid in aqueous and micelle systems
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Differential pulse voltammetric behaviour of uric acid has been studied in phosphate buffers of pH 2.3- 11.2 at glassy carbon electrode. A well-defined 2e, 2H+, pH-dependent oxidation peak has been noticed and the conjugate base found as the electroactive species. Cationic surfactants have been found to significantly affect the Ep and ip. At concentrations below CMC, the change in Ep is predominantly due to adsorption effects, whereas at concentrations above CMC electrocatalysis and micellar catalysis shifted the Ep towards less positive potentials. The voltammetric behaviour in the presence of surfactants indicates that the 2e oxidation proceeds in two le steps and the formation of cationic free radical is suggested. The products of electrode reaction have been found to be the same in aqueous and micelle systems.
- Goyal, Rajendra N.,Ahmad, Shakeel,Rastogi, Arshi
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p. 283 - 289
(2007/10/03)
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- Significance of uricase in oxidase-induced oxidative coloring reaction of p-phenylenediamine
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Uricase (urate oxidase, UOD) is found to induce the oxidative polymerization of p-phenylenediamine (PPD) effectively, which is a key reaction of color development in hair-dyeing and fur-dyeing practices. The significance of uricase is described by comparison to glucose oxidase (GOD), which also produces hydrogen peroxide as an oxidizing agent of PPD. In contrast to UOD, GOD inhibits the polymerization reaction. Spectroscopic and electrochemical study has revealed that the inhibition effect of GOD is ascribed to the glucose dehydrogenase activity, in which p-benzoquinonediimine (BQI) as the two-electron oxidized form of PPD works as an efficient electron acceptor to be reduced back to PPD, resulting in the inhibition of the subsequent polymerization of BQI. On the other hand, the UOD reaction does not compete with the polymerization of BQI owing to the lack of urate dehydrogenase activity in UOD. In addition, it has been found that UOD catalyzes the oxidation of PPD in the presence of uric acid by PPD oxidase-like and PPD peroxidase-like activities. These properties of UOD are favorable toward the oxidative generation of BQI from PPD and are responsible for the prominent ability in the oxidative coloring of PPD.
- Aoki, Masahiro,Tsujino, Yoshio,Kano, Kenji,Ikeda, Tokuji
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p. 5610 - 5616
(2007/10/03)
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- Electrochemical oxidation and kinetics of the decay of UV-absorbing intermediate of uric acid oxidation at pyrolytic graphite electrodes
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The electrochemical oxidation of uric acid has been studied in 0.50 M NaCl and phosphate buffers of ionic strength 0.05 to 0.5 M. The kinetics of the decay of UV-absorbing intermediate generated during electrooxidation of uric acid (7,9-dihydro-1H-purine-2,6,8-(3H)-trione) has been also studied. On the basis of pseudo first order rate constants observed in different media, it has been concluded that the diimine species formed from the 2e,2H+ oxidation of uric acid is attacked by water to give diol, which then decomposes in a series of reactions to give allantoin as the major product at pH 7.2. The possibility of attack by phosphate on diimine has been ruled out.
- Goyal, Rajendra N.,Mittal, Alok,Agarwal, Divya
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p. 1668 - 1674
(2007/10/02)
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- Oxidation Chemistry of Adenine and Hydroxyadenines at Pyrolytic Graphite Electrodes
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The electrochemical oxidation of adenine and hydroxyadenines has been studied in aqueous solutions in the pH range 3.0-11.2 using a pyrolytic graphite electrode.The initial course of the electrode reaction has been deduced to involve a 2e, 2H+ reaction to give 2- and not 8-hydroxyadenine, further oxidation of which gives 2,8-dihydroxyadenine and then diimine species which undergo a series of chemical reactions to give different products.The major products of oxidation at pH 3.0 and parabanic acid (imidazolidinetrione)> and at pH 7.0 have been isolated using HPLC and column chromatography and their structures elucidated by spectroscopic techniques.The probable EC mechanisms for their formation have also been suggested.
- Goyal, Rajendra N.,Kumar, Anoop,Mittal, Alok
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p. 1369 - 1375
(2007/10/02)
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- Electrochemical oxidation of xanthine at solid electrodes
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The electrochemical oxidation of xanthine at glassy carbon electrode (GCE) and pyrolytic graphite electrode (PGE) has been studied in the pH range of 1.8-10.7 and found to proceed in a 4e, 4H(+) reaction via the formation of uric acid at both the electrodes.Cyclic voltammetric behaviour, spectral studies, observed first order rate constant and product identification clearly point out that xanthine initially oxidises in a 2e, 2H(+) reaction to give uric acid which on subsequent oxidation gives the final products.The first order decay of UV absorbing intermediate has been monitored and the products have been identified.A plausible mechanism is suggested on the basis of observed experimental behaviour.
- Goyal, R. N.
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p. 467 - 471
(2007/10/02)
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- On-line electrochemistry/thermospray/tandem mass spectrometry as a new approach to the study of redox reactions: the oxidation of uric acid.
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The electrochemical oxidation pathway of uric acid was determined by on-line electrochemistry/thermospray/tandem mass spectrometry. Intermediates and products formed as a result of electrooxidation were monitored as the electrode potential was varied. Several reaction intermediates have been identified and characterized by tandem mass spectrometry. The tandem mass spectrometric results provide convincing evidence that the primary intermediate produced during the electrooxidation of uric acid has a quinonoid diimine structure. The results indicate that once formed via electrooxidation, the primary intermediate can follow three distinct reaction pathways to produce the identified final products. The final electrochemical oxidation products observed in these studies were urea, CO2, alloxan, alloxan monohydrate, allantoin, 5-hydroxyhydantoin-5-carboxamide, and parabanic acid. The solution reactions that follow the initial electron transfer at the electrode are affected by the vaporizer tip temperature of the thermospray probe. In particular, it was found that at different tip temperatures either hydrolysis or ammonolysis reactions of the initial electrochemical oxidation products can occur. Most importantly, the results show that the on-line combination of electrochemistry with thermospray/tandem mass spectrometry provides otherwise difficult to obtain information about redox and associated chemical reactions of biological molecules such as the structure of reaction intermediates and products, as well as providing insight into reaction pathways.
- Volk,Yost,Brajter-Toth
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p. 1709 - 1717
(2007/10/02)
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- THE MECHANISM FOR THE CONVERSION OF URIC ACID INTO UROXANATE AND ALLANTOIN A NEW BASE-INDUCED 1,2-CARBOXYLATE SHIFT
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Alkaline permanganate oxidation of uric acid (1), particularly the late stages of the transformation into uroxanate (7) and allantoin (3), was studied by means of isotope-position labelling.A clear-cut degradation procedure developed for distinguishing among carbonyl and α-aminal carbon atoms in these products demonstrated conclusively that the carboxylic carbon of 7 and the 4-carbonyl carbon of 3 have their origin in C(5) of uric acid (1).None of the mechanisms that have been proposed for this reaction would have predicted this result.Isotope-labelling evidence, in combination with other data, revealed the sequence of events and identities of species involved in oxidative transformation of 1; the carbon-skeleton rearrangement of the first transient intermediate 4 must occur by a 1,2-carboxylate shift to give allanatoin-5-carboxylate (6) which either decarboxylates to allantoin (3) or else undergoes hydrolytic ring opening to uroxanate (7).
- Poje, M.,Sokolic-Maravic, Lea
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p. 6723 - 6728
(2007/10/02)
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- THE MECHANISM FOR THE CONVERSION OF URIC ACID INTO ALLANTOIN AND DEHYDRO-ALLANTOIN. A NEW LOOK AT AN OLD PROBLEM
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The reaction of uric acids 1 with iodine in alkaline solution yields, on acidification, new dehydroallantoins 11, or normal oxidation products, allantoins 13, depending on whether an excess or a stoichiometric amount of oxidant was used.The structure and regiochemistry of dehydro-allantoins 11 was established by chemical, spectroscopic, and 14C-labelling methods.These experimental results, in combination with other data, generate a new mechanistic scheme for the uricolytic pathway to allantoin, ruling out the intervention of a symmetrical transition state prior to the decarboxylation step.
- Poje, M.,Sokolic-Maravic, Lea
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p. 747 - 752
(2007/10/02)
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