Conical intersection in a bilirubin model – A possible pathway for phototherapy of neonatal jaundice

Phototherapy of neonatal jaundice involves Z–E-isomerisation around an exocyclic double bond in bilirubin. Our results of a CASSCF study on dipyrrinone, a bilirubin model, show a conical intersection between the ground and first excited singlet states associated with the Z–E-isomerisation. The conical intersection, located ca. 50 kJ/mol below the Franck–Condon-point, together with the S₁ minimum, ca. 50 kJ/mol below the conical intersection, are able to explain the available time-resolved spectroscopic data (the very short lifetime of the initially excited state and transient ‘dark state’ intermediate) as well as bilirubin’s very low fluorescence quantum yield and the medium-efficient photoisomerisation reaction.     

Synthesis,photooxidation and Z ⇌ E photoisomerization of benzalpyrrolinones

In contrast to oxodipyrromethenes and bilirubin, benzalpyrrolinones (H, P-OCH₃, p-Cl, p-N(CH₃)₂ and o-CH₃) and α-pyridalpyrrolinones appear not to undergo dye-sensitized photo-oxygenation. They do, however, undergo an unsensitized E ? Z photoisomerization reminiscent of stilbene photoisomerization, and the photostationary state varies with substituent. Intramolecular H-bonding is implicated in the α-pyridalpyrrolinone isomerization. In each case, the Z isomers are the therrnodynamically more stable ones, but the corresponding E isomers have been isolated and characterized following photoirradialion.     

竹红菌甲素光敏氧化反应机制

本文报道了在竹红菌甲素的光敏氧化反应中, 原初反应产生了^1O2、O2和H2O2,在一些还原性底物(5-羟基色氨酸、色氨酸、组氨酸、蛋氨酸和赖氨酸等)的存在下, 体系中形成的O2量大大增加。证明了体系中的^1O2是通过三重态的竹红菌甲素和基态氧进行能量传递形成的, O2是体系中的竹红菌甲素负离子自由基和基态氧进行单电子转移的结果, H2O2是体系中存在的竹红菌甲素二价负离子还原基态氧的产物。在一些底物存在下, 次级反应产生了.OH。我们也发现竹红菌甲素具有弱的抽氢能力而生成一些有机自由基, 这些有机自由基的形成促进了各种活泼态氧的相互转化, 因此我们认为竹红甲素的光敏氧化是各种活泼态氧和一些有机自由基综合反应的结果。     

The mechanism of photooxidation of the menaquinones

Irradiation of menaquinone-1 under nitrogen gives rise to metastable isomeric o-quinone methides, one of which reacts with oxygen in the dark to form the same hydroperoxide obtained previously from direct or dye sensitized photolysis of the quinone in the presence of oxygen.     

Bilirubin photoisomerization products in serum and urine from a Crigler-Najjar type I patient treated by phototherapy

The relative compositions of the photoisomers of bilirubin-1X alpha (4Z, 15Z-bilirubin) in serum and urine of a patient with Crigler-Najjar type I syndrome treated by phototherapy are reported. High-performance liquid chromatography analysis reveals the presence of high serum levels of the configurational bilirubin photoisomer (4Z,15E-bilirubin) before the beginning of phototherapy (between 12 and 16% of the total bilirubin). The configurational photoisomer value increases during phototherapy with blue fluorescent lamps up to a photoequilibrium of about 25%, similar to that obtained in a bilirubin solution in vitro irradiated by the same lamps. This evidence suggests an inefficient serum excretion of the 4Z,15E-bilirubin. Indeed, its average half-life in serum of the Crigler-Najjar patient is found to be about 8 h. No detectable traces of the bilirubin structural isomer, lumirubin, are found in the serum. On the other hand, lumirubin represents the dominant bilirubin isomer excreted in the urine, as both 15Z and 15E configurations. Smaller amounts of 4Z,15E-bilirubin, 4E,15Z-bilirubin and native 4Z,15Z-bilirubin are observed in urine. The presence in urine of 4Z,15Z-bilirubin is probably due to a fast reversion of the configurational photoisomers to their native form. The half-life of the configurational photoisomers in urine kept at 38 degrees C is found to be of the order of a few minutes. Our study indicates that in Crigler-Najjar type I patients, mechanisms exist to excrete all bilirubin photoisomers. The lumirubin pathway seems to contribute markedly to bilirubin excretion in the urine, as occurs in jaundiced babies under phototherapy. However, the contribution of configurational isomers cannot be neglected.     

Updated chemical mechanism for atmospheric photooxidation of toluene

A new reaction mechanism describing the atmospheric photochemical oxidation of toluene is formulated and tested against environmental chamber data from the University of California, Riverside, Statewide Air Pollution Research Center (SAPRC). On simulations of toluene—NO_x and toluene—benzaldehyde—NO_x irradiations, the average predicted O₃ and PAN maxima are within 3% of the experimental values. Simulations performed with the new mechanism are used to investigate various mechanistic paths, and to gain insight into areas where our understanding is not complete. Specific areas that are investigated include benzaldehyde photolysis, organic nitrate formation, alternate ring fragmentation pathways, and conjugated γ-dicarbonyl condensation to the aerosol phase.     

Studies on the photooxidation mechanism of polymers. I. Photolysis and photooxidation of polystyrene

A new mechanism has been proposed for the photooxidation of polystyrene as film and in benzene. The initial stage of the photooxidative degradation may involve reactions of singlet oxygen with polystyrene molecules. Singlet oxygen may be formed in the reaction between excited benzene ring in polystyrene molecule and molecular oxygen. The addition of singlet oxygen quenchers such as 1,3-cyclohexadiene or β-carotene reduces the rate of polymer degradation in benzene solution. The mechanisms of the photolysis of polystyrene as film and in benzene solution, in vacuo and in the presence of oxygen, are discussed and interpretations proposed. The pronounced yellowing of polystyrene during the photooxidation process is interpreted as a reaction involving benzene ring-opening photooxidation in polystyrene molecule. These results were obtained by comparing ultraviolet and infrared spectra in experiments of photooxidation of pure liquid benzene and polystyrene film.     

Quantum-chemical model for the photoisomerization of protonated schiff bases of formaldehyde and acrolein

By means of potential energy curves in the excited states we discuss the mechanism of trans-cis isomerization of protonated imine and enumine. For the higher homologues isomerization about the CN bond is highly improbable     

On the mechanism of the photoisomerization reaction of aryl substituted indenes

The mechanism of the phototransposition reaction of a number arylalkyl substituted indenes has been found to be markedly dependent on the nature and location of the substituent groups.     

The photoisomerization mechanism of azobenzene: a semiclassical simulation of nonadiabatic dynamics

We have simulated the photoisomerization dynamics of azobenzene, taking into account internal conversion and geometrical relaxation processes, by means of a semiclassical surface hopping approach. Both n-->pi* and pi-->pi* excitations and both cis-->trans and trans-->cis conversions have been considered. We show that in all cases the torsion around the N==N double bond is the preferred mechanism. The quantum yields measured are correctly reproduced and the observed differences are explained as a result of the competition between the inertia of the torsional motion and the premature deactivation of the excited state. Recent time-resolved spectroscopic experiments are interpreted in the light of the simulated dynamics.     

Relationship between photoisomerization path and intersection space in a retinal chromophore model

A low-lying segment of the intersection space (IS) between the excited-state and the ground-state energy surfaces of a retinal chromophore model has been mapped using ab initio CASSCF computations. Analysis of the structural relationship between the computed IS cross-section and the excited state Z --> E isomerization path shows that these are remarkably close both in energy and in structure. Indeed, the IS segment and the Z --> E path remain roughly parallel and merge only when the double bond reaches a 70 degree twisting. This finding supports the idea that, in certain chromophores, a more extended segment of IS, and not a single conical intersection, contributes to the decay to the ground state.     

Kinetics and mechanism of the vitamin B2-sensitized photooxidation of isoproterenol

The sensitized photooxidation promoted by daylight-absorbing compounds appears as a plausible course to produce the photodegradation of catecholamines. We report the kinetics and mechanism of vitamin B2 (riboflavin [Rf])-sensitized photooxidation of isoproterenol (Iso), a synthetic sympathomimetic drug structurally related to epinephrine, using water as a solvent. A weak dark complex Rf-Iso is formed, only detectable at relatively high Iso concentrations (>10 mM), with a mean value of 13 +/- 3 M(-1) for the apparent association constant. Under aerobic sensitizing conditions (Rf approximately 0.02 mM and Iso approximately 0.5 mM) two oxidative mechanisms operate, mediated by singlet molecular oxygen (O2(1delta g)) and superoxide radical anion (O2*-). Our analysis shows that the main reaction pathway is an electron transfer-mediated quenching of Rf excited triplet state (3Rf*) by Iso. It produces the species Iso*+ and Rf*-. The latter, in a subsequent reaction path, generates O2*-, which is mainly responsible for Iso photooxygenation. In a less-important process, energy transfer of the 3Rf* to dissolved oxygen generates O2(1delta g). The kinetic balance between chemical and physical quenching of O2(1delta g) by Iso indicates that the process is largely dominated by the physical, not chemical, interaction. The results, which can be extrapolated to an in vivo condition, show the susceptibility of Iso to undergo visible light-induced photodegradation in the presence of dye sensitizers present in the environment.     

Study of the mechanism of the photochemical oxidation of bilirubin by high-performance liquid chromatography

The photochemical degradation of bilirubin was studied in vitro using high-performance liquid chromatography and spectrophotometry. Attention was centered on the formation of biliverdin, which is produced as an intermediate in the photooxidation catalysed by riboflavin. approximate values of the overall and partial relative rate constants were calculated using the physiochemical criteria for a pseudo-monomolecular reaction. A more precise evaluation was made by non-linear regression programmes on a Hewlett-Packard 9835 A computer. In addition to the formation of biliverdin, attention was also paid to accompanying processes affecting the reaction mechanism, which were explained as aggregation and dimerization of bilirubin and biliverdin. It was also found that during phototherapy of newborn babies suffering from hyperbilirubinaemia the level of biliverdin in the plasma increased. The results are discussed from the point of view of phototherapy.     

Systematic control of photochemistry: the dynamics of photoisomerization of a model cyanine dye

On-the-fly CASSCF nonadiabatic dynamics have been used to model the trans-cis isomerization of a model cyanine dye. Our results show that the photochemical generation of the trans versus cis product is dynamically controlled by the presence of an extended cis-trans conical intersection seam that persists along all torsional angles. This in turn suggests that the photochemistry could be completely controlled by controlling the distribution of momentum components in a wave packet excited by laser photolysis in a coherent control experiment.     

Study of adsorption behavior of bilirubin on human-albumin monolayer using a quartz crystal microbalance

The quartz crystal microbalance was employed to study the adsorption behavior of bilirubin on human-albumin layer, which was chemically bound to the self-assembled monolayer of 4-aminothiophenol on the surface of a gold electrode of the crystal via glutaraldehyde. A long-time adsorption process of bilirubin that took place on a human-albumin-modified surface was observed, and the adsorption kinetic parameters were estimated from the in situ frequency measurements. The amount of adsorbed bilirubin increased with increasing of both hydrogen ions and bilirubin concentration and was larger than that estimated based on the conclusion that there are two affinity sites for bilirubin per albumin molecule. With the present method, the displacement of bilirubin from an albumin layer caused by aspirin was also examined. QCM measurement provides a facile method for in situ monitoring of the adsorption/desorption of bilirubin on proteins layers.     

Model for the photooxidation of parylenes

Parylenes belong to a family of polymers that have been investigated for use in electronic and medical applications. The photooxidation of these materials is of interest both to prevent degradation and to induce targeted chemical changes. This article describes a transport and reaction model for the photooxidation of parylenes. This model is based on existing polymer photooxidation mechanisms that have been adapted to this system. The model has been compared with existing parylene photooxidation data for this system and shows qualitative agreement with surface oxidation profiles and oxidation depth profiles. On the basis of the results of the model comparison, it has been determined that the key parameters that appear to affect the photooxidation of parylenes are the diffusion coefficient of oxygen in these films and the concentration of oxygen initially present in these films. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2666–2677, 2004     

N2(OH)2光致异构化机理

偶氮化合物 ,如偶氮芳烃、偶氮冠醚、偶氮环糊精等作为有机光化学开关分子具有重要的应用价值 ,已引起了人们的广泛重视[1] 。这类分子作为光化学开关的基本原理是反式的偶氮化合物在光照射下异构化为顺式构型 ,如下图所示 :长期以来 ,人们对该反应的机理持有两种不同看法[2 ,3] :一是分子平面内按角旋转机制 ,二是绕Ｎ =Ｎ键旋转的二面角机制。前者受氮上孤对电子的“阻碍” ,而后者将破坏共轭双键 ,显然两种机制在基态都很难发生 ,而如果光激发后发生ｎ→π 或π→π 跃迁 ,则两种机制均有可能发生。本文用量子化学从头算方法研究基态和激…