Hydrogen-Bonding Interaction Regulates Photoisomerization of a Single-Bond-Rotation Locked Photoactive Yellow Protein Chromophore in Protein.

We have employed the QM(CASPT2//CASSCF)/MM method to explore the excited-state isomerization and decay mechanism of a single-bond-rotation locked photoactive yellow protein (PYP) chromophore in wild-type and mutant proteins. The S1 state is a spectroscopically bright state in the Franck-Condon region. In this state, there exist two excited-state isomerization pathways separately related to the clockwise and anticlockwise rotations of the C=C bond. The clockwise path is favorable because of a small barrier of 2 kcal/mol and uses a novel bicycle-pedal unidirectional photoisomerization mechanism in which the involved two dihedral angles rotate asynchronously because of the reinforced hydrogen-bonding interaction between the chromophore and Cys69. Near the twisted S1 minimum, the chromophore hops to the S0 state via the S1/S0 conical intersection. Finally, the R52A mutation has small effects on the excited-state properties and photoisomerization of the locked PYP chromophore. The present work provides new insights for understanding the photochemistry of PYP chromophores in protein surroundings.

Exposure to phototoxic NSAIDs and quinolones is associated with an increased risk of melanoma.

PURPOSE: Ultraviolet radiation exposure is the most important exogenous risk factor for cutaneous malignancies. It is possible that phototoxic drugs promote the development of cutaneous melanoma (CM) by intensifying the effect of ultraviolet light on the skin. We investigated the association between the use of common systemic phototoxic drugs and development of CM. METHODS: This study was a case-control study in a Dutch population-based cohort. The drug dispensing data was obtained from PHARMO, a Dutch drug dispensing and hospital admissions registry, and linked to PALGA, the nationwide pathology network of the Netherlands. The cases were patients diagnosed with pathologically confirmed primary CM between 1991 and 2004. Controls were sampled from the PHARMO population. Exposure to systemic phototoxic drugs was measured and included antimicrobial agents, diuretics, antipsychotic drugs, antidiabetic drugs, cardiac drugs, antimalarials and nonsteroidal anti-inflammatory drugs (NSAIDs). A multivariate conditional logistic regression analysis was performed to study the association between exposure to phototoxic drugs and CM. RESULTS: The study population included 1,318 cases and 6,786 controls. Any phototoxic drug during the study period was dispensed for 46 % of the cases and 43 % of the controls (p = 0.012). The use of quinolones [odds ratio (OR) 1.33, 95 % confidence interval (CI) 1.01-1.76] and propionic acid derivative NSAIDs (OR 1.33, 95 % CI 1.14-1.54) had a positive association with CM. CONCLUSIONS: Our study shows that the use of phototoxic drugs is associated with an increased risk of developing CM. Even a short-term use of phototoxic quinolones and propionic acid derivative NSAIDs may increase the risk for CM. Patient education to promote sun-protective behaviour is essential to avoid immediate adverse effects and possible long-term effects of phototoxic drugs.

Trimethylaluminium mediated amide bond formation in a continuous flow microreactor as key to the synthesis of rimonabant and efaproxiral.

A safe, functional-group-tolerant and high-throughput version of the trimethylaluminium mediated amide bond formation reaction has been developed in a microreactor system; rimonabant and efaproxiral were prepared to illustrate the utility of the method.

Photosensitized conversion of 9,10-deoxytridachione to photodeoxytridachione.

[reaction: see text] The photochemical conversion of 9,10-deoxytridachione to photodeoxytridachione has been photosensitized. The conversion was also quenched by piperylene. Photodeoxytridachione was produced in good yields under conditions in which only the cyclohexadiene group is sensitized. The results show that some, and perhaps all, of the photoreactions of 9,10-deoxytridachione occur through a triplet excited state. The mechanistic and biosynthetic implications of these results are discussed.

Triplet photoreactivity of the diaryl ketone tiaprofenic acid and its decarboxylated photoproduct. Photobiological implications.

The 2-benzoylthiophene chromophore of the photosensitizing drug tiaprofenic acid and of its decarboxylated derivative is characterized by a unusually high energy gap between the T1 (pi, pi*) and T2 (n, pi*) excited states, which makes this a unique system to study the intrinsic photoreactivity of the two states. Weak fluorescence and phosporescence emission were detected at room temperature. Tiaprofenic acid undergoes photodecarboxylation from the triplet manifold as the main reaction. The photoprocess is temperature dependent with activation energy of 7-10 kcal/mol, close to the energy gap between T1 and T2. The decarboxylated product abstracts hydrogen in type I reactions. The involvement of T2 in the above processes is proposed. Moreover the decarboxylated derivative exhibits reactivity toward phenols, consistent with a participation of the T1 state as electron acceptor. The observed photoprocesses can account for biological photosensitization reactions, like membrane damage and protein modification.

Photodegradation and photobinding of tiaprofenic acid: in vitro versus in vivo.

The photoreactivity of the photosensitizing nonsteroidal anti-inflammatory drug tiaprofenic acid (TA) and its photoproduct decarboxytiaprofenic acid (DTA) was studied both in the presence and in the absence of bovine serum albumin (BSA). The photoproduct DTA was found to be photostable in buffered aqueous solution at pH 7.4, but photodecomposed when BSA was present in the reaction medium. Both TA and DTA underwent irreversible photobinding to BSA in an almost quantitative way, as evidenced by radioactivity measurements using labeled (3H) compounds. In the case of TA, it has been proven that photobinding is mainly attributable to the phototoreactivity of in situ-generated DTA. The photodegradation and photobinding of TA were also investigated in the epidermis in vivo. Rats were exposed to UVA after application of TA to their shaven dorsal skin. During the initial periods of irradiation, the amount of TA decreased sharply, and the yield of the corresponding photoproduct (DTA) reached a maximum. Prolonged irradiation led to photodegradation of DTA. In vivo photobinding was studied using 3H-TA. Photobinding took place slowly at the beginning, but its rate increased sharply after complete photoconversion of TA, when the photoproduct DTA reached the maximum concentration. Thereafter, the decrease of DTA was more pronounced than that of TA. This indicates that-also in vivo-DTA rather than TA is responsible for the photobinding to biomacromolecules in the viable layer of the epidermis. Overall, the above results suggest that irradiation of TA in buffered aqueous solution, in the presence of proteins, is a reasonable model system to study the photodegradation and photobinding behavior of this drug in vivo. From the qualitative point of view, the main conclusion is that DTA plays a key role both in vivo and in vitro: it is the major photoproduct, it undergoes further photodegradation upon prolonged irradiation, and it appears to be responsible for the photobinding process.

Light-induced vitamin deficiency in Drosophila melanogaster.

Illumination by visible light (400 Ix) of cultures containing larvae of Drosophila melanogaster can reduce survival (Bruins et al., Insect Biochemistry 21:535-539, 1991). Here we show that the effect of light depends on the presence of propionic or acetic acid in the food medium. We also show that survival is far more affected by illumination of the yeast food media than by direct illumination of the eggs and developing larvae. It is shown that addition of antioxidants to the food prevents light induced mortality. The action of antioxidants suggests that free radicals are important in light induced mortality. We also showed that both yeast and riboflavin (vitamin B2) solutions illuminated with visible light (400 Ix) generate hydrogen peroxide. Other vitamin and amino acid solutions do not produce peroxide in measurable amounts. However, the concentration of photogenerated hydrogen peroxide is far too low to explain the death of eggs and developing larvae upon exposure to light. A 400 Ix light treatment destroys the capability of yeast food media to support survival of larvae. Addition of vitamin C, carotene, tryptophan, nipagin, uric acid, or sucrose to the light treated medium does not restore viability. It is restored when riboflavin is added to the photo-inactivated yeast. A high concentration of pyridoxine also produced an improvement in survival. When riboflavin is treated with light, it cannot support survival on synthetic food media nor can it restore survival on light treated yeast food media. These results show that riboflavin (or a derivative) is a major light sensitive compound of yeast, which can be degraded by light. Light induced loss of riboflavin leads to mortality, because this is an essential dietary vitamin. The vitamin degradation can be prevented by dietary antioxidants. A chromatographic analysis confirms this conclusion.

Involvement of drug-derived peroxides in the phototoxicity of naproxen and tiaprofenic acid.

Photodegradation of naproxen and tiaprofenic acid in aqueous buffered solutions leads to decarboxylated products with ethyl, 1-hydroxyethyl and/or acetyl side chains. The photomixtures obtained in the presence of oxygen were clearly more toxic to cultured hepatocytes than those obtained under anaerobic conditions. This effect was more noticeable in the case of naproxen. Based on the composition of the oxygenated photomixtures and the relative toxicity of the different photoproducts, it is possible to account for most of the observed toxicity in the case of tiaprofenic acid but not in the case of naproxen. This is explained as a result of the presence of drug-derived peroxidic species in the photomixtures and their contribution to the observed toxicity. Peroxides were determined by the peroxidase-catalyzed oxidation of dichlorodihydrofluorescein to its fluorescent analog. The amount of peroxides present in naproxen photomixtures was much higher than in the case of tiaprofenic acid. A dose-dependent depletion of intracellular glutathione was observed when hepatocytes were incubated with peroxide-containing naproxen photomixtures. This effect was prevented by the addition of catalase or N-acetylcysteine to the culture medium.

Photodegradation of nalidixic and tiaprofenic acids and nifedipine in aerobic conditions.

Since nalidixic acid had been previously studied in acidic and basic media, nifedipine had been investigated in anaerobic conditions and under ultraviolet light and tiaprofenic acid had not been studied at all, their photodegradation was carried out in this laboratory under milder conditions, with methanol as the solvent and using visible light. The role of oxygen was demonstrated and the photoproducts were isolated and identified spectroscopically.