Method of kinetic analysis of photodegradation: nifedipine in solutions

A rate equation for photodegradation was derived from Lambert-Beer's law and Grotthus-Draper's law: -dc/dt=k1(1-exp(-(k2c+k3(c0-c))))k2c/(k2c+k3(c0-c)) where c is the concentration of reactant, c0 is the initial concentration of reactant, t is time, k1 is the rate constant, and k2 and k3 are the absorption coefficient of reactant and its photodegradation product, respectively. In a case where the photodegradation products have no photoabsorption, k3 assumes the value of zero in the above general equation. In a case where the photodegradation products have the same spectrum and molar absorptivity as that of the reactant, k3 assumes the value of k2, and hence the photodegradation is not a first-order reaction; however, the equation itself gives the pseudo-first-order reaction rate equation. In a case where the concentration of reactant is high enough, the equation approaches a zero-order reaction rate equation. The photodegradation rate of nifedipine in solutions under a germicidal lamp, near an ultraviolet fluorescent lamp and a fluorescent lamp was analyzed using the above equation. The photodegradation rate was directly proportional to the amount of light absorbed, and fitted well with the equation. The above theoretical equation was substantiated by the photodegradation of nifedipine, and hence is expected to apply to other photosensitive drugs.     

Antioxidant product analysis of Folium Hibisci Mutabilis

Folium Hibisci Mutabilis, a new member of Chinese Pharmacopoeia, can treat some diseases induced by reactive oxygen species. The study prepared a lyophilized aqueous extract of Folium Hibisci Mutabilis (LAFHM). LAFHM was found to enrich eight flavonoids (i.e., quercetin, luteolin, hyperoside, isoquercitrin, rutin, kaempferol, tiliroside, and vitexin) by HPLC analysis. These flavonoids were further compared using antioxidant assays, where triliroside and vitexin always exhibited higher IC₅₀ values than the others. In ultra-performance liquid chromatography coupled with electrospray ionization quadrupole time-of-flight tandem mass spectrometry analysis, these flavonoids could basically give two characteristic m/z values (226 and 196) and their corresponding double m/z values (i.e., 602, 570, 926, 926, 570, 1186, and 862), when treated by 1,1-diphenyl-2-picryl-hydrazl radical (DPPH^?). Finally, the coupling products of DPPH^?-treated triliroside were investigated using computational chemistry. It was found that the –OH in para-coumaroyl moiety to have the lowest bond disassociation energy among all phenolic -OHs in the triliroside. In conclusion, Folium Hibisci Mutabilis contains the above eight antioxidant flavonoids. Despite of the different antioxidant levels, they can generally produce flavonoid-radical coupling product and flavonoid-flavonoid homodimer during antioxidant process. Especially, tiliroside uses para-coumaroyl as linker to construct a tiliroside-radical coupling product at the meta-carbon atom.     

Metachromatic effects and photodegradation of basic blue on nanocrystalline titania films

Titania nanocrystalline films have been deposited on solid substrates by a sol-gel procedure carried out in Triton X-100 reverse micelles. When the dye Basic Blue is adsorbed on these films, it demonstrates a strong metachromatic effect; that is, it aggregates, resulting in a blue shift of its absorption spectrum. Metachromasy in this system is related to the hydrophilicity of the film surface and to the humidity of the film environment. Films composed of 67% titania and 33% silica gave an intense and reversible metachromatic effect that can be exploited to make a handy humidity sensor. Photodegradation of Basic Blue on titania films is faster in humid environments than in dry environments, and this goes in parallel with metachromatic effects.     

LC/MS/MS study for identification of entacapone degradation product obtained by photodegradation kinetics

Entacapone is indicated for clinical use as an adjunct to levodopalcarbidopa to treat patients with idiopathic Parkinson's Disease who experience the signs and symptoms of end-of-dose wearing-off. The aim of this study was to determine the photodegradation kinetics and to elucidate the structure of the main degradation product. The stability of entacapone was studied in order to investigate the degradation kinetics of this drug using LC as a stability indicator. Entacapone was subjected to accelerated photodegradation. This study was carried out with methanolic solutions, prepared from coated tablets, in quartz cells under UV light at 254 nm. The degradation process of entacapone in solutions can be described by second-order kinetics under the experimental conditions used in this study. The LC/MS/MS determinations revealed that in the above conditions the photodegraded product formed the geometric isomer of entacapone (Z-entacapone). The obtained results show the importance of appropriate light protection during the drug development process, storage, and handling.     

Solvents effects in the photodegradation and reactivity of the various ionic forms of haematoporphyrin

The dependence of the ionic forms of haematoporphyrin(LX) dihydrochloride (HpdiCl) on solvent composition was investigated. In 2.8 x 10(-4) M solutions of HpdiCl in apolar (C6H6) and polar (CH3CN) solvents, HpdiCl exists in dicationic form. In hydrogen-bonding solvents, such as CH3OH, HpdiCl can exist in neutral, monocationic and dicationic forms. In C6H6-CH3OH solvent mixtures, the ionic forms in which HpdiCl is present depend on the composition of the solvent and on the acidity of the solution. The rate of oxidative photodegradation of HpdiCl excitation in its Q bands (WBI) and the ability to produce free radicals are different for the different ionic species. The highest values correspond to the dicationic form of HpdiCl and the lowest values correspond to the neutral species. In the absence of oxygen, the formation of free radicals due to the reaction of 3(Hp dication) is detected in the following solvent mixtures: CH3OH-toluene, CH3OH-ethylbenzene, CH3OH-hexane. The data obtained indicate that interaction of 3(Hp dication) with methine groups is an intermediate step in the formation of free radicals. In the HpdiCl concentration range studied, the presence of a phenolic antioxidant, such as beta-naphtol, inhibits the oxidative photodegradation of the dicationic form in a treated solution, but has little effect on the oxidative photobleaching of the monocation. The rate of oxidative photodegradation of the monocationic form increases with the addition of propionic acid to the solution.     

Riboflavin photodegradation and photosensitizing effects are highly dependent on oxygen and ascorbate concentrations

Riboflavin (RF) is a normal component of the eye lens which triggers a strong photosensitizing activity when exposed to light. Upon irradiation with short wavelength radiations below 400 nm, RF-photosensitized damage may occur. However, vitamin C is present at high concentrations in the normal lens and plays an important role in inhibiting these photosensitization processes. An in vitro simple model was used with the objective of understanding better the relationships between vitamin C and oxygen concentrations on the mechanisms of RF-mediated photodegradation of tryptophan (Trp), a target particularly sensitive to photo-oxidation. Under nitrogen, the RF decomposition reached its maximal value, and vitamin C and Trp photo-oxidation was negligible. When increasing oxygen pressure, RF photodegradation dropped and vitamin C photo-oxidation strongly increased and was maximal at 100% O2. RF-induced photodegradation of Trp first increased with oxygen concentration, up to 40 microM O2, and then decreased. RF and Trp degradation were significantly protected by vitamin C so that no more than 20% of the substrates concentration were oxidized in the presence of vitamin C higher than 0.8 mM. From our results we conclude that in the specific conditions of the normal lens, the high vitamin C concentration (2 mM) is compatible with the UVA radiation hazard, despite the presence of RF. However, if lenticular vitamin C decreases below 0.8 mM, photodegradation of RF may occur and Trp may therefore be photo-oxidized by a Type-I mechanism.     

Determination of rofecoxib,in the presence of its photodegradation product,in pharmaceutical preparations by micellar electrokinetic capillary chromatography

A simple and rapid micellar electrokinetic capillary chromatographic (MEKC) method for analysis of rofecoxib (ROF) and its photodegradation product (PDP) in pharmaceutical preparations has been developed and validated. Analyses were conducted in a fused silica capillary (72 cm effective length, 50 m i.d.) with a background electrolyte consisting of 25 mmol L^–1 borate buffer at pH 7.0 containing 15 mmol L^–1 sodium dodecyl sulfate (SDS) and 10% acetonitrile (ACN). The separation was performed by voltage-controlled system, applying 30 kV at 30 °C, detecting at 225 nm; injection was hydrodynamic at 50 mbar for 2 s. Nifedipine was used as internal standard (IS). Under the optimum conditions ROF, PDP, and IS were well separated with in 10 min. The method was validated with regard to linearity, limit of detection and quantitation, precision, accuracy, specificity, and robustness. The detection limit of the method was low, 0.8 g mL^–1, and the linearity range was wide, 2.5 to 125 g mL^–1. The method was highly efficient—5×10⁵ plates m^–1 for ROF. The method was applied to the tablet form of ROF-containing pharmaceutical preparations. The data were compared with those from the voltammetric method described in literature. No statistically significant difference was found.     

Depth profile of polymer photodegradation

The availability of the recently published rate constants for H abstraction by methyl radicals, during the photolyses of polymer films, has made it possible to calculate the amounts of methane produced as product at various depths from the film surface facing the radiation. The parameters used in the calculations are derived from the experimental values obtained in the short wave (λ = 254 nm) photolysis of poly(p-methoxystyrene) and the long wave (λ ⩾ 300 nm) irradiation of poly(vinyl acetophenone) under high vacuum. The results of the calculation are presented and discussed in terms of the CH₄ yield as a function of depth of the film.     

Interactions between photodegradation components

ABSTRACT: BACKGROUND: The interactions of p-cresol photocatalytic degradation components were studied by response surface methodology. The study was designed by central composite design using the irradiation time, pH, the amount of photocatalyst and the p-cresol concentration as variables. The design was performed to obtain photodegradation % as actual responses. The actual responses were fitted with linear, two factor interactions, cubic and [left-to-right mark] quadratic model to select an appropriate model. The selected model was validated by analysis of variance which provided evidences such as high F-value (845.09), very low P-value (<.0.0001), non-significant lack of fit, the coefficient of R-squared (R2 = 0.999), adjusted R-squared (Radj2 = 0.998), predicted R-squared (Rpred2 = 0.994) and the adequate precision (95.94). RESULTS: From the validated model demonstrated that the component had interaction with irradiation time under 180 min of the time while the interaction with pH was above pH 9. Moreover, photocatalyst and p-cresol had interaction at minimal amount of photocatalyst (< 0.8 g/L) and 100 mg/L p-cresol. CONCLUSION: These variables are interdependent and should be simultaneously considered during the photodegradation process, which is one of the advantages of the response surface methodology over the traditional laboratory method.     

Ultraviolet photodegradation of folic acid

The vitamin folate is vital for all living creatures. Scientists have suggested that ultraviolet degradation of folate in vivo played a role in the evolution of mankind. In order to better understand the photodegradation of folate, we have provided a spectroscopic study of the ultraviolet photodegradation of aqueous folic acid under aerobic conditions. We found strong indications that the folic acid molecule is cleaved into p-aminobenzoyl-L-glutamic acid and 6-formyl pterin when exposed to ultraviolet radiation. When the irradiation continues, 6-formyl pterin is degraded to pterin-6-carboxylic acid. The photodegradation of folic acid is divided into three phases. In the first phase, the formation of photoproducts follows a zero order rate law. In the second phase the presence of photoproducts sensitizes the degradation of folic acid and the degradation process is accelerated. In the third phase the degradation of 6-formyl pterin to pterin-6-carboxylic acid is the dominating process. This reaction follows a first order rate law. We show that both 6-formyl pterin and pterin-6-carboxylic acid sensitize the photodegradation of folic acid. However, experiments performed in heavy water indicate that generation of singlet oxygen is probably not the explanation for the photosensitizing of folic acid.     

The photodegradation of a zinc phthalocyanine

A water-soluble zinc phthalocyanine (Pc), ZnPc (3), bearing 12 dimethylamino groups, which enhance the solubility of the macrocycle was synthesized and characterized. Photobleaching of the compound was examined both in vivo and in vitro. Laser irradiation causes photo-oxidation of the newly synthesized ZnPc. A photobleachable phthalocyanine can be an alternative in imaging; phthalocyanine dyes are used in imaging the cardiovascular system. Besides, it can be used in fluorescein angiography in some cases. When compared to stable ones, a photobleaching ZnPc (3) might be an attractive compound for imaging in medicine.     

Wax treatment of wood slows photodegradation

Waxes and wax emulsions are one of the most important solutions for non-biocidal wood protection. Wax treated wood is designed for outdoor use, therefore it is of considerable importance to elucidate the influence of weathering on the photodegradation processes. It is presumed, that wax treatment will reduce water uptake and thus reduce or slow down photodegradation processes. In order to test this hypothesis, three types of wax emulsions at two different concentrations were vacuum impregnated into Norway spruce wood specimens: an emulsion of montan wax (LGE), an emulsion of polyethylene (WE1) and an emulsion of oxidized polyethylene (WE6) wax. The samples were exposed to artificial accelerated weathering (AAW) for 500 cycles. Before and after AAW colour, the contact angle of water and moisture content were determined. Chemical and morphological changes at exposed surfaces were also investigated with FTIR spectroscopy and SEM. In parallel, the uptake of wax emulsions and water into the axial surfaces of samples was determined with a tensiometer. The moisture content measurements showed that the best hydrophobic effect was achieved with LGE treated wood, where the lowest colour and FTIR changes were observed as well. These results were supported by tensiometer measurements as well. Presented data clearly showed that high loadings of waxes reduce or at least slow down weathering. Among tested waxes, montan wax was found the most effective.     

Thermal degradation of wood during photodegradation

In this study, wood samples were exposed to light irradiations (direct sunlight, xenon lamp, mercury vapour lamp) and thermal treatments were carried out in dry- and in humid conditions at 90°C. One part of the samples was covered by an aluminium plate during light irradiation. The samples under the aluminium plate also suffered considerable chemical changes, monitored by infrared technique and colour measurement. The sunlight produced greater colour change under the aluminium plate than the artificial light sources. During light irradiation, the carbonyl band having two maximum at 1700 and 1,746 cm(-1) increased and the peak of the aromatic skeletal vibration arising from lignin (1,510 cm(-1)) decreased together with the guaiacyl vibrations at 1,275 cm(-1). There was absorption decrease at 1,174 cm(-1) because of the ether band splitting. Under the covered surface only the ether band at 1,174 cm(-1) decreased and one carbonyl band increased with a maximum at 1,715 cm(-1). Degradation of lignin was negligible for the covered surface. Colour change generated by thermal degradation was much greater in humid condition than in dry condition.     

Fractional wavelet analysis for the simultaneous quantitative analysis of lacidipine and its photodegradation product by continuous wavelet transform and multilinear regression calibration

Fractional wavelet transform (FWT) was applied to the original absorption spectra of lacidipine (LAC) and its photodegradation product (LACD), and the resulting FWT spectra were processed by continuous wavelet transform (CWT) and multilinear regression calibration (MLRC) for the simultaneous quantitative analysis of both products in their binary mixtures. These methods do not require any chemical separation step and chemical complex reaction to obtain a detectable signal for the degradation product. By using the Mexican hat function, 2 calibration functions for LAC and LACD were obtained by measuring the CWT transformed signals at 416.1 nm for LAC and 414.6 nm for LACD, after FWT processing of the original absorption spectra. The calibration graphs were linear in the concentration range of 5.08-40.64 microg/mL for LAC and 0.51-8.16 microg/mL for LACD. The limit of detection and the limit of quantitation were found to be 0.289 and 0.956 microg/mL for LAC and 0.036 and 0.118 microg/mL for LACD, respectively. For comparison, the MLRC algorithm was applied to the linear regression functions for the individual drug and its photoproduct. In this approach, a set of linear regression functions was obtained from the relationship between concentrations and FWT signals in the wavelength range 411.0-412.4 nm. Both methods were applied to the quantitative evaluation of LAC and LACD in laboratory and pharmaceutical samples, and produced very satisfactory results.     

脲对丙烯酰胺聚合及产物结构的影响

本文研究了添加脲（添加量为丙烯酞胺的０．０６３到０．２００质量比）对丙烯酰胺聚合的影响。脲的加入能取代丙烯酞胺的双分于氢键缔合，使聚丙烯酰胺的水溶性速度提高，玻璃化温度下降。用红外光谱分析和差热分析的结构分析证实了上述机理。     

Two-photon heterogeneous photodegradation of an aromatic polyurethane

Glass, zinc, copper, or tin supports were shown to have a sensitizing action on the photochemical degradation of a polyurethane prepared from tolylene diisocyanate and trimethylol propane. It was shown that photodegradation of the polyurethane was linked with the breakdown of the isocyanate groups. It was concluded that a two-photon mechanism of heterogeneous photodegradation of the polyurethane operated.     

Monitoring of phenol photodegradation by ultraviolet spectroscopy

Advanced oxidation processes (AOPs) have been developed as an emerging technology for hazardous organic treatment in industrial wastewater. In this paper, the contribution of ultraviolet (UV) spectroscopy to follow phenol photodegradation was studied in a laboratory photochemical reactor equipped with a low pressure mercury lamp. It has been observed that a multicomponent approach is efficient for the evolution estimation of the initial product or intermediate compounds formed during the photodegradation.     

Influences of extractives on the photodegradation of wood

Wood is susceptible to photodegradation when exposed to light. In addition to three main components of wood, the influence of extractives on photodegradation is not fully understood. This study examines the role of wood extractives in photodegradation process, and so the variations in the surface properties of non-extracted and extracted Cryptomeria japonica and Acacia confusa heartwood were examined after exposure to ultraviolet light. FTIR and UV-visible techniques were used to analyze the variations in chemical characteristics on the wood surfaces after irradiation. Following the lightfastness test, the contents of quinonoids and carbonyl groups derived from the lignin increased. However, the wood surface deteriorated less when the extractives existed. In conclusion, extractives play an essential role in the photodegradation of wood, and the rate of wood degradation was lessened by the presence of extractives.     

Frequency-domain measurement of the photodegradation process of fluorescein

The frequency-domain technique is applied to measure the photodegradation rate of fluorescein in aqueous solutions. The illuminating light is modulated, and the changes in fluorescence from the illuminated region are detected synchronously. A constant flow rate is imposed on the fluorescein solution to control the mass transport of fluorescein into the illuminated region. The fluorescence response is described by a model that assumes that photodegradation occurs from the triplet excited state. The predictions of the model are consistent with the observed variations in the fluorescence response with flow rate, modulation frequency and incident power. We discuss in this article how the dependence of the model parameters on experimental conditions can be used to infer the photodegradation rate as well as some of the details of the photodegradation mechanism. The results are consistent with the known mechanism of photodegradation of fluorescein. The frequency-domain technique gives a photodegradation rate of 53 s(-1) in an air-saturated solution and 37 s(-1) in solutions purged with argon gas.     

Influence of copper-phthalocyanine on the photodegradation of polycarbonate

Dyes and pigments are extensively used in polymer materials to confer colour-changing properties. However, these additives can significantly affect polymer stability against degradation. While the mechanism of stabilization of polymers by some pigments, such as carbon black, has been studied and is well known, the action of chromatic colorants, mainly in the sensitization of the degradation process, remains unclear. Cu-phthalocyanine dye can stabilize polymers against degradation as well as accelerate degradation in other situations. Cu-phthalocyanine incorporated into polycarbonate resulted in an acceleration of the degradation when the material was submitted to photochemical aging. The possible mechanism to explain the photodegradative behavior of PC containing Cu-phthalocyanine is based on the hypothesis that specific interactions among excited states of PC and Cu-phthalocyanine take place and enhance the formation of reactive species in polycarbonate. Excited states of Cu-phthalocyanine may abstract hydrogen atoms from methyl groups in polycarbonate, increasing the formation of free radicals P, which are the starting points for the sequential photo-oxidation reactions that lead to the degradation of the polycarbonate. Electron transfer sensitization is also a possible mechanism: the excited state of Cu-Ph abstracts an electron from PC to form the Cu-Ph radical anion and the PC radical cation. These reactive species in the presence of oxygen can cause oxidation of the aromatic ring.