A. Keith Davies

Photochemical oxidation of tetracycline in aqueous solution

Irradiation of an aqueous solution of tetracycline at pH 9 with u.v. light from a medium pressure mercury lamp (filtered through Pyrex glass to give wavelengths >290 nm) produces a red product with λmax. 534 nm. The red product is only produced in the presence of oxygen, one mole of oxygen being absorbed per mole of tetracycline photolysed. When the photolysis is carried out at ca. 3 °C an intermediate compound is produced (λmax. 357 nm). On warming this irradiated solution the red product is formed in a dark reaction for which the activation energy is 90 kJ mol–1. The red product has a pKa value of 7.6, the protonated form being orange-yellow (λmax. 442 nm). The red product is reduced by sodium dithionite solution to a leuco form which is reoxidised to the original compound by oxygen of the air. The red product is also reduced photochemically; flash photolysis reveals the intermediacy of a semiquinone radical in this process. These properties are typical of a quinone ring system. The spectral and chemical properties of the red product are consistent with its being 4a, 12a-anhydro-4-oxo-4-dedi-methylaminotetracycline. The mechanism of formation of the red product involves photodeamination followed by reaction of the tetracycline radical with molecular oxygen to form a peroxyl radical, which abstracts a hydrogen atom to give a hydroperoxide. Decomposition of the hydroperoxide by loss of water yields an oxo group at position 4. In the final dark reaction, water is lost across the C(4a)–C(12a) bond to form the red quinone.

Laser flash photolysis study of the photoionisation of chlorpromazine and promazine in solution

Solutions of promazine hydrochloride and chlorpromazine hydrochloride have been flash photolysed with 25 ns pulses of 347.1 nm light from a frequency-doubled ruby laser. In addition to the excited triplet states of these molecules, the cation radicals and the solvated electron were observed. The ionisation of promazine and chlorpromazine was found to occur by a monophotonic process; a possible mechanism for this process is considered. The quantum yield for photoionisation of chlorpromazine in aqueous solution was determined to be (1.0 ± 0.2)× 10–1 and the extinction coefficient of the chlorpromazine cation radical was determined to be 9.3 × 102 m2 mol–1.

Photochemistry of chlorpromazine [2-chloro-N-(3-dimethylaminopropyl)phenothiazine] in propan-2-ol solution

In oxygen-free propan-2-ol solution electronically excited chlorpromazine [2-chloro-N-(3-dimethylaminopropyl)phenothiazine] undergoes a carbon–chlorine bond fission to form free radicals which react with the solvent to produce promazine, isopropoxypromazine, hydrogen chloride, and acetone. In oxygen-saturated solution energy transfer occurs from excited chlorpromazine to molecular oxygen to produce excited singlet oxygen. Under these conditions chlorpromazine does not undergo any permanent photochemical change.

Factors influencing the photosensitizing properties and photoluminescence of thioflavin T

Abstract The fluorescence intensity of aqueous solutions of thioflavin T greatly increases when the dye is bound to ribonucleic acid and to single-stranded poly-nucleotides containing purine bases. Binding is due to both ionic and hydrophobic interactions. Thioflavin is an inefficient photosensitizer for the oxidation of histidine but the photosensitizing power of thioflavin increases considerably when the dye is bound to polynucleotides. These effects are interpreted in terms of a stabilization of the excited singlet and triplet states of the dye in the bound state.

Factors influencing the photofading of commercial anthraquinone dyes in solution

Abstract The photofading of two anthraquinone dyes has been studied in aqueous solution using ultraviolet-visible absorption spectroscopy and flash photolysis. The influence of alcohol concentration, pH, atmosphere, photosensitisers and stabilisers have been examined and these lead to some important conclusions on the mechanism of dye fading. Essentially, the results indicate the photoexcited triplet state of the dye undergoes a process of either electron or hydrogen-atom abstraction depending on the nature of the environment. Other factors such as aggregation and singlet oxygen also appear to play an important role in solution photofading. The relevance of these results to photofading in a polymeric phase is discussed.

Pulse radiolysis study of chlorpromazine and promazine free radicals in aqueous solution

·OH radicals may react with chlorpromazine in four different ways, viz.(i) electron transfer to produce the cation radical, (ii) addition to the sulphur atom followed by acid-catalysed OH– elimination to yield the cation radical, (iii) addition to the aromatic rings to produce cyclohexadienyl type radicals, and (iv) abstraction of hydrogen atoms from the —CH2— which is in the α position to the ring nitrogen. Electron transfer and addition to the sulphur atom each account for 40% of the ·OH radicals. Similar considerations apply to promazine.The hydrated electron may attach itself to either the sulphur atom in promazine or its aromatic system with equal probability. Addition to the sulphur atom probably leads to immediate cleavage of the carbon–sulphur bond whereas addition to the aromatic system probably produces an anion radical. The lifetime of this anion radical is sufficiently long to enable electron transfer between itself and another promazine molecule to occur to produce the promazine cation radical.With chlorpromazine, the hydrated electron probably reacts solely with the aromatic system to eliminate chloride ions by the end of the reaction. The promazine radical which is produced simultaneously may add on to another chlorpromazine molecule to produce a cyclohexadienyl type radical.

A comparison of the ultra-violet radiation stabilities of hyaluronic acid and chondroitin-4-sulphate in aqueous solution

Abstract The degradation of aqueous solutions of hyaluronic acid and chondroitin-4-sulphate by ultra-violet radiation (184·9 nm, 194·2 nm and 253·7 nm) has been investigated. From measurements of changes in viscosity, hexuronic acid, hexosamine, sulphate, reducing end group and ability to induce metachromasia with acridine orange, it was concluded that the presence of the sulphate group makes scission at the glycosidic linkage less likely.

Factors influencing the photostability of reactive dyes in polymers

Abstract This paper examines several important factors that are believed to affect the photostability of reactive dyes in polymer substrates. The light stability of the dye-polymer covalent bond was found to be influenced by several factors such as the nature of the dyeing process, the nature of the polymer and humidity. In the former the presence of residual washing powder played and important role, whereas in the latter two, although photohydrolysis occurred to a greater extent in nylon 6,6 than in cellulose, humidity had an important influence only in a cellulose environment. In agreement with these findings the presence of a hydroxy radical trap, potassium thiocyanate, inhibited photohydrolysis in cellulose, whereas the presence of an electron trap, cadmium sulphate, inhibited photohydrolysis in nylon 6,6. On the other hand, the influence of the covalent bond on photofading of the dyes was found to be variable. However, the presence of the reactive triazinyl group appeared to stabilise the dye.

Fluorescence quenching of 2-piperidinoanthraquinone

Fluorescence solvent shift data indicate the first excited singlet state of 2-piperidinoanthraquinone [2-PA] to be considerably more polar than its ground state. Amines quench the fluorescence of [2-PA] in cyclohexane by electron transfer. Alcohols also quench the fluorescence but by a different mechanism involving hydrogen bonding between the alcohol and the ground state of [2-PA] to form a complex that is itself fluorescent.

Photochemistry of methoxyanthraquinones

Die Blitzphotolyse der Chinone (II) in luftfreiem Benzol-Isopropanolgemisch fuhrt zur Bildung der Hydrochinone (I).