SPECTROSCOPIC STUDIES OF CUTANEOUS PHOTOSENSITIZING AGENTS-XIII. pH DEPENDENCE OF THE PHOTOCHEMISTRY OF PHOTO ALLERGENS BITHIONOL AND FENTICHLOR: AN ELECTRON spIN RESONANCE STUDY OF THE FREE RADICAL PHOTOPRODUCTS

Abstract The photoallergens bithionol (BT) and fentichlor (FT) generated free radical photoproducts upon UV photolysis which were observable by direct electron spin resonance (ESR). Both the yield and the type of free radical photoproducts were affected by pH, and to some extent, concentration of oxygen and concentrations of the photosensitizers. At pH 8.5, bithionol (0.9 mM) generated a semiquinone type free radical (BI) via a mechanism which probably involves substitution of the 4-chlorine by hydroxyl to form the corresponding hydroquinone followed by oxidation. The photolysis of 4-chlorophenol, 4-chlorocatechol and 2,2'-methylene-bis(4-chlorophenol) also generated the corresponding semiquinone radicals, suggesting that this mechanism is shared by other 4-chlorophenols. At pH 8.5, only photoproduct BI was observed during the irradiation of BT; FT related photoproducts were not observed at this pH. However, at higher pH values (pH 10.7 or pH 12), FT photoproducts were also observed in addition to BI upon prolonged irradiation. Moreover, the yield of BI increased drastically at higher pH. Oxygen did not play any role at pH 10.7, although it enhanced the yield of BI at pH 8.5. At pH 8.5, irradiated fentichlor generated, in roughly equal amounts, a semiquinone radical (Fla) and an unidentified species which contained two inequivalent protons (FII). At higher pH values (pH 10.7 and pH 12), at least four species were observed. All of the species are believed to be semiquinone radicals and two have been unambiguously identified. The yield of FI increased by a factor of 50 as the pH was increased from 8.5 to 12. Oxygen played only a minor role at pH 10.7 and above. However, at pH 8.5, it also enhanced the yield of FI.     

Modelling moisture uptake in a cereal grain

auther to whom correspondence should be addressed. Email: k.landman{at}ms.unimelb.edu.au.E-mail: cpp{at}maths.soton.ac.uk Recent experimental data have revealed the spatial and temporalstructure of moisture content within a cereal grain immersedin boiling water. A simple model of the water's motion is presented,guided by the observed behaviour, which allows for nonlinear(exponential) diffusion within the grain and a constant mass-transfercoefficient to represent the pericarp on the outer surface.Numerical results are presented illustrating the close relationshipof the predictions to the experimental results, with the mass-transfercoefficient as a fitting parameter. The model is studied usingasymptotic analysis, in the limit of large activation energyin the diffusion coefficient and large mass transfer. The analysisgives insight into the three phases of the process, consistingof initial linear diffusion, linear motion of the moisture frontinto the grain, and slow filling of the grain in a relativelyuniform manner. The problem is also studied using mean-action-timeanalysis to derive simple expressions for the time for the grainto saturate.     

THE STEPWISE BIPHOTONIC PHOTOIONIZATION OF CHLORPROMAZINE AS SEEN BY LASER FLASH PHOTOLYSIS

It is generally accepted that both promazine (PZ) and chlorpromazine (CPZ) photionize monophotonically to their respective cation radicals and the corresponding hydrated electrons. It is also supposed that this photoinization has a role in the phototoxic effects of these drugs. However, using laser flash photolysis, we have observed that photoionization of CPZ during S1 excitation (lambda greater than 300 nm) is a stepwise biphotonic process. In the case of PZ our flash photolysis results are less clearcut, but are consistent with stepwise biphotonic photoionization for S1 excitation. We demonstrate, using computer simulation of the intramolecular kinetics, that the estimated triplet state lifetime of CPZ is sufficiently long (23 ns at room temperature) to account for the apparent monophotonic photoionization that has been observed by others at high light intensities and short pulse times. Our laser flash photolysis results also suggest that the photo-ionization mechanism of PZ and CPZ is wavelength-dependent. Both drugs exhibit apparent monophotonic photoionization when they are excited at 266 nm under conditions of laser pulse width and intensity similar to those at 355 nm. We suggest that photoionization is not an important mechanism in the observed phototoxic and photoallergic effects of PZ and CPZ in sunlight.     

PHOTOACOUSTIC CALORIMETRY OF CYCLOHEPTENE DERIVATIVES. RELAXED TRIPLET ENERGIES OF 1-PHENYLCYCLOHEPTENE AND 1, 3-CYCLOHEPTADIENE AND THE ENERGETICS OF GROUND-STATE cis-trans ISOMERIZATION

Abstract— The heat of isomerization E _ct of cis -1-phenylcycloheptene ( c -PC7) to its highly strained trans isomer has been determined by a combination of a quantitative study of acid-catalyzed trapping of the trans isomer with time-resolved photoacoustic calorimetry. The relaxed triplet energy has been determined for PC7 and for cis, cis -1, 3-cycloheptadiene. The triplet energies are in line with previous studies of other alkenes but the E _ct value is significantly higher than that calculated by molecular mechanics calculations for cycloheptene. The significance of the results for cis-trans thermal isomerization is discussed.