A study of photoinduced transformations of sunscreen chemical absorbers.
Solar ultraviolet radiation is known to have deleterious effects on human skin and is a major cause of skin cancer. Therefore, the topical application of sunscreen preparations has gained wide usage for skin protection. These preparations typically contain a variety of chemical absorbers that absorb ultraviolet (UV) radiation and physical blockers that scatter, absorb and reflect UV light. The efficacy of sunscreens can be estimated by the Sun Protection Factor (SPF) which depends on the UV filters present in the formulations. However, although some of these commercial sunscreens have beneficial effects, they can also have undesirable results. It is known that the sunscreens undergo electronic excitation when exposed to UV light which may make them susceptible to photochemical modification. The production of reactive intermediates (e.g. free radicals) and stable photoproducts, either due to photoisomerisation or photofragmentation is a major concern because these species may be toxic and may lead to a reduction in efficacy. Hence a study of the photochemistry of these chemical absorbers found in commercial sunscreens is of great importance. Photostability and broad-spectrum studies of some Australian commercial sunscreen products were undertaken by means of spectrophotometric and chromatographic methods. The sunscreen products dissolved in methanol solutions were irradiated using simulated solar radiation. High performance liquid chromatography (HPLC) was used to identify and quantify the active chemical ingredients. UV spectrophotometry was used to monitor the spectral absorbance before and after UV exposure of the formulations. Our results show that some of the evaluated photoactive chemical absorbers currently used in sunscreens are unstable upon UV radiation. This was mainly due to either photoisomerisation and/or photofragmentation of some active chemical ingredients. An examination of the photochemistry of 2-ethylhexyl-p-methoxycinnamate (2- EHMC), an ultraviolet B absorber that was found in all the suncare products investigated in this study was undertaken. Irradiation of dilute (~ 10-6 M) solutions of EHMC with wavelengths of light greater than 300 nm results in trans - cis photoisomerisation leading to a photostationary equilibrium mixture. However, pure or concentrated solutions of 2-EHMC upon prolonged irradiation showed additional photoproducts. These were isolated by preparative high performance liquid chromatography (HPLC) and characterised by nuclear magnetic resonance (1H NMR) spectroscopy, which was used to identify them as [2 +2] cycloadducts of 2-EHMC. There are 13 possible dimers formed via a [2+2] cycloaddition reaction mechanism across the ethylenic double bond, however only the stable and energetically favoured isomers were isolated. In addition, ab initio molecular orbital calculations have been used to investigate the structures and the transition states of the various dimers resulting from the cycloaddition reactions. Geometry optimizations and energy calculations were performed with the Gaussian 98 program, using the B3LYP density functional and 6-31+G (d) basis set. GaussView was used to visualize the transition state structures. The theoretical calculations predicted the most stable dimer forms. The trans-trans configuration at the cyclobutane ring of the 2-EHMC adduct gave relatively more stable photoproducts. The theoretical results have been confirmed by HPLC isolation experiments, which together with the UV spectra of the different products; verify the presence of the different conformers of 2-EHMC.