A study of photoinduced transformations of sunscreen chemical absorbers.
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Date
2009
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Abstract
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.
Description
Thesis (Ph.D.)-University of KwaZulu-Natal, Westville, 2009.
Keywords
Photochemistry., Sunscreens (Cosmetics), Theses--Chemistry.