Measuring sunscreen protection against solar-simulated radiation-induced structural radical damage to skin using ESR/spin trapping: Development of an ex vivo test method

Abstract

The in vitro star system used for sunscreen UVA-testing is not an absolute measure of skin protection being a ratio of the total integrated UVA/UVB absorption. The in vivo persistent-pigment-darkening method requires human volunteers. We investigated the use of the ESR-detectable DMPO protein radical-adduct in solar-simulator-irradiated skin substitutes for sunscreen testing. Sunscreens SPF rated 20+ with UVA protection, reduced this adduct by 40–65% when applied at 2 mg/cm².

SPF 15 Organic UVA-UVB (BMDBM-OMC) and TiO₂-UVB filters and a novel UVA-TiO₂ filter reduced it by 21, 31 and 70% respectively. Conventional broad-spectrum sunscreens do not fully protect against protein radical-damage in skin due to possible visible-light contributions to damage or UVA-filter degradation. Anisotropic spectra of DMPO-trapped oxygen-centred radicals, proposed intermediates of lipid-oxidation, were detected in irradiated sunscreen and DMPO. Sunscreen protection might be improved by the consideration of visible-light protection and the design of filters to minimise radical leakage and lipid-oxidation.

Keywords::

• electron-spin-resonance spectroscopy
• free radical
• solar radiation
• sunscreen
• 5,5-dimethyl-1-pyrroline N-oxide

Acknowledgements

We would like to thank the Restoration of Appearance and Function Trust (RAFT, Registered Charity No. 299811) for funding this work.

Declaration of interest The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.

This paper was first published online on Early Online on 9 February 2012.