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Monitoring 2-Ethylhexyl-4-Methoxycinnamate Photoisomerization on Skin Using Attenuated Total Reflection Fourier Transform Infrared Spectroscopy

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Abstract:

Photoisomerization and photodimerization of a widely used UVB filter, 2-ethylhexy-4-methoxycinnamate (EHMC) on a ZnSe surface and baby mouse (Mus musculus Linn.) skin were monitored using attenuated total reflection Fourier transform infrared spectroscopy (ATR-FT-IR). Differentiation between the E- and the Z-EHMC could be achieved by examining the infrared (IR) peak at 981 cm−1 (b peak), which corresponds to the CH rocking deformation vibration of Ph–CH=CH– detected only in the E configuration. By plotting the ratios of the peak area of the b peak and an internal standard peak (1060–998 cm−1) against mole percentage of Z-isomer in the E-Z mixtures, a linear calibration plot was obtained. Thus, a simple estimation of the mole percentage of each configuration in a sample was obtained. At the same UVB exposure, photostationary equilibrium of the E/Z isomerization on the surface varied with the applied amounts of EHMC. Photoisomerizations on ZnSe and on baby mouse skin were comparable. Less than 10% of E-EHMC changed configuration when the mouse skins applied with 1.0–4.0 mg/cm2 E-EHMC were exposed to sunlight for 60 min (UVB radiant exposure of ∼0.30 J/cm2). This corresponded to less than 5% loss in UV filtering efficiency. However, at a typical EHMC skin coverage (∼0.2 mg/cm2), 0.30 J/cm2 UVB exposure induced ∼50% photoisomerization resulting in 25% loss of UV filtering efficiency. No photodimerization was detected even at the extreme EHMC coverage of 4.0 mg/cm2 after a UVB exposure of 0.90 J/cm2.

Keywords: 2-ETHYLHEXY-4-METHOXYCINNAMATE; ATR-FT-IR; ATTENUATED TOTAL REFLECTION FOURIER TRANSFORM INFRARED SPECTROSCOPY; EHMC; PHOTOISOMERIZATION; SUNSCREEN

Document Type: Research Article

DOI: http://dx.doi.org/10.1366/000370207779947648

Affiliations: Sensor Research Unit, Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok, Thailand

Publication date: February 1, 2007

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