UVA light and singlet oxygen quantum yield of endogenous photosensitizers determined directly by its luminescence

Jürgen Baier, Claudia Pöllmann, Tim Maisch, Max Maier and Wolfgang Bäumler
(J. Baier, C. Pöllmann, T. Maisch, M. Maier and W. Bäumler)

The UVA component of solar radiation has been shown to produce deleterious biological effects in which singlet oxygen plays a major role. In tissue the UVA light is only weakly absorbed by a limited number of molecules, which may act then as photosensitizer. After UVA light absorption, the photosensitizer molecules cross over to its triplet state and transfers energy to generate singlet oxygen. To provide doubtless evidence for a correlation of UVA damage in tissue and singlet oxygen, it must be shown that these endogenous photosensitizers generate singlet oxygen to a sufficient extent. Comparable to exogenous photosensitizers, the efficacy of singlet oxygen generation (quantum yield) must be determined. In the present experiments flavins, NADH/NADPH, urocanic acid or different fatty acids where investigated.
These endogenous photosensitizers were excited in the range of UVA light using a Nd:YAG laser at 355 nm. Singlet oxygen was detected directly by its time resolved luminescence at 1270 nm. The respective decay rates and rate constants of singlet oxygen were determined, in particular at different oxygen concentrations. The singlet oxygen quantum yield could be calculated. For e.g. riboflavin in fully aerated solution of H2O, a singlet oxygen quantum yield of 0.54 ± 0.07 was determined. That value is comparable to exogenous photosensitizers used in photodynamic therapy (Photofrin = 0.33). The singlet oxygen quantum yield depends critically on the oxygen concentration, i.e. the oxygen partial pressure (pO2) in the respective experimental setup. That is important when comparing experiments of in vitro (pO2 ~ 150 mmHg) and conditions in vivo such as the skin (pO2 < 20 mmHg). The results show a decrease of FD with decreasing oxygen concentration.
Our investigations provide clear evidence that UVA light at 355 nm generates singlet oxygen in endogenous sensitizers such as flavins, urocanic acid or fatty acids.

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