Ability of the Putative Decomposition Products of 2,3‐dioxetanes of Indoles to Photosensitize Cyclobutane Pyrimidine Dimer (CPD) Formation and its Implications for the “Dark” (Chemisensitized) Pathway to CPDs in Melanocytes^†

Abstract

The formation of cyclobutane pyrimidine dimers (CPDs) by a "dark" pathway in melanocytes has been attributed to chemisensitization by dioxetanes produced from peroxynitrite oxidation of melanin or melanin precursors. These dioxetanes are proposed to decompose to triplet state compounds which sensitize CPD formation by triplet-triplet energy transfer. To determine whether such compounds are capable of sensitizing CPD formation, the putative decomposition products of 2,3dioxetanes of variously substituted indoles were synthesized and their triplet state energies determined at 77 K. Their ability to photosensitize CPD formation was determined by an enzyme-coupled gel electrophoresis assay in comparison with norfloxacin (NFX) which has the lowest triplet energy known to sensitize CPD formation. The decomposition products of 2,3-dioxetanes of 5-hydroxy and 5,6-dimethoxy indoles used as models for melanin precursors had lower triplet energies and were incapable of photosensitizing CPD formation. Theoretical calculations suggest that the decomposition products of the 2,3-dioxetanes of melanin precursors DHI and DHICA will have similarly low triplet energies. Decomposition products of the 2,3-dioxetanes of indoles lacking oxygen substituents had higher triplet energies than NFX and were capable of photosensitizing CPD formation, suggesting that peroxynitrite oxidation of tryptophan could play a hitherto unrecognized role in the dark pathway to CPDs.

Significance of melanin distribution in the epidermis for the protective effect against UV light

“…However, some studies have yielded conflicting results, failing to simulate the inverse dependence of DNA damage on skin type. Recently, significant CPD formation has been described in mouse and in vitro models 2–3 h (retained up to 24 h) after exposure to broadband UV-A or narrowband UV-B via delayed melanin sensitization 2 , 35 . Therefore, the hypothesis that melanin may be a photosensitizer for the formation of dCPD in tanned RHE could not be excluded.…”

Endogenous Photosensitizers in Human Skin

“…81 The decomposition of 1,2dioxetanes, 501 formed from the oxidation of melanin and its precursors by peroxynitrite, has been proposed to produce triplet excited carbonyl compounds that are able to undergo energy transfer to T, creating the excited state necessary for cycloaddition. 81 The carbonyl products of thermolysis of model indole dioxetanes have triplet energies similar to that of norfloxacin, which has the lowest E T known to promote photosensitized CPD formation, 502 and triplet quenchers have been suggested as antioxidants to prevent damage caused by excited state formation as a result of photo-or chemiexcitation. 503 Nevertheless, the precise role played by chemically generated excited states in biological systems has yet to be elucidated.…”

Chemiexcitation: Mammalian Photochemistry in the Dark†

“…The precise chemistry leading to melanin‐related dioxetanes has not yet been solved. For example, photoexciting putative degradation products of dioxetanes on indoles did not induce dCPDs; nor were the measured triplet energies of these products high enough to do so (42). This result suggests that melanin‐like reactants might be creating dCPDs by a reaction other than the indole ring dioxetane proposed in reference (32).…”