Li

Node

Gril ci c

Antibonding it ii

Bonding n (Ground State)

Figure 3 (as from 43, p. 59) Two p orbitals combine to form two B orbits. The bonding B orbital is of a lower energy and contains both B electrons (with opposite spins) in the ground state. In comparison, the higher energy antibonding B* orbital is not occupied by electrons in the ground state. Excitation of the molecule by light or radiation of a suitable frequency can promote an electron from the lower energy level to the higher energy level.

Oxygenated Products

Radicals or * radical ions interact covalently with\ specific biomolecules )

Sens

Substrate or solvent t

Sens

Figure 4 (as from ref. 37, p. 406). Type I phototoxic mechanism of action that occurs in coumarin. UV-A radiation renders the greatest toxicity of the excited coumarin compound.

Coumarin is an example of a compound that has an abundance of B electrons and can be excited by UV-A radiation. Downum37 suggested that the phototoxic mechanism of coumarins is via a Type I mechanism (Figure 4). In the excited state coumarin can react with a suitable substrate to form radicals (Figure 5). Hydroxylated derivatives of coumarin can be scavengers of free radicals in vitro.21

Figure 5. The radicals produced by the excitation of coumarin by UV-A radiation. Dioxygen (O2) is dangerous in excess (i.e. toxic to the cell) and may combine with other radicals or broaden the effects of other radicals.34 Rice et al.34 stated that singlet oxygen reacts as an electrophile whereas superoxide is a weak base and has low reactivity as a radical. The hydroxyl radicals can damage DNA whereas the peroxide radicals damage unsaturated membrane components.

Figure 5. The radicals produced by the excitation of coumarin by UV-A radiation. Dioxygen (O2) is dangerous in excess (i.e. toxic to the cell) and may combine with other radicals or broaden the effects of other radicals.34 Rice et al.34 stated that singlet oxygen reacts as an electrophile whereas superoxide is a weak base and has low reactivity as a radical. The hydroxyl radicals can damage DNA whereas the peroxide radicals damage unsaturated membrane components.

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Photostabilities of Several Chemical Compounds used as Active Ingredients in Sunscreens

John M. Allen, Sandra K. Allen and Brian Lingg Department of Chemistry Indiana State University Terre Haute, IN 47809, USA

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