Study 6.31 Biochemistry photocatalytic oxidation of DNA guanine

Case Study 6.31 Biochemistry - photocatalytic oxidation of DNA guanine 

Explain the following concepts and keywords photochromism photoswitching photoinitiator photoaffinity labelling photo-Wolff rearrangement photoactivatable compound Barton reaction photochemical trigger photolabile linker photoamination redox photosensitization. 

Entry Starting material3 Product(s) Mechanism Section 

Similarly to carbonyl compounds (Section 6.3.1), thiocarbonyl compounds abstract hydrogen upon irradiation however, both n,7t and n,n excited states are reactive and the hydrogen atom can be added to either the sulfur (Table 6.17, entry 1) or carbon (entry 2) atoms of the C=S bond. Aliphatic and aromatic thiocarbonyl compounds can also undergo photocycloaddition to unsaturated compounds from both singlet or triplet excited states to form thietanes (analogously to the Paterno Biichi reaction see Section 6.3.2) (entry 3) or 1,4-dithianes. On the other hand, fragmentation of the S C bond is a typical primary process observed in excited sulfones and sulfonates (entry 4), followed by efficient SO2 extrusion from the radical intermediate. 

In contrast to excited carbonyl compounds, which abstract hydrogen efficiently from the n,Jt state (Section 6.3.1), thiocarbonyl compounds react from both n,Jt and n,n states and the hydrogen atom can be added to either sulfur or carbon atom of the C=S bond.1259,1269,1270 The sulfur atom is larger than oxygen, therefore the reaction can occur 

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