Acid-Base Photon-Transfer Reactions

Fig. 8. Reaction mechanism of (6-4) photolyase. The enzyme binds to DNA containing a (6-4) photoproduct and flips out the dinucleotide adduct into the active site cavity, where the open form of the photoproduct is converted to the oxetane intermediate by a light-independent general acid-base mechanism. Catalysis is initiated by light MTHF absorbs a photon and transfers energy to FADH , which then transfers an electron to the oxetane intermediate bond rearrangement in the oxetane radical regenerates two canonical pyrimidines, and back-electron transfer restores the flavin radical to catalytically competent FADH form. The repaired dipyrimidine flips back into the DNA duplex, and the enzyme is dissociated from the substrate.

For the studies of the excited-state proton transfer reactions of aromatic compounds, kinetic analyses by means of fluorimetry, single-photon counting, and laser photolysis methods are very important to obtain the exact data. Their acid-base properties in the excited states can be understood on the bases of thermodynamic analyses and electronic structures. Large changes in the acidity constant of organic compounds upon electronic excitation may be applicable to various fields, especially to biochemistry. 

---