ABSTRACT
The ultraviolet (UV) component of sunlight irradiation causes
DNA damage by inducing the formation of cyclobutane pyrimidine
dimer (CPD) and pyrimidine-pyrimidone (6-4) photoproduct (6-
4PP), which are mutagenic and finally lead to mutations and cell
death. Photolyase, which exists in many organisms, is a flavoprotein
that can restore the damaged DNA. With femtosecond spectroscopy,
the dynamics and mechanisms of repairs of CPD and 6-4PP by
photolyases were revealed. After electron injection, the CPD anion
radical splits in two sequential steps within 90 picoseconds before
the electron returns to the cofactor to complete the photocycle. For
6-4PP repair, the anionic 6-4PP proceeds to a critical cyclic proton
transfer with a nearby histidine residue to close the photocycle.
Figure 3.1 Structures of undamaged thymine bases and two major DNA photolesions.
