A doorway state leads to photostability or triplet photodamage in thymine DNA.
Ultraviolet irradiation of DNA produces electronic excited states that predominantly eliminate the excitation energy by returning to the ground state (photostability) or following minor pathways into mutagenic photoproducts (photodamage). The cyclobutane pyrimidine dimer (CPD) formed from photodimerization of thymines in DNA is the most common form of photodamage. The underlying molecular processes governing photostability and photodamage of thymine-constituted DNA remain unclear. Here, a combined femtosecond broadband time-resolved fluorescence and transient absorption spectroscopies were employed to study a monomer thymidine and a single-stranded thymine oligonucleotide. We show that the protecting deactivation of a thymine multimer is due to an ultrafast single-base localized stepwise mechanism where the initial excited state decays via a doorway state to the ground state or proceeds via the doorway state to a triplet state identified as a major precursor for CPD photodamage. These results provide new mechanistic characterization of and a dynamic link between the photoexcitation of DNA and DNA photostability and photodamage.
Kwok
Wai-Ming
WM
Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China. bcwmkwok@polyu.edu.hk
Ma
Chensheng
C
Phillips
David Lee
DL
D004247:DNA / Q000737:chemistry*; D004249:DNA Damage / Q000528:radiation effects*; D010777:Photochemistry; D011740:Pyrimidine Dimers / Q000737:chemistry / Q000528:radiation effects; D013057:Spectrum Analysis; D013941:Thymine / Q000737:chemistry*; D013997:Time Factors
D011740:Pyrimidine Dimers; D004247:DNA; D013941:Thymine