A Doorway State Leads to Photostability or Triplet Photodamage in Thymine DNA

Abstract

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.

Identification of charge separated states in thymine single strands

“…18 In the other case the decay was ascribed to the quenching of triplet states eventually leading to the formation of cyclobutane pyrimidine dimer (CPD) lesions. 14 Neither of the studies reported direct evidence for the interpretations due to the lack of conclusive spectral signatures in the UV and visible spectral range monitored in these experiments. In this study, we applied time resolved IR spectroscopy (TRIR) to reinvestigate the picosecond dynamics in the allthymine single strand (dT) 18 .…”

“…Reaction dynamics on the 100 ps time scale have been observed in two independent studies on (dT) 18 and (dT) 20 . 14,18 In one case the decay was tentatively assigned to a singlet np* state. 18 In the other case the decay was ascribed to the quenching of triplet states eventually leading to the formation of cyclobutane pyrimidine dimer (CPD) lesions.…”

Excited-state hydrogen bonding effect on dynamic fluorescence of coumarin 102 chromophore in solution: A time-resolved fluorescence and theoretical study

“…Therefore, the nature of hydrogen bond in solution is of particular interest and has been investigated extensively by various experimental and theoretical methods, since solute-solvent interactions play a fundamental role in molecular nonequilibrium processes in liquids [11][12][13][14][15][16][17][18][19][20][21][22][23]. However, little is known about electronic excited-state hydrogen bond.…”

Aerobic oxidation of alcohols on Au/TiO2 catalyst: new insights on the role of active sites in the oxidation of primary and secondary alcohols

“…This photolesion, which can take place in both the singlet and triplet manifolds, has been extensively investigated spectroscopically [2][3][4][5][6][7] and computationally [8][9][10][11][12][13][14][15]. The triplet pathway is a much slower process [7] and exhibits a smaller yield [6,16] than the singlet mechanism due to inefficient intersystem crossing. As a consequence, this pathway yields very weak spectroscopic signals that preclude unambiguous statements regarding the mechanism [5][6][7].…”

“…The triplet pathway is a much slower process [7] and exhibits a smaller yield [6,16] than the singlet mechanism due to inefficient intersystem crossing. As a consequence, this pathway yields very weak spectroscopic signals that preclude unambiguous statements regarding the mechanism [5][6][7]. In order to enhance the triplet signals, photosensitization is commonly used, increasing the ThiT dimerization yield [5,[17][18][19].…”