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Efficient Photosensitized Splitting of Thymine Dimer by a Covalently Linked Tryptophan in Solvents of High Polarity
Author(s) -
Song QinHua,
Tang WenJian,
Hei XiaoMing,
Wang HongBo,
Guo QingXiang,
Yu ShuQin
Publication year - 2005
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.200400631
Subject(s) - chemistry , dimer , photochemistry , tryptophan , radical ion , electron transfer , photoinduced electron transfer , moiety , chromophore , solvent , stereochemistry , ion , organic chemistry , amino acid , biochemistry
Tryptophan‐thymine dimer model compounds used to mimic the repair reaction of DNA photolyase have been synthesized. The photosensitized cleavage of the dimer by the covalently linked tryptophan is strongly solvent‐dependent with the reaction rates increasing in increasingly polar solvents, for example, the quantum yield Φ = 0.004 in THF/hexane (5:95) and 0.093 in water. The fluorescence of the tryptophan residue is quenched by the dimer moiety by electron transfer from the excited tryptophan to the dimer. Fluorescence‐quenching studies indicated that the electron transfer was efficient in polar solvents. The splitting efficiency of the dimer radical anion within the tryptophan · + –dimer · – species is also remarkably solvent‐dependent and increases with the polarity of the solvents. The back‐electron‐transfer reaction in the charge‐separated species, which competes with cleavage, was suppressed in polar solvents. These results are in contrast to those of earlier solvent‐dependent studies of indole‐dimer systems, but they can be rationalized in terms of the differences in the distances between the chromophore unit and the attached dimer. The pH‐dependent measurements of the splitting reaction and the deuterium isotope effect showed that the tryptophan radical cation within the charge‐separated species does not deprotonate prior to the cleavage of the dimer radical anion. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)