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Impressum
(c) 2002 BMO

DNA repair by Photolyase
UV radiation causes dimerization of adjacent pyrimidine bases in DNA (see Fig. 1), which can lead to proliferation or cell death. With means of QM/MM methods, we want to investigate recognition, binding and repair of this defect by photolyase, which is hardly understood by now.

Reaction mechanism

The enzyme photolyase (PL) catalyzes monomerization of the dimers by light-induced electron transfer from an FAD cofactor to the dimer (see Fig. 2) Quantum chemical calculations in vacuo indicate, that the C5-C5' bond of the dimer is the first to split after electron transfer leading to an anionic radical dimer intermediate.

The subsequent splitting of the C6-C6' bond is assumed to occur in a thermal reaction catalyzed by the enzyme. Electron transfer from the anionic pyrimidine back to the FAD cofactor finally restores the native state of the DNA and the initial state of the photolyase.

As an alternative reaction path, a simultaneous splitting of both bonds has also been proposed, as former quantum mechanical calculations in vacuo have revealed a high activation barrier for the cleavage of the anionic dimer radical.

Vibrational analysis

Time-resolved FTIR-spectroscopy is a promising approach to resolve the discussion on possible reaction intermediates and reaction constants by experimental means.

The isolated spectral positions and the large intensities of the C=O stretching modes between 1600 and 1800 cm-1 (see Fig. 3) suggest that they might represent suitable candidates for the desired marker modes.

Our calculations are accompanied by experiments carried out by E. Schleicher and G. Richter in the group of A. Bacher (Biochemical Institute, TU München).



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Pyrimidine Dimer
Fig. 1: UV-damaged DNA strand. Two thymine bases (blue) are interlinked by two bonds between the respective C5 and C6 atoms. The DNA backbone is drawn black, two neighboring adenine bases are coloured yellow.



Repair Mechanism
Fig. 2: Possible reaction mechanism of catalyzed monomerization. Anionic structures are coloured red, neutral structures are coloured blue.



Calculated IR spectrum
Fig. 3: IR spectrum of thymine, calculated with a BLYP functional and a 6-31G(d) basis set.

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