Structures of pyrimidine dimers

When DNA is exposed to radiation at wavelengths of about 260 nm, adjacent pyrimidines become covalently linked by the formation of a four-member ring structure resulting from the saturation of their respective C5, C6 double bonds. The structure formed by this photochemical cycloaddition is referred to as a pyrimidine dimer. The pyrimidine bases in DNA are thymine and cytosine. 

The total energies of ToT and ToT( +) are shown in Table 1. The value of S of ToT( +) is 0.782 it is surprisingly close to the ideal value (0.75) for a doublet, as we could have expected a larger spin contamination due to the 7C orbitals of the moities. The orbital energy of the highest occupied molecular orbital (HOMO) of the neutral ToT is -0.407 au, while the vertical ionization potential of ToT is 11.1 eV according to Koopmans theorem. The adiabatic ionization potential after taking account of orbital relaxation (ASCF method) is 

which is the difference between the total energies of the optimized T T and the optimized ToT( -F). The adiabatic ionization potential calculated with the ASCF method is usually underestimated by around 10 %. Therefore, the adiabatic ionization potential is predicted to be around 9.3 eV. There are no experimental data directly comparable. Table 1 Total Energies (in au) and bond lengths (in A) around the four-member ring of thymine dimer (RHF/6-31G) and its radical cation (UHF/6-31G)  

Selected bond angles of thymine dimer (RHF/6-31G) and its radical cation (UHF/6-31G)  

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Scheme 1. Structures of pyrimidine dimers (I) electrochemically generated hydrodimer of pyri-midone-2 (II) photochemically generated pyrimidine adducts (III) photochemically generated cyclobutane dimers...

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