Mixed pyrimidine dimer

The total energies of the mixed pyrimidine dimers and their radical cations are listed in Table 15. Each of the adiabatic ionization potential, which is the difference between the total energies of the optimized pyrimidine dimer and its radical cation, is 8.25, 7.64, 7.93 and 7.90 eV, for T<>T, CoC, T<>C and Cot, respectively. It is predicted that the adiabatic ionization potential is largest in ToT, and smallest in CoC, and those of the mixed pyrimidine dimers are in-between. No experimental data are available for the ionization potentials of the pyrimidine dimers. 

The bond lengths around the four-member rings of the mixed pyrimidine dimers and their radical cations are summarized in Tables 16 and 18. The dihedral angles around the four-member rings are summarized in Tables 17 and 19. The spin densities on the C6A and C6B atoms in the pyrimidine dimer radical cations are listed in Table 20. 

The lifetime of Pyr +oPyr seemed to depend on the structure of the parent dimer. It was observed that the presence of a trimethylene bridge linking both pyrimidine units at N(l) and N(l ), as in the mixed thymine-uracil dimer 4 and in the thymine dimer 5, leads to significant stabilization of the corresponding dimer radical cation, whereas in unbridged dimers a maximum lifetime of 10-1°-10-9 s was proposed for their dimer radical cations [6]. It was further shown that in bridged pyrimidines dimers of types 4 and 5 the interconversion of dimer and... 

Figure 7. Measured and modeled profiles of cyclobutane pyrimidine dimers (CPD) for water columns with varying intensities of vertical mixing (a) Gulf of Mexico, 8 September 1994, mixed layer 20 m, wind speed low (2 m s ) (b) Gulf of Mexico, 7 September 1994, mixed layer 20 m, wind speed high (8 m s ) (c) Gerlache Strait, Antarctica, 6 October 1996, mixed layer 25 m, wind speed high (8 m s 0- Modeled profiles are for damage (CPD formation) only, damage plus photoreactivation only, and damage and all repair mechanisms (photoreactivation and excision repair). [From Jeffrey et al. and Huot et al. 82.]...

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