Ab initio Calculation for Photochemistry of Psoralens

The psoralen-pyrimidine base complexes resulting from interstrand cross-linking through C4-cycloaddition is studied by ab initio and DFT methods. The results indicate that in the case of the molecular complex formation between psoralens and pyrimidine base, the most probable photocycloadditions are 8-MOP< > Thy, Ps< > Cyt and Ps< > Thy. The geometries of photoadducts were optimized at the HF levels and Delta G degrees were calculated. The photocycloadduct was inferred to be a C4-cycloaddition product with the stereochemistry of trans-syn 8-MOP< > Thy, trans-anti Ps(3, 4)< > Cyt, trans-anti Ps(12, 13)< > Cyt, trans-syn Ps(3, 4)< > Thy, trans-syn Ps(12, 13)< > Thy, trans-anti Ps(12, 13)< > Ps(12, 13), cis syn, cis anti Thy< >(3, 4) Ps(12, 13)< > Thy.