Computational Study of Oxidation of Guanine by Singlet Oxygen (1 Δg ) and Formation of Guanine:Lysine Cross-Links

Oxidation of guanine in the presence of lysine can lead to guanine-lysine cross-links. The ratio of the C4, C5 and C8 crosslinks depends on the manner of oxidation. Type II photosensitizers such as Rose Bengal and methylene blue can generate singlet oxygen, which leads to a different ratio of products than oxidation by type I photosensitizers or by one electron oxidants. Modeling reactions of singlet oxygen can be quite challenging. Reactions have been explored using CASSCF, NEVPT2, DFT, CCSD(T), and BD(T) calculations with SMD implicit solvation. The spin contamination in open-shell calculations were corrected by Yamaguchi's approximate spin projection method. The addition of singlet oxygen to guanine to form guanine endo- peroxide proceeds step-wise via a zwitterionic peroxyl intermediate. The subsequent barrier for ring closure is smaller than the initial barrier for singlet oxygen addition. Ring opening of the endoperoxide by protonation at C4-O is followed by loss of a proton from C8 and dehydration to produce 8-oxoG^ox . The addition of lysine (modelled by methylamine) or water across the C5=N7 double bond of 8-oxoG^ox is followed by acyl migration to form the final spiro products. The barrier for methylamine addition is significantly lower than for water addition and should be the dominant reaction channel. These results are in good agreement with the experimental results for the formation of guanine-lysine cross-links by oxidation by type II photosensitizers.

Reactions of 1,3-cyclohexadiene with singlet oxygen. A theoretical study

A thorough study of the reaction of singlet oxygen with 1,3-cyclohexadiene has been made at the B3LYP/6-31G(d) and CASPT2(12e,10o) levels. The initial addition reaction follows a stepwise diradical pathway to form cyclohexadiene endoperoxide with an activation barrier of 6.5 kcal/mol (standard level = CASPT2(12e,10o)/6-31G(d); geometries and zero-point corrections at B3LYP/6-31G(d)), which is consistent with an experimental value of 5.5 kcal/mol. However, as the enthalpy of the transition structure for the second step is lower than the diradical intermediate, the reaction might also be viewed as a nonsynchronous concerted reaction. In fact, the concertedness of the reaction is temperature dependent since entropy differences create a free energy barrier for the second step of 1.8 kcal/mol at 298 K. There are two ene reactions; one is a concerted mechanism (DeltaH(double dagger) = 8.8 kcal/mol) to 1-hydroperoxy-2,5-cyclohexadiene (5), while the other, which forms 1-hydroperoxy-2,4-cyclohexadiene (18), passes through the same diradical intermediate (9) as found on the pathway to endoperoxide. The major pathway from the endoperoxide is O-O bond cleavage (22.0 kcal/mol barrier) to form a 1,4-diradical (25), which is 13.9 kcal/mol less stable than the endoperoxide. From the diradical, two low-energy pathways exist, one to epoxyketone (29) and the other to the diepoxide (27), where both products are known to be formed experimentally with a product ratio sensitive to the nature of substitutents. A significantly higher activation barrier leads to C-C bond cleavage and direct formation of maleic aldehyde plus ethylene.