Geometry optimization calculations were performed using the B3LYP/6‐31+G* method on the complexes of
1O
2 and
3O
2 molecules with a stacked dimer of planar guanine, varying the distance (D) between the planes of the guanine molecules. In this process, geometries of the guanine molecules were held fixed, D was fixed at different values, while the bond lengths of
1O
2 and
3O
2 as well as their orientations with respect to the guanine molecules were optimized for each value of D. The complexes in their most stable geometries were solvated in water using the integral equation formalism of the polarized continuum model of the self‐consistent reaction field theory. In gas phase, the most stable complex between
1O
2 and the guanine dimer (2G.
1O
2) is formed when D is about 6 Å, while the most stable complex between
3O
2 and the guanine dimer (2G.
3O
2) is formed when D is about 3.75 Å. In the minimum total energy geometry of 2G.
1O
2,
1O
2 is located between the guanine molecules, above the imidazole ring of one of them. However, in the minimum total energy geometry of 2G.
3O
2,
3O
2 is located outside the stack of guanine molecules, near the amino group of one of them. The solvation calculations showed that in aqueous media,
1O
2 would bind with the stacked guanine dimer more strongly than in gas phase, while
3O
2 would not bind with the same. The mode of binding of
1O
2 with the stacked guanine dimer is such that it seems that
1O
2 would replace one basepair in DNA, as happens in the intercalative mode of binding of drugs and other molecules, and it can lead to the formation of 8‐oxoguanine that has a mutagenic nature. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005
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