Interactions of molecules with nucleic acids. X. Covalent intercalative binding of the carcinogenic BPDE I(+) to...

A theoretical model is proposed for the covalent binding of (+) 7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10- tetrahydrobenzo[a]pyrene denoted by BPDE I(+), to N2 on guanine. The DNA must kink a minimum of 39 degrees to allow proper hybrid configurations about the C10 and N2 atoms involved in bond formation and to allow stacking of the pyrene moiety with the non-bonded adjacent base pair. Conservative (same sugar puckers and glycosidic angles as in B-DNA) and non-conservative (alternating sugar puckers as in intercalation sites) conformations are found and they are proposed structures in pathways connecting B-DNA, an intercalation site, and a kink site in the formation of a covalently intercalative bound adduct of BPDE I(+) to N2 on guanine. Stereographic projections are presented for (3') and (5') binding in the DNA. Experimental data for bending of DNA by BPDE, orientation of BPDE in DNA and unwinding of superhelical DNA is explained. The structure of a covalent intercalative complex is predicted to result from the reaction. Also, an anti----syn transition of guanine results in a structure which allows the DNA to resume its overall B-form. The only change is that guanine has been rotated by 200 degrees about its glycosidic bond so that the BPDE I(+) is bound in the major groove. The latter step may allow the DNA to be stored with an adduct which may produce an error in the genetic code.