Effects of N-alkyl and ammonium groups on the hydrolytic cleavage of DNA with a Cu(II)TACH (1,3,5-triaminocyclohexane) complex. Speciation, kinetic, and DNA-binding studies for reaction mechanism

cis,cis-1,3,5-Triaminocyclohexane (c-TACH), its N-alkyl-derivatives (alkyl = methyl, ethyl), and trans,cis-1,3,5-triaminocyclohexane (t-TACH) were prepared, and speciation and DNA cleaving property of Cu(II) complexes of these ligands were investigated. All of the complexes efficiently promote the hydrolytic cleavage of supercoiled plasmid DNA under physiological conditions without further additives. The DNA cleavage rate (Vobs) trend at pH values between 8 and 9 is N-Me3 = N-Et1 < t-TACH < c-TACH < N-Et2 < N-Et3. At pH 7, the trend is c-TACH < N-Et3 = N-Et2 < N-Et1 < N-Me3 ≪ t-TACH. The cleavage rate constants at 35 °C, for the c-TACH complex are 3 × 10−1 h−1 at pH 8.1 and 2 × 10−1 h−1 at pH 7.0 ([DNA] = 7 μM, [Cu(II)-complex] = 105 μM). The hydrolytically active species at pH > 8 is CuL(H2O)(OH)+ in which L coordinates to Cu(II) as a tridentate ligand for all complexes except for t-TACH. The hydrolytically active species at pH 7 is CuLH(H2O)33+ or CuLH(H2O)43+ in which LH coordinates as bidentate ligand. DNA-binding constants of c-TACH and t-TACH complexes are presented and the effects of N-alkyl and ammonium groups are discussed in light of the proposed reaction mechanism.