DNA Binding of Ruthenium Tris(1,10-phenanthroline): Evidence for the Dependence of Binding Mode on Metal Complex Concentration.

The interaction with calf thymus DNA, poly(dA-dT)(2) and poly(dG-dC)(2) of the two enantiomers (Lambda and Delta) of [Ru(1,10-phenanthroline)(3)](2+), denoted PHEN, and of [Ru(4,7-dimethyl-1,10-phenanthroline)(3)](2+), denoted [4,7], [Ru(5,6-dimethyl-1,10-phenanthroline)(3)](2+), denoted [5,6], and [Ru(3,4,7,8-tetramethyl-1,10-phenanthroline)(3)](2+), denoted [3,4,7,8], has been investigated by normal absorption, linear dichroism (LD), circular dichroism (CD), and computer modeling. These studies have been performed at the saturated binding limit and the "isolated" limit where the DNA is in excess. The binding mode is dependent upon the enantiomer (Lambda or Delta), the DNA base sequence, the ring substituent pattern, and, for the Delta enantiomer, the relative concentrations of DNA and metal complex. Both the Lambda and Delta enantiomers of PHEN and [4,7] show at least two binding regimes. One binding regime operates below a metal complex:DNA phosphate mixing ratio, R, of 1:4-6. The average site size (number of DNA bases per bound metal complex) also decreases from 8-12 bases per metal complex at low R to 3 bases at high R. The average angle (alpha(eff)) between the metal complex 3-fold axis and the DNA helical axis was derived from the LD. At high R (saturated metal complex binding) for both enantiomers of both compounds, this angle is 55 degrees +/- 3 degrees. For low R (isolated metal complex binding), the average binding orientations for the enantiomers are different for PHEN (Lambda, alpha(eff) = 59 degrees; Delta, alpha(eff) = 38 degrees ) and for [4,7] (Lambda, alpha(eff) = 84 degrees; Delta, alpha(eff) = 42 degrees ). Under the low-R conditions the Delta enantiomer of both compounds binds to calf thymus DNA more strongly than the Lambda enantiomer. [3,4,7,8] binds to DNA but is not oriented in the LD experiment. There is no evidence that [5,6] binds to DNA. To explain the LD results for PHEN several possible binding orientations were considered in computer modeling studies. These have the metal complex located with (i) a single phenanthroline chelate approximately parallel to the base pair planes in the major groove (referred to as partially inserted); (ii) a single chelate along the minor groove (referred to as slotted); (iii) two chelates in the minor groove (referred to as minor facial). Using orientations adopted in energy-minimized complexes it was possible to deduce the approximate relative occupancy of the different modes. For Lambda-PHEN the partially inserted mode is favored at all mixing ratios. For Delta-PHEN at low-R minor groove binding is preferred for most sequences with most metal complexes adopting a minor facial orientation. However, at high R (close packed metal complexes) the slotted mode becomes more favorable and some major groove partial insertion also occurs. For both Delta- and Lambda-[4,7] the minor facial mode is favored at low R. As R increases, the slotted mode becomes more favorable for both enantiomers of [4,7].