The DNA-binding properties of Ru(bpy)(2)(eilatin)(2+) have been investigated to determine if the sterically expansive eilatin ligand confers specificity for destabilized single-base mismatches in DNA. Competitive DNA photocleavage experiments employing a sequence-neutral metallointercalator, Rh(bpy)(2)(phi)(3+) (phi = 9,10-phenanthrenequino-nediimine), and a mismatch-specific metalloinsertor, Rh(bpy)(2)(chrysi)(3+) (chrysi = chrysene-5,6-quinonediimine), reveal that the eilatin complex binds to a CC mismatched site with an apparent binding constant of 2.2(2) x 10(6) M-1. Nonetheless, the selectivity in binding mismatched DNA is not high: competitive titrations with Rh(bpy)(2)(phi)(3+) show that the complex binds also to well-matched B-form sites. Thus, Ru(bpy)(2)(eilatin)(2+), despite containing the extremely expansive eilatin ligand, displays lower selectivity for the mismatch than does Rh(bpy)(2)(chrysi)(3+), a metalloinsertor containing the smaller, though still bulky, chrysene-5,6-quinonediimine ligand. In summary, the size and shape of the eilatin ligand allow stacking with both well-matched and mismatched DNA.