Making the right ligand selection: DFT calculations of Gibbs energies for the one-electron photooxidation of guanine by six Ru(II)-polypyridine complexes are reported. The theoretical predictions are compared with new EPR spectra. Our theoretical calculations confirm the experimental observations that the direct photooxidation of guanine by [Ru(bpz)(3)](2+), [Ru(tap)(3)](2+), and [Ru(bpz)(2)(dppz)](2+) is thermodynamically favorable, but is unfavorable with [Ru(bpy)(3)](2+), [Ru(phen)(3)](2+), and [Ru(bpy)(2)(dppz)](2+) (see figure).