We performed fully first principles quantum mechanical calculations of the ground- and excited-state properties of the [cis (NCS)(2)-Ru(Il)-bis(2,2'-bipyridine-4,4'-dicarboxylate)] dye, N719, adsorbed onto a model TiO2 nanoparticle. Our study confirms an indirect electron injection mechanism for Ru(II) dyes on TiO2 and indicates a remarkable effect of dye protonation on the electronic properties of N719-sensitized TiO2 nanoparticles. We find that two different electron injection mechanisms (adiabatic and nonadiabatic) may be present in DSSCs employing dyes carrying a different number of protons. Despite such differences, the absorption spectra corresponding to strongly and weakly coupled dye/TiO2 excited states are remarkably similar, so that a discrimination of the two electron injection regimes does not appear to be feasible based on inspection of the absorption spectra.