Ruthenium drugs are potent anti-
cancer agents, but inducing
drug selectivity and enhancing their modest activity remain challenging. Slow Ru
ligand loss limits the formation of free sites and subsequent binding to
DNA base pairs. Herein, we designed a
ligand that rapidly dissociates upon irradiation at low pH. Activation at low pH can lead to
cancer selectivity, since many
cancer cells have higher metabolism (and thus lower pH) than non-cancerous cells. We have used the pH sensitive
ligand, 6,6'-dihydroxy-2,2'-bipyridine (66'bpy(
OH)2), to generate [
Ru(bpy)2(66'(bpy(
OH)2)](2+), which contains two acidic
hydroxyl groups with pKa1=5.26 and pKa2=7.27. Irradiation when protonated leads to photo-dissociation of the 66'bpy(
OH)2
ligand. An in-depth study of the structural and electronic properties of the complex was carried out using X-ray crystallography, electrochemistry, UV/visible spectroscopy, and computational techniques. Notably, RuN bond lengths in the 66'bpy(
OH)2 complex are longer (by ~0.3Å) than in polypyridyl complexes that lack 6 and 6' substitution. Thus, the longer bond length predisposes the complex for photo-dissociation and leads to the anti-
cancer activity. When the complex is deprotonated, the 66'bpy(O(-))2
ligand molecular orbitals mix heavily with the
ruthenium orbitals, making new mixed
metal-
ligand orbitals that lead to a higher bond order. We investigated the anti-
cancer activities of [
Ru(bpy)2(66'(bpy(
OH)2)](2+), [
Ru(bpy)2(44'(bpy(
OH)2)](2+), and [
Ru(bpy)3](2+) (44'(bpy(
OH)2=4,4'-dihydroxy-
2,2'-bipyridine) in HeLa cells, which have a relatively low pH. It is found that [
Ru(bpy)2(66'(bpy(
OH)2)](2+) is more cytotoxic than the other
ruthenium complexes studied. Thus, we have identified a pH sensitive
ruthenium scaffold that can be exploited for photo-induced anti-
cancer activity.