Long-Lifetime Ru(II) Complexes as Labeling Reagents for Sulfhydryl Groups

We report the synthesis and spectral properties of two long-lifetime highly luminescent Ru(II) complexes containing either a sulfhydryl reactive iodoacetamido group or a less reactive chloroacetamido group, [Ru(bpy)2(5-iodoacetamido-1,10-phenanthroline)](PF6)2and [Ru(bpy)2(5-chloroacetamido-1,10-phenanthroline)](PF6)2, respectively, where bpy is 2,2′-bipyridine. Ru(bpy)2(phen-IA)](PF6)2was covalently linked to human serum albumin (HSA) and human immunoglobulin G (IgG). The photoluminescence lifetime of protein-bound probes approaches 1 μs under ambient conditions. In the absence of rotational motions, this probe displayed an anisotropy of 0.18 for excitation at 472 nm. Anisotropy decay data were used to determine the overall rotational correlation times of HSA and IgG. These long-lifetime sulfhydryl-reactive probes can be used to recover microsecond rotational motions and/or domain motions of proteins and/or macromolecular complexes.