A long-lived, highly luminescent rhenium(I) metal-ligand complex as a probe of biomolecules

A luminescent rhenium(I) metal-ligand complex, [Re(bcp)(CO)(3)(4-COOHPy)](ClO4), where bcp is 2,9-dimethyl-4,7-diphenyl-1,10-phenanthrolin and 4-COOHPy is isonicotinic acid, has been synthesized and characterized. This complex displays high quantum yields and long excited-state lifetimes (0.3 - 10 mu s) in fluid solution at room temperature. The metal-to-ligand charge transfer (MLCT) emission from this compound exhibits sensitivity to microenvironment. [Re(bcp)(CO)(3)(4-COOHPy)](+) displays highly polarized emission with a maximum anisotropy near 0.3 in the absence of rotational diffusion. This Re(I) MLCT complex was conjugated to several biomolecules, including the proteins human serum albumin (HSA) and bovine immunoglobulin G (IgG) as well as an amine containing lipid. When bound to a protein or lipid, the decay time is near 3 mu s and the quantum yield is similar to 0.12 in aqueous air-equilibrated solution at room temperature. The unique spectral properties and reactive carboxylic acid functionality of [Re(bcp)(CO)(3)(4-COOHPy)](+), allowed utilization of this probe in numerous biophysical and biomedical applications.