Near-Infrared Emitters and Their Noncovalent Adducts with Human Serum Albumin: Synthesis and Photophysical and Computational Study

Near-infrared (NIR) molecular emitters based on transition-metal complexes have attracted growing attention due to their potential application for in vivo and in vitro bioimaging experiments. Their photophysical characteristics (large Stokes shift and lifetime in the microsecond domain) offer some important advantages in comparison to organic fluorophores and may provide better imaging resolution and higher sensitivity: for example, in mapping the oxygen concentration in biological objects. We have synthesized a series of [Ir(N<^>C)(2)(N<^>N)](+) complexes with emission in the NIR region (N<^>C = (2-benzothienyl)-phenanthridine and 6-(2-benzothienyl)phenanthridine-2-carboxylic acid; N<^>N = functionalized pyridine-triazole chelates), which also display a considerable red shift of their excitation spectra to the window of transparency. The flexible protocol for the synthesis of the N<^>N ligands makes possible wide variations in the peripheral ligand environment: e.g., insertion of hydrophilic carboxyl group and further attachment of the other biologically relevant functions. The compounds obtained were completely characterized using spectroscopic methods, and their ground-state structures and photophysical properties were studied by DFT and TD DFT methods. To analyze the behavior of these emitters in biological systems, we investigated their interaction with human serum albumin (HSA), as the most abundant serum protein. It was found that these complexes readily form noncovalent {HSA-complex} adducts by embedding into hydrophobic cavities of this protein that also induced its partial aggregation. The complexes demonstrated preferential redistribution toward aggregated forms of HSA; the complex:HSA molar ratio did not exceed 1:3 for nonaggregated species. It was also shown that interaction of the hydrophobic complexes with albumin and the resulting aggregation dramatically change their important photophysical parameters such as emitter lifetime and its sensor response onto molecular oxygen.