Rhenium(I) Polypyridine Complexes as Luminescence-Based Sensors for the BSA Protein

Karina P M Frin,Verônica M Nascimento

doi:10.5935/0103-5053.20150268

Abstract

The binding interaction of rhenium(I) complexes fac-[Re(CO)3(NN)(py)]+, py = pyridine and NN = 1,10-phenanthroline (phen), 4,7-diphenyl-1,10-phenanthroline (ph2phen) or 4,7-dichloro-1,10-phenanthroline (Cl2phen), and bovine serum albumin (BSA) was investigated at physiological pH using emission intensity variation and circular dichroism (CD) spectroscopy. The photophysical investigations showed that in the presence of BSA, the metal-to-ligand-charge transfer (3MLCT) emission of the rhenium(I) complexes was quenched due to entrapment of the complex within the protein environment. Additionally, high Stern-Volmer (KSV) and binding (Kb) constants were determined from luminescence data, revealing the occurrence of a strong interaction and/or association. The differences in KSV values can be tentatively associated with an electron-withdrawing constant (σ) defined by Hammett equation. The CD results showed that the extent of α-helicity of the BSA decreased upon the addition of rhenium complexes, which provided further support for the interaction of rhenium(I) complexes and the protein.

Highlights

The use of coordination compounds in the design of photosensors offers a vast range of applications from small molecule probes to biomolecule probes, such as proteins and DNA.[1,2,3,4] In particular, the emissive property of rhenium(I) polypyridyl complexes, fac-[Re(CO)3(NN)L]n (n = 0 or +1), is generally ascribed to the metal-to-ligand charge transfer (3MLCT) excited state
In the luminescence intensity and a small hypsochromic shift, which can be due to changes in local environment of the ReI complexes promoted by the protein environment
Since the substituent constant of phenyl group is very close to the non-substituted phen constant, the KSV values obtained for both complexes are very similar

Summary

Introduction

The use of coordination compounds in the design of photosensors offers a vast range of applications from small molecule probes to biomolecule probes, such as proteins and DNA.[1,2,3,4] In particular, the emissive property of rhenium(I) polypyridyl complexes, fac-[Re(CO)3(NN)L]n (n = 0 or +1), is generally ascribed to the metal-to-ligand charge transfer (3MLCT) excited state This excited state can be modulated by changing the polypyridine ligand, NN, as well as the spectator ligand, L, and can be conveniently employed in the development of luminescent sensors.[5,6,7,8]. The rhenium(I) complexes have several advantages over organic compounds employed for the same purpose:[3,6] long emission lifetimes, which enhance the detection sensitivity in time-resolved techniques; photostability; the nature of their emission-phosphorescence with a large Stokes shift, which can minimize self-quenching; environmentsensitive emission, among other benefits. Such complexes exhibit high membrane permeability, being stable under physiological conditions.[3,8,9,10,11] While there are several investigations on the photophysical behavior of these complexes in fluid, usually in acetonitrile and dichloromethane, rigid media,[12,13,14,15,16,17] and in the presence of

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