Design of luminescent rhenium (I) diimine complexes as new probes for biomolecules

Abstract

A series of luminescent rhenium(I) diimine indole complexes, [Re(N–N)(CO)3(py-L-indole)](CF3SO3) and their indole-free counterparts [Re(N–N)(CO)3(py-3-CONH-Et)](CF3SO3) have been synthesized, characterized and their photophysical and electrochemical properties studied. The X-ray crystal structures of [Re(Me4-phen)(CO)3(py-3-CONHC5H10CONH- C2H4-indole)](CF3SO3) and [Re(Me2-phen)(CO)3(py-3-CONH-C2H4- indole)](CF3SO3) have been investigated. Upon irradiation, most of the complexes exhibited triplet metal-to-ligand charge-transfer (3MLCT) (dπ(Re) → π*(diimine)) emission in fluid solutions at 298 K and in low-temperature glass. The structural features and long emission lifetimes of the Me4-phen complexes in solutions at room temperature suggested that the excited state of these complexes exhibited substantial triplet intra-ligand (3IL) (π → π*) (Me4-phen) character. The interactions of these complexes with indole-binding proteins including bovine serum albumin and tryptophanase have been examined by emission titrations and enzyme inhibition assays. Three luminescent rhenium(I) amidodipyridoquinoxaline biotin complexes [Re(dpqa)(CO)3(py-L-biotin)](PF6) and their biotin-free counterpart [Re(dpqa)(CO)3(py)](PF6) have been synthesized and characterized. Excitation of these complexes resulted in intense and long-lived 3MLCT (dπ(Re) → π*(dpqa)) emission in fluid solutions at 298 K and in alcohol glass at 77 K. The emission became much weaker in aqueous buffer, probably due to interactions of water molecules with the amide substituent of the dpqa ligand. The avidin-binding properties of the new biotin complexes have been studied by 4’-hydroxyazobenzene-2-carboxylic acid (HABA) assays, emission titrations, and competitive association and dissociation assays. Most importantly, the complexes showed a profound increase in emission intensities upon binding to avidin. New homogeneous assays for biotin using these complexes, avidin and anthracene-labeled avidin have been designed. To develop new bifunctional probes for nucleic acids and protein, four luminescent rhenium(I) complexes containing an extended planar diimine ligand and a biotin moiety [Re(N–N)(CO)3(py-L-biotin)](PF6) have been synthesized. Upon photoexcitation, all the complexes exhibited intense and long-lived greenish yellow to orange photoluminescence in fluid solutions at 298 K and in low-temperature glass. The luminescence is assigned to a 3IL (π → π*) (diimine) excited state. However, the emissive state of the dppn complexes is likely to possess substantial 3MLCT (dπ(Re) → π*(dppn)) character. The interactions of these complexes with double-stranded calf thymus DNA have been studied by absorption and emission titrations. The binding of the complexes to avidin has also been investigated by HABA assays and emission titrations. Additionally, a new homogeneous assay for biotin using complex 1 and avidin has been designed. A series of luminescent rhenium(I) estradiol conjugates [Re(N–N)(CO)3(py-L-est)](CF3SO3) and [Re(bpy-est)(CO)3(py)](CF3SO3) that can function as probes for estrogen-binding proteins have been designed. The X-ray crystal structure of [Re(phen)(CO)3(py-est)](CF3SO3) has been investigated. The photophysical and electrochemical properties of all the complexes have been studied. Most of the complexes exhibited intense and long-lived 3MLCT (dπ(Re) → π*(N–N)) emission in fluid solutions at 298 K and in low-temperature glass upon irradiation. The emissive state of the Me4-phen complexes possessed some 3IL (π → π*) (Me4-phen) character. The lipophilicity of these complexes has been determined by reversed-phase HPLC. In addition, the binding of these complexes to estrogen receptor α has been investigated by emission titrations.

Date of Award	3 Oct 2006
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Kam Wing Kenneth LO (Supervisor)