Synthesis, photophysics, electrochemistry, and protein-binding studies of luminescent rhenium(I) polypyridine indole and biotin complexes

Abstract

A series of luminescent rhenium(I) dipyridoquinoxaline indole complexes [Re(N–N)(CO)3(py-spacer-indole)](CF3SO3) (N–N = dipyrido[3,2-f:2’,3’-h]quinoxaline (dpq), 2-(n-butylamido)dipyrido[3,2-f:2’,3’-h]quinoxaline (dpqa); py-spacer-indole = N-(3-pyridoyl)tryptamine (py-3-CONHC2H4-indole), N-(N-(3-pyridoyl)-6-aminohexanoyl)tryptamine (py-3-CONHC5H10CONHC2H4-indole)) and their indole-free counterparts [Re(N–N)(CO)3(py-3-CONH-Et)](CF3SO3) (N–N = dpq, dpqa; py-3-CONH-Et = N-ethyl-(3-pyridyl)formamide) have been synthesized and characterized. All the complexes exhibited triplet metal-to-ligand charge-transfer (3MLCT) (dπ(Re) → π*(diimine)) emission in fluid solutions at 298 K and in alcohol glass at 77 K. The emission quantum yields of the complexes were reduced upon changing the solvent from CH2Cl2 to aqueous buffer. The reduction for the dpqa complexes was much more significant compared to that for the dpq complexes due to the hydrogen-bonding interactions of the amide substituent of the dpqa ligand with water molecules. The interactions of these rhenium(I) complexes with indole-binding proteins including bovine serum albumin (BSA) and tryptophanase (TPase) have been studied by emission titrations and inhibition assays, respectively. The first class of luminescent biotinylating reagents that are derived from rhenium(I) polypyridine complexes [Re(N–N)(CO)3(py-biotin-NCS)](PF6) (N–N = 3,4,7,8-tetramethyl-1,10-phenanthroline (Me4-phen), 4,7-diphenyl-1,10-phenanthroline (Ph2-phen); py-biotin-NCS = 3-isothiocyanato-5-(N-((2-biotinamido)ethyl)aminocarbonyl)pyridine) including a biotin unit and an isothiocyanate moiety have been synthesized from the precursor amine complexes [Re(N–N)(CO)3(py-biotin-NH2)](PF6) (N–N = Me4-phen, Ph2-phen; py-biotin-NH2 = 3-amino-5-(N-((2-biotinamido)ethyl)aminocarbonyl)pyridine). To investigate the amine-specific reactivity of the isothiocyanate complexes, they have been reacted with a model substrate ethylamine, resulting in the formation of the thiourea complexes [Re(N–N)(CO)3(py-biotin-TU-Et)](PF6) (N–N = Me4-phen, Ph2-phen; py-biotin-TU-Et = 3-ethylthioureidyl-5-(N-((2-biotinamido)ethyl)aminocarbonyl)pyridine). All the rhenium(I) complexes have been characterized and their photophysical and electrochemical properties have been studied. The avidin-binding properties of the thiourea complexes have been examined by 4’-hydroxyazobenzene-2-carboxylic acid (HABA) assays and emission titrations. Additionally, BSA has been biotinylated with the isothiocyanate complexes. The emission and avidin-binding properties of the resultant rhenium-BSA bioconjugates have been investigated.