Design of Rhenium(I) Polypyridine Biotin Complexes as a New Class of Luminescent Probes for Avidin

This paper describes the design of a series of luminescent rhenium(I) polypyridine biotin complexes containing different spacer-arms, [Re(N−N)(CO)3(py-4-CH2−NH−biotin)](PF6) (py-4-CH2−NH−biotin = 4-(biotinamidomethyl)pyridine; N−N = 1,10-phenanthroline, phen (1a), 3,4,7,8-tetramethyl-1,10-phenanthroline, Me4-phen (2a), 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, Me2−Ph2-phen (3a), dipyrido[3,2-f:2‘,3‘-h]quinoxaline, dpq (4a)), [Re(N−N)(CO)3(py-3-CO−NH-en-NH−biotin)](PF6) (py-3-CO−NH-en-NH−biotin = 3-(N-((2-biotinamido)ethyl)amido)pyridine; N−N = phen (1b), Me4-phen (2b), Me2−Ph2-phen (3b), dpq (4b)), and [Re(N−N)(CO)3(py-4-CH2−NH-cap-NH−biotin)](PF6) (py-4-CH2−NH-cap-NH−biotin = 4-(N-((6-biotinamido)hexanoyl)aminomethyl)pyridine; N−N = phen (1c), Me4-phen (2c), Me2−Ph2-phen (3c), dpq (4c)). Upon irradiation, all of the rhenium(I)−biotin complexes exhibited intense and long-lived triplet metal-to-ligand charge-transfer (3MLCT) (dπ(Re) → π*(diimine)) emission in fluid solutions at 298 K. The interactions of these biotin-containing complexes with avidin have been studied by 4‘-hydroxyazobenzene-2-carboxylic acid (HABA) assays, emission titrations, and competitive association and dissociation assays. On the basis of the results of these experiments, homogeneous assays for biotin and avidin have been designed.