Design of luminescent biotinylation reagents derived from cyclometalated iridium(III) and rhodium(III) bis(pyridylbenzaldehyde) complexes.

A new class of luminescent biotinylation reagents derived from cyclometalated iridium(III) and rhodium(III) bis(pyridylbenzaldehyde) biotin complexes, Ir(pba)(2)(bpy-C6-biotin) (1), Ir(pba)(2)(bpy-TEG-biotin) (2), and Rh(pba)(2)(bpy-C6-biotin) (3), together with their biotin-free counterparts Ir(pba)(2)(bpy-Et) (4) and Rh(pba)(2)(bpy-Et) (5) [Hpba = 4-(2-pyridyl)benzaldehyde, bpy-C6-biotin = 4-[(6-biotinamido)hexylaminocarbonyl]-4'-methyl-2,2'-bipyridine, bpy-TEG-biotin = 4-[(13-biotinamido-4,7,10-trioxa)tridecylaminocarbonyl]-4'-methyl-2,2'-bipyridine, bpy-Et = 4-(ethylaminocarbonyl)-4'-methyl-2,2'-bipyridine], have been synthesized and characterized and their photophysical and electrochemical properties studied. Upon photoexcitation, the iridium(III) complexes 1, 2, and 4 exhibited intense and long-lived orange-yellow luminescence in fluid solutions at 298 K and in rigid glass at 77 K. The rhodium(III) complexes 3 and 5 were weakly emissive in fluid solutions at 298 K but showed intense luminescence in low-temperature glass. In view of the structured emission profiles and the long lifetimes, the emission of all of the complexes has been assigned to a triplet intraligand ((3)IL) (pi --> pi*) (pba) excited state, which was probably mixed with some triplet metal-to-ligand charge-transfer ((3)MLCT) [dpi(Ir or Rh) --> pi*(pba)] character. To investigate the reactivity of the aldehyde groups, complex 2 was reacted with n-butylamine, resulting in the formation of the complex Ir(ppy-CH(2)NHC(4)H(9))(2)(bpy-TEG-biotin) (2a) [Hppy-CH(2)NHC(4)H(9) = 2-[4-[N-(n-butyl)aminomethyl]phenyl]pyridine]. All of the aldehyde complexes have been used to biotinylate bovine serum albumin (BSA) to form bioconjugates 1-BSA-5-BSA. The bioconjugates have been isolated, purified, and characterized and their photophysical properties studied. Upon photoexcitation, all of the bioconjugates were luminescent and the emission has been attributed to a (3)MLCT [dpi(Ir) --> pi*(N(wedge)N)] state for the iridium(III) conjugates and a mixed (3)IL (pi --> pi*) (N(wedge)N and N(wedge)C)/(3)MLCT [dpi(Rh) --> pi*(N(wedge)N)] state for the rhodium(III) conjugates. The avidin-binding properties of complexes 1, 2, 2a, and 3 and bioconjugates 1-BSA-3-BSA have been investigated using the 4'-hydroxyazobenzene-2-carboxylic acid assay. Emission titrations showed that complex 2a displayed a significant change of the emission profile upon binding to avidin. Additionally, the cytotoxicity of all of the iridium(III) and rhodium(III) complexes toward the human cervix epithelioid carcinoma cells has been examined by the 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyltetrazolium bromide assay. Furthermore, the cellular uptake properties of the complexes and bioconjugate 2-BSA have been investigated by laser-scanning confocal microscopy.