Luminescent transition metal complex biotin conjugates

The avidin–biotin system is a classic protein–ligand model and is widely employed in bioanalytical applications. Although biotin has been linked to various reporter units, such as fluorescent organic compounds, luminescent transition metal-based biotin conjugates have not been explored. We have recently incorporated biotin into a series of luminescent rhenium(I), iridium(III) and ruthenium(II) polypyridine complexes to form new sensors for avidin. The most important observations were the enhanced emission intensities and extended lifetimes of these luminescent transition metal biotin complex conjugates when they bound to the protein. These changes resulted from the increased hydrophobicity and rigidity of the local surroundings of the probes after the binding event. The effects of the polypyridine and cyclometallating ligands and spacer-arms between the luminophores and biotin on protein-binding were examined. On the basis of the characteristic photophysical properties of these luminescent transition metal biotin complexes, new assays for avidin and biotin were developed.