Minocycline‐Based Europium(III) Chelate Complexes: Synthesis, Luminescent Properties, and Labeling to Streptavidin

Two chelate ligands for europium(III) having minocycline (=(4 S ,4a S ,5a R ,12a S )‐4,7‐bis(dimethylamino)‐1,4,4a,5,5a,6,11,12a‐octahydro‐3,10,12,12a‐tetrahydroxy‐1,11‐dioxonaphthacene‐2‐carboxamide; 5 ) as a VIS‐light‐absorbing group were synthesized as possible VIS‐light‐excitable stable Eu 3+ complexes for protein labeling. The 9‐amino derivative 7 of minocycline was treated with H 6 TTHA (=triethylenetetraminehexaacetic acid=3,6,9,12‐tetrakis(carboxymethyl)‐3,6,9,12‐tetraazatetradecanedioic acid) or H 5 DTPA (=diethylenetriaminepentaacetic acid= N , N ‐bis{2‐[bis(carboxymethyl)amino]ethyl}glycine) to link the polycarboxylic acids to minocycline. One of the Eu 3+ chelates, [Eu 3+ (minocycline‐TTHA)] ( 13 ), is moderately luminescent in H 2 O by excitation at 395 nm, whereas [Eu 3+ (minocycline‐DTPA)] ( 9 ) was not luminescent by excitation at the same wavelength. The luminescence and the excitation spectra of [Eu 3+ (minocycline‐TTHA)] ( 13 ) showed that, different from other luminescent Eu III chelate complexes, the emission at 615 nm is caused via direct excitation of the Eu 3+ ion, and the chelate ligand is not involved in the excitation of Eu 3+ . However, the ligand seems to act for the prevention of quenching of the Eu 3+ emission by H 2 O. The fact that the excitation spectrum of [Eu 3+ (minocycline‐TTHA)] is almost identical with the absorption spectrum of Eu 3+ aqua ion supports such an excitation mechanism. The high stability of the complexes of [Eu 3+ (minocycline‐DTPA)] ( 9 ) and [Eu 3+ (minocycline‐TTHA)] ( 13 ) was confirmed by UV‐absorption semi‐quantitative titrations of H 4 (minocycline‐DTPA) ( 8 ) and H 5 (minocycline‐TTHA) ( 12 ) with Eu 3+ . The titrations suggested also that an 1 : 1 ligand Eu 3+ complex is formed from 12 , whereas an 1 : 2 complex was formed from 8 minocycline‐DTPA. The H 5 (minocycline‐TTHA) ( 12 ) was successfully conjugated to streptavidin (SA) ( Scheme 5 ), and thus the applicability of the corresponding Eu 3+ complex to label a protein was established.