Abstract
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.