The Binding of Yb(III) to Adriamycin. A ¹H NMR Relaxation Study

The binding of Yb(III) to adriamycin in methanol was monitored by UV-vis absorption spectroscopy. A Job ratio plot based on UV-vis binding data indicated that the complex had a stoichiometry of 1:1. The formation of the complex was found to be dependent on the hydrogen ion concentration present in methanol. The ¹H NMR spectrum of the Yb(III)-adriamycin complex was found to contain two sets of resonances arising from the complex and the free adriamycin indicating the rate of exchange of adriamycin between its free and complexed state was slow on the ¹H NMR chemical shift time scale. Transfers of magnetization experiments were carried out in order to assign the resonances arising from the paramagnetic complex. The spin-lattice relaxation times of the assigned resonances were determined. These relaxation times were analyzed in terms of the distances from the Yb(III) ion to various protons in the drug. With use of these distances and the coordinates of the hydrogen atoms obtained from an X-ray structure, the site of metal complexation was found to be at the C₁₁ and C₁₂ oxygens of the antibiotic characterized by an oxygen-Yb(III) bond length of 2.5 Å.