A proton nuclear magnetic resonance study of the interaction of zinc(II) with the antitumour drug streptonigrin

The interaction of zinc(II) with the antitumour drug streptonigrin in [H-2(8)]tetrahydrofuran has been studied by 400 MHz variable-temperature H-1 NMR spectroscopy. kf 190 K, and in the presence of an excess of zinc(II) trifluoromethanesulfonate (triflate); (10-15 equivalents);streptonigrin forms a 1:1 complex Ia(6%) and two diastereomeric 2: 1 complexes, Ib (15%) and Ic (79%). The relative concentrations of these complexes are highly temperature dependent and at room temperature only la is detected. A fourth minor complex Id was detected (< 5%) in the range 240-260 K. In CD3CN and less than 5 equivalents of zinc(II) triflate, la-ld are detected, but in the presence of 10-15 equivalents of zinc(rr) triflate la is the predominant complex (> 93%) in the range 235-300 K. Implications for the role of metal ions in the mechanism of the antitumour action of streptonigrin are discussed.