METAL-COMPLEXES OF ANHYDROTETRACYCLINE .1. A SPECTROMETRIC STUDY OF THE CU(II) AND NI(II) COMPLEXES

Anhydrotetracycline (AHTC), one of the major toxic decomposition produ cts of the antibiotic tetracycline, contains several potential binding sites to metal ions. The acidity constants of the ligand were calcula ted in aqueous medium (I = 0.1 M) at 25 and 37 degrees C. We found pK( a1) = 3.23 +/- 0.08, pK(a2) = 5.94 +/- 0.09, and pK(a3) = 8.48 +/- 0.0 2 at 25 degrees C and pK(a1) = 3.12 +/- 0.09, pK(a2) = 5.86 +/- 0.03, and pK(a3) = 8.38 +/- 0.04 at 37 degrees C. The coordination of AHTC t o Cu(II) and NI(II) ions was studied in the solid state as well as in buffered aqueous solution at pH 10.0. At this pH, the formation of the two CuL(2) and CUL species was indicated (log beta(1) = 8.41 +/- 0.04 and log beta(2) = 12.55 +/- 0.05), but only the formation of the NiL complex (log beta = 5.74 +/- 0.04) was identified. Spectroscopic data confirm the previous assignment of the C11 and C12 oxygens as the coor dination sites, yielding six-membered ring chelates and excluding comp lexation through any of the potential binding positions on ring A.