The coordination sites of tetracyclines to Cu
2+ in a solution at pH 5 were examined by the methods of circular dichroism (CD) spectra, absorption spectra and optical rotation. The results obtained were as follows : 1) The CD spectrum of tetracycline (TC)-Cu
2+ complex showed a positive peak at 400 nm, whereas that of tetracyclinenitrile (TN)-Cu
2+ complex didn't show any positive peaks in the range over 350 nm. 2) Bathochromic shifts in the absorption spectra of TC and TN induced by the addition of Cu
2+ at pH 5 were observed. 3) With an increase in the molar ratio of Cu
2+ to TC or oxytetracycline (OT), the optical rotation changed from negative to positive. On the other hand, the optical rotation of TN-Cu
2+ complex was independent of the molar ratio of Cu
2+ to TN. 4) The CD spectrum of TC-Cu (ethylenediamine)
2+ or OT-Cu (ethylenediamine)
2+ complex showed a positive peak at 400 nm or 390 nm. 5) The curves obtained by the plots of the optical rotation against the molar of Cu (ethylenediamine)
2+ to TC or OT were similar to that obtained by the plots against the molar ratio of Cu
2+ to OT. Therefore, at pH 5, tetracyclines may bind to Cu
2+ at both C
11-C
12 β-diketone site and C
2 carboxamide site through bending of A ring over B and C rings.
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