The Complexation of Tetracycline and Anhydrotetracycline with Mg2+ and Ca2+: A Spectroscopic Study

Citations Over Time

Abstract

Steady-state absorption and emission, circular dichroism (CD), and time-of-flight secondary-ion-mass-spectroscopic (TOF−SIMS) measurements were performed to study the complexation of tetracycline (TC) and anhydrotetracycline (AHTC) with Mg2+ and Ca2+ ions, respectively, in aqueous solutions at pH 8.02. The results obtained suggest that Ca2+ forms a 1:2 ligand:metal complex with TC via chelation through O10−O11 and O12−O1 and induces thereby the extended conformation A of TC, which is stabilized through hydrogen bonding between the deprotonated dimethylamino nitrogen, N4, and OH12a. pH titrations provide evidence that N4 deprotonates in the presence of a 164-fold molar excess of Ca2+ at approximately pH ⩾ 7.7 (cTC = 2.1 × 10-5 M). In contrast to Ca2+, Mg2+ binds to N4−O3 and thereby stabilizes the twisted conformation B of TC. TOF−SIMS measurements indicate that a 1:2 ligand:metal complex is formed in addition to the 1:1 complex. The Mg2+-induced increase in the fluorescence intensity and the observed changes in the absorption spectra provide evidence that the other Mg2+ ion binds to the BCD ring system through the deprotonated O11. In contrast to TC, which adopts the twisted conformation B in aqueous solution at pH 8.02, AHTC exhibits the extended conformation A due to slightly lower deprotonation constants. In the presence of Mg2+, however, the conformational equilibrium is shifted toward the twisted conformation B due to binding of Mg2+ to N4. TOF−SIMS measurements suggest that a 2:2 ligand:metal complex is formed. AHTC remains in conformation A upon addition of Ca2+; complexation through O10 can be excluded on the basis of absorption spectroscopic data.

Related Papers

• → Partially and fully deprotonated sulfuric acid in H2SO4(H2O) (n=6–9) clusters(2004)45 cited
• → Deprotonation of Iron Vinylidene Complexes Containing a dppe Ligand(2007)15 cited
• → Deprotonation and acidity characterization of biomass sugars: a first-principles study(2016)6 cited
• → Deprotonation of lipid-depleted bacteriorhodopsin.(1988)29 cited
• Deprotonation reactions of the non-rigid bis-(2,5-diamidopyrrole) induced by anions(2009)