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Ti(IV) and the Siderophore Desferrioxamine B: A Tight Complex Has Biological and Environmental Implications

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Version 2 2017-01-25, 22:03
Version 1 2017-01-24, 19:20
posted on 2017-01-24, 00:00 authored by Kayleigh E. Jones, Kathleen L. Batchler, Célia Zalouk, Ann M. Valentine
The siderophore desferrioxamine B (DFOB) binds Ti­(IV) tightly and precludes its hydrolytic precipitation under biologically and environmentally relevant conditions. This interaction of DFOB with Ti­(IV) is investigated by using spectro-potentiometric and spectro-photometric titrations, mass spectrometry, isothermal titration calorimetry (ITC), and computational modeling. The data from pH 2–10 suggest two one-proton equilibria among three species, with one species predominating below pH 3.5, a second from pH 3.5 to 8, and a third above pH 8. The latter species is prone to slow hydrolytic precipitation. Electrospray mass spectrometry allowed the detection of [Ti­(IV) (HDFOB)]2+ and [Ti­(DFOB)]+; these species were assigned as the pH < 3.5 and the 3.5 < pH < 8 species, respectively. The stability constant for Ti­(IV)-DFOB was determined by using UV/vis-monitored competition with ethylenediaminetetraacetic acid (EDTA). Taking into consideration the available binding constant of Ti­(IV) and EDTA, the data reveal values of log β111 = 41.7, log β110 = 38.1, and log β11–1 = 30.1. The former value was supported by ITC, with the transfer of Ti­(IV) from EDTA to DFOB determined to be both enthalpically and entropically favorable. Computational methods yielded a model of Ti-DFOB. The physiological and environmental implications of this tight interaction and the potential role of DFOB in solubilizing Ti­(IV) are discussed.