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.

Quorum sensing (QS) regulates important bacterial behaviors such as virulent protein production and biofilm formation. QS requires that molecular signals are exchanged between cells, extracellularly, where environmental conditions influence signal stability. In this work, we present a novel complexation between metal cations (Ag and Cu) and a QS autoinducer signal, N-hexanoyl- L-homoserine lactone (HHL). The molecular interactions were investigated using mass spectrometery, attenuated total reflectance-Fourier transform infrared spectroscopy, and computational simulations. Results show that HHL forms predominantly 1:1 complexes with Ag ( K = 3.41 × 10 M) or Cu ( K = 1.40 × 10 M), with the coordination chemistry occurring on the oxygen moieties. In vivo experiments with Chromobacterium violaceum CV026 show that sublethal concentrations of Ag and Cu decreased HHL-regulated QS activity. Furthermore, when Ag was preincubated with HHL, Ag toxicity to CV026 decreased by an order of magnitude, suggesting HHL:metal complexes alter the bioavailability of the individual constituents.

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