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Complexes Formed in Solution Between Vanadium(IV)/(V) and the Cyclic Dihydroxamic Acid Putrebactin or Linear Suberodihydroxamic Acid

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journal contribution
posted on 16.12.2015, 19:22 by Amalie A. H. Pakchung, Cho Zin Soe, Tulip Lifa, Rachel Codd
An aerobic solution prepared from V(IV) and the cyclic dihydroxamic acid putrebactin (pbH2) in 1:1 H2O/CH3OH at pH = 2 turned from blue to orange and gave a signal in the positive ion electrospray ionization mass spectrometry (ESI-MS) at m/zobs 437.0 attributed to the monooxoV(V) species [VVO(pb)]+ ([C16H26N4O7V]+, m/zcalc 437.3). A solution prepared as above gave a signal in the 51V NMR spectrum at δV = −443.3 ppm (VOCl3, δV = 0 ppm) and was electron paramagnetic resonance silent, consistent with the presence of [VVO(pb)]+. The formation of [VVO(pb)]+ was invariant of [V(IV)]:[pbH2] and of pH values over pH = 2–7. In contrast, an aerobic solution prepared from V(IV) and the linear dihydroxamic acid suberodihydroxamic acid (sbhaH4) in 1:1 H2O/CH3OH at pH values of 2, 5, or 7 gave multiple signals in the positive and negative ion ESI-MS, which were assigned to monomeric or dimeric V(V)– or V(IV)–sbhaH4 complexes or mixed-valence V(V)/(IV)–sbhaH4 complexes. The complexity of the V-sbhaH4 system has been attributed to dimerization (2[VVO(sbhaH2)]+ ↔ [(VVO)2(sbhaH2)2]2+), deprotonation ([VVO(sbhaH2)]+ – H+ ↔ [VVO(sbhaH)]0), and oxidation ([VIVO(sbhaH2)]0 e ↔ [VVO(sbhaH2)]+) phenomena and could be described as the sum of two pH-dependent vectors, the first comprising the deprotonation of hydroxamate (low pH) to hydroximate (high pH) and the second comprising the oxidation of V(IV) (low pH) to V(V) (high pH). Macrocyclic pbH2 was preorganized to form [VVO(pb)]+, which would provide an entropy-based increase in its thermodynamic stability compared to V(V)–sbhaH4 complexes. The half-wave potentials from solutions of [V(IV)]:[pbH2] (1:1) or [V(IV)]:[sbhaH4] (1:2) at pH = 2 were E1/2 −335 or −352 mV, respectively, which differed from the expected trend (E1/2 [VO(pb)]+/0 < VV/IV–sbhaH4). The complex solution speciation of the V(V)/(IV)–sbhaH4 system prevented the determination of half-wave potentials for single species. The characterization of [VVO(pb)]+ expands the small family of documented V–siderophore complexes relevant to understanding V transport and assimilation in the biosphere.