Abstract
Soluble Mn(III)–L complexes appear to constitute a substantial portion of manganese (Mn) in many environments and serve as critical high-potential species for biogeochemical processes. However, the inherent reactivity and lability of these complexes—the same chemical characteristics that make them uniquely important in biogeochemistry—also make them incredibly difficult to measure. Here we present experimental results demonstrating the limits of common analytical methods used to quantify these complexes. The leucoberbelin-blue method is extremely useful for detecting many high-valent Mn species, but it is incompatible with the subset of Mn(III) complexes that rapidly decompose under low-pH conditions—a methodological requirement for the assay. The Cd-porphyrin method works well for measuring Mn(II) species, but it does not work for measuring Mn(III) species, because additional chemistry occurs that is inconsistent with the proposed reaction mechanism. In both cases, the behavior of Mn(III) species in these methods ultimately stems from inter- and intramolecular redox chemistry that curtails the use of these approaches as a reflection of ligand-binding strength. With growing appreciation for the importance of high-valent Mn species and their cycling in the environment, these results underscore the need for additional method development to enable quantifying such species rapidly and accurately in nature.
Highlights
The chemistry of manganese associated with biomass, Earth surface environments, and geological materials is characterized by redox conversions among three common oxidation states [1]
It has been reported that some Mn(III)–L complexes, most notably Mn(III)–desferrioxamine B (DFOB), do not react with leucoberbelin blue (LBB) [24]
We attempted to measure Mn(III)–DFOB by using LBB and did not observe the spectral change at 624 nm characteristic of LBB oxidation (Figure 2A), as is consistent with previous reports that Mn(III)–DFOB does not react with LBB [24]
Summary
The chemistry of manganese associated with biomass, Earth surface environments, and geological materials is characterized by redox conversions among three common oxidation states [1]. Even under the assumption that the only Mn(III) chemistry occurring in these reactions is the proposed mechanism of a simple ligand-exchange reaction, deconvolving reactions from multiple unknown Mn complexes is a formidable challenge Using both the LBB and TCPP methods, differences in the responses from standard solutions of different Mn(III) complexes have been observed [24,32]. In the case of ligand-exchange extractions, we caution that, since DFOB is not a redox stable ligand and its reactivity exhibits a strong pH dependence [17], this compound (and others like it) should not be used as reference species to examine thermodynamic binding strength of unknown molecules in environmental samples In all cases, these issues stemmed from the inherent reactivity and lability of Mn(III)–L complexes
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