Comparative Binding Studies of the Chelators Methylolanthanin and Rhodopetrobactin B to Lanthanides and Ferric Iron

The question of lanthanide (Ln) uptake in Ln-using bacteria has gained a lot of attention in recent years, and the existence of specific Ln-binding metallophores, termed lanthanophores, has been postulated. Here, the recently isolated metallophore methylolanthanin (MLL), which is shown to be involved in Ln metabolism of Methylobacterium extorquens AM1 along the structurally related siderophore rhodopetrobactin B (RPB B), is investigated. The total synthesis of both chelators as well as Ln-binding investigations employing a multitude of spectroscopic methods is reported. Compared to MLL, RPB B has a higher binding affinity for Fe³⁺. Unexpectedly, both metallophores seem to precipitate Lns under biologically relevant conditions (pH and concentration range). Therefore, a solubility product of -12.07 ± 0.24 mol² L^-2 for the precipitated Eu³⁺-MLL complex is reported. Furthermore, a combination of single-cell inductively coupled plasma mass spectrometry and Liquid Chromatography-Mass Spectrometry (LC-MS) analysis of bacterial supernatant to investigate the Nd accumulation as well as MLL secretion under Fe limitation in M. extorquens AM1 is used. Finally, ion mobility spectrometry-mass spectrometry and quantum chemical calculations are used to investigate the RPB B and MLL complexation in the gas phase with Fe³⁺ and all rare earth elements (except Pm). The results challenge the classical siderophore-like Ln uptake (via simple solubilization) through MLL and underline again a potential complex interplay between Fe³⁺ and Ln³⁺ in microbial Ln uptake.