Full Paper Chemistry—A European Journal doi.org/10.1002/chem.202002653 & Coordination Chemistry The Earlier the Better: Structural Analysis and Separation of Lanthanides with Pyrroloquinoline Quinone Henning Lumpe+,[a] Annika Menke+,[a] Christoph Haisch,[b] Peter Mayer,[a] Anke Kabelitz,[c] Kirill V. Yusenko,[c] Ana Guilherme Buzanich,[c] Theresa Block,[d] Rainer Pçttgen,[d] Franziska Emmerling,[c] and Lena J. Daumann*[a] Abstract: Lanthanides (Ln) are critical raw materials, howev- ronment. The complex crystallizes as an inversion symmetric er, their mining and purification have a considerable nega- dimer, Eu2PQQ2, with binding of Eu in the biologically rele- tive environmental impact and sustainable recycling and vant pocket of PQQ. LnPQQ and Ln1Ln2PQQ complexes separation strategies for these elements are needed. In this were characterized by using inductively coupled plasma study, the precipitation and solubility behavior of Ln com- mass spectrometry (ICP-MS), infrared (IR) spectroscopy, 151Eu- plexes with pyrroloquinoline quinone (PQQ), the cofactor of Mçssbauer spectroscopy, X-ray total scattering, and extend- recently discovered lanthanide (Ln) dependent methanol de- ed X-ray absorption fine structure (EXAFS). It is shown that a hydrogenase (MDH) enzymes, is presented. In this context, natural enzymatic cofactor is capable to achieve separation the molecular structure of a biorelevant europium PQQ com- by precipitation of the notoriously similar, and thus difficult plex was for the first time elucidated outside a protein envi- to separate, lanthanides to some extent. Introduction also called “vitamins, or seeds of technology” and the global demand of rare earth oxides is growing steadily.[1] Unlike their Rare earth elements (REE) include the elements 21Sc, 39Y and name suggests, REE are not particularly rare and the occur- 57La, in addition to the 14 lanthanides (Ln) from 58Ce to 71Lu. rence of the two least abundant ones, Tm and Lu, is even Due to their extensive usage in modern technologies, they are higher than the one of silver.[2] Mining of those elements is, however, a challenge, due to their dispersion and low concen- trations in REE containing ores. In addition, extraction methods [a] Dr. H. Lumpe,+ A. Menke,+ Dr. P. Mayer, Prof. Dr. L. J. Daumann Department of Chemistry include strong acids or bases and produce large scales of radi- Ludwig-Maximilians-University Munich oactive and heavy metal contaminated waste.[3] Separations of Butenandtstraße 5–13, 81377 Mnchen (Germany) the chemically similar REE are energy-intensive and challeng- E-mail: [email protected] ing.[4] However, several exciting new directions for REE separa- [b] Prof. Dr. C. Haisch Chair of Analytical Chemistry and Water Chemistry tion have been presented recently. The group of Schelter used 3À Technical University of Munich the size-sensitive ligand TriNOx (Figure 1B), which is able to Marchioninistraße 17, 81377 Mnchen (Germany) form a self-associative equilibrium out of REE mixtures and can [c] Dr. A. Kabelitz, Dr. K. V. Yusenko, Dr. A. Guilherme Buzanich, be used for REE separation by leaching.[5] Sun, Bnzli and co- Dr. F. Emmerling workers used a supramolecular approach with a tris-tridentate Division Structure Analysis Federal Institute for Materials Research and Testing (BAM) ligand, which forms 4-nuclear cages preferentially with the [6] Richard-Willsttter-Straße 11, 12489 Berlin (Germany) smaller, late REE (Figure 1A). [d] T. Block, Prof. Dr. R. Pçttgen With a modification of the ligand, using long alkyl chains, Institut fr Anorganische und Analytische Chemie the group was able to perform a liquid-liquid extraction of late Universitt Mnster (WWU) REE, while the early ones remained in the aqueous phase.[8] Re- Corrensstraße 30, 48149 Mnster (Germany) cently, also magnetic field driven REE separations have been [+] These authors contributed equally to this work. reported.[9] Among the REE, the early lanthanides (La–Eu) are Supporting information and the ORCID identification number(s) for the au- thor(s) of this article can be found under: now recognized as biorelevant for methylotrophic bacteria ha- https://doi.org/10.1002/chem.202002653. bituating a number of different ecosystems (plant phyllo- 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. spheres, volcanic mudpots, soil and aquatic environments).[10] This is an open access article under the terms of Creative Commons Attri- Those bacteria use Ln containing enzymes (methanol dehydro- bution NonCommercial License, which permits use, distribution and repro- genases, MDH, active site shown in Figure 2A) in their C1 me- duction in any medium, provided the original work is properly cited and is not used for commercial purposes. tabolism. The active site of the Ln-containing enzymes includes Part of a Special Collection to commemorate young and emerging scien- redox cofactor PQQ (Figure 2B) that coordinates the central tists. To view the complete collection, visit: Young Chemists 2020. metal in a tridentate fashion.[11] Chem. Eur. J. 2020, 26,1–8 1 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim && These are not the final page numbers! ÞÞ Full Paper Chemistry—A European Journal doi.org/10.1002/chem.202002653 be exploited for Ln-separation. In addition, we report the first crystal structure of PQQ with a biorelevant metal ion (Eu) out- side of a MDH protein environment and without synthetic co- coordinating ligands or modified PQQ precursors. Results and Discussion Ln1PQQ complexes: characterization In MDH, PQQ acts as a tridentate ligand for lanthanides, using 7’-O; 6-N and 5-O (see numbering in Figure 2B). Without the protein environment, all carboxyl groups, as well as 4-O can participate in metal coordination, often complicating analy- sis.[17] From aqueous solutions of the sodium salt of the cofac- tor (Na2PQQ, 2, isolated from vitamin capsules) complexes with Figure 1. A) Tris-tridentate ligand and supramolecular Ln complex reported lanthanides rapidly precipitate after mixing, showing 1:1 stoi- [6] [17a] by Sun, Bnzli et al. and B) H3TriNOx ligand and Ln TriNOx THF complex chiometry, even with lanthanides added in excess (6 equiv). from Schelter et al.[7] Single crystals from the first Eu2PQQ2 complex outside the MDH protein environment were obtained for X-ray structural analysis (Figure 3). To this end, very few crystal structures of PQQ complexes have been reported[18] and none of them con- tained metals relevant for MDH activity (Ca, Ln). A structure of a 1:1 complex with a PQQ derivative (here the trimethylester [18a] PQQMe3 and copper(I) were used) was reported by Kaim. 2+ Kojima reported a structure of PQQMe3 with Ru bearing a terpyridine coligand. A structure of PQQ (1) with Cu2+ and the same coligand was reported by Suzuki.[18b,c] Crystals of a Eu2PQQ2 complex were derived after several days from a mix- ture of aqueous Na2PQQ and EuCl3 solutions at 808C, which Figure 2. A) Active site of a Ln-dependent MDH (PDB 6FKW). B) Structure of was allowed to slowly cool down to room temperature (see PQQ and related species. Water adduct 3 forms readily in aqueous solution. Numbering scheme according to Unkefer et al.[12] Supporting Information). The Eu-structure is consistent with a 1:1 stoichiometry but surprisingly reveals a dimer with head to tail coordination of PQQ. While no other co-coordinating li- Remarkably, early lanthanides are taken up more quickly and gands other than water were necessary to crystallize the com- preferentially by bacteria than the later ones. It was shown plex, a carboxylic acid of a second PQQ molecule is needed to that Methylorubrum extorquens AM1 can even grow with Nd- complete the coordination sphere. Modified PQQ derivatives, containing hard-drive magnets as the only source of Ln, where the carboxylic acid moiety at the pyrrole ring is either making those bacteria interesting for bioleaching or biomin- blocked by alkylation to yield an ester or replaced entirely by ing.[13] With early Ln, bacteria grow faster and their respective MDH enzymes are more efficient in turning over methanol. Seemingly, natural systems have been tuned specifically by evolution to work best with the earlier, larger, and more abun- Lena J. Daumann obtained her Diploma in dant lanthanides. The reasons for the preference of natural sys- Chemistry in 2010 from the University of Hei- delberg working with Peter Comba. After an tems for early lanthanides remain somewhat elusive. However, internship at BASF she completed her PhD in factors such as changing coordination numbers across the Ln- 2013 with Lawrie Gahan at the University of series, lack of efficient activation and negative impact on redox Queensland in Australia working on pesticide- cycling of PQQ by certain Ln in the active site have been pro- degrading enzymes and biomimetic com- plexes. Her postdoctoral work as a Feodor- posed.[14] PQQ is one of the few pincer ligands existing in Lynen fellow with Ken Raymond at UC Berke- nature and coordinates via a carboxylic acid moiety, a pyridyl ley involved luminescent lanthanide com- nitrogen and a quinone oxygen atom.[15] Similar binding motifs plexes with siderophore-inspired ligands. In have been used in ligands employed in the separation of lan- 2016, she took up a position as W2 Professor for bioinorganic and coordination chemistry thanides and actinides. 2,6-pyridine dicarboxylic acid (PDCA) at the Ludwig-Maximilians-Universitt and derivatives have been widely used for solvent extraction Munich, where she is exploring the bioinorganic chemistry of lanthanides and and ion chromatography.[16] The tris-tridentate ligand shown in the role of high valent iron species in epigenetic processes. She has won nu- Figure 1A also features an ONO binding pocket. Here, we merous Awards for her research and teaching. Among them the Prinzessin Therese von Bayern-Preis and the Ars legendi-Fakulttenpreis in Chemistry, show that the MDH-cofactor PQQ by itself preferentially forms both in 2019.