PQQ-Aza-Crown Ether Complexes as Biomimetics for Lanthanide and Calcium Dependent Alcohol Dehydrogenases

Understanding the role of metal ions in biology can lead to the development of new catalysts for

several industrially important transformations. Lanthanides are the most recent group of metal ions

that have been shown to be important in biology i.e. - in quinone-dependent methanol

dehydrogenases (MDH). Here we evaluate a pyrroloquinoline quinone and 1-aza-15-crown-5 based

ligand platform as scaffold for Ca2+ , Ba2+ , La3+ and Lu3+ biomimetics of MDH and we evaluate the

importance of ligand design, charge, size, counterions and base for the alcohol oxidation reaction

using NMR spectroscopy. In addition, we report a new straightforward synthetic route (3 steps

instead of 11 and 33% instead of 0.6% yield) for biomimetic ligands based on PQQ. We show that

when studying biomimetics for MDH, larger metal ions and those with lower charge in this case

promote the dehydrogenation reaction more effectively and that this is likely an effect of the ligand

design which must be considered when studying biomimetics. To gain more information on the

structures and impact of counterions of the complexes, we performed collision induced dissociation

(CID) experiments and observe that the nitrates are more tightly bound than the triflates. To resolve

the structure of the complexes in the gas phase we combined DFT-calculations and ion mobility

measurements (IMS). Furthermore, we characterized the obtained complexes and reaction mixtures

using Electron Paramagnetic Resonance (EPR) spectroscopy and show the emergence of a quinone-

based radical during the reaction with substrate and base.

Abstract