Ion Mobility Studies of Pyrroloquinoline Quinone Aza-Crown Ether–Lanthanide Complexes

Abstract

Lanthanide-dependent enzymes and their biomimetic complexes have arisen as an interesting target of research in the past decade. These enzymes, specifically, pyrroloquinoline quinone (PQQ)-bearing methanol dehydrogenases, efficiently convert alcohols to the respective aldehydes. To rationally design bioinspired alcohol dehydrogenation catalysts, it is imperative to understand the species involved in catalysis. However, given the extremely flexible coordination sphere of lanthanides, it is often difficult to assess the number and nature of the active species. Here, we show how such questions can be addressed by using a combination of ion mobility spectrometry, mass spectrometry, and quantum-chemical calculations to study the test systems PQQ and lanthanide−PQQ−crown ether ligand complexes. Specifically, we determine the gas-phase structures of [PQQH 2 ] − , [PQQH 2 +H 2 O] − , [PQQH 2 +MeOH] − , [PQQ−15c5+H] + , and [PQQ−15c5+Ln+NO 3 ] 2+ (Ln = La to Lu, except Pm). In the latter case, a trend to smaller collision cross sections across the lanthanide series is clearly observable, in line with the well-known lanthanide contraction. We hope that in the future such investigations will help to guide the design and understanding of lanthanidebased biomimetic complexes optimized for catalytic function.

Ion Mobility and Fourier Transform Ion Cyclotron Resonance Collision Cross Section Techniques Yield Long-Range and Hard-Sphere Results, Respectively

“…The multi-CRAFTI measurements are sufficiently sensitive to observe subtle structural effects such as these, whereas our IM-MS measurements show very little variation as the metal ions vary, perhaps because these small effects are masked by long-range interactions. Interestingly, we note recent trapped ion mobility-time-of-flight (TIMS-TOF) measurements that did successfully measure differences due to the different sizes of lanthanide ions, so at least in some cases mobility measurements at sufficiently high mobility resolution are able to detect these types of subtle structural effects.…”

A perspective on the role of lanthanides in biology: Discovery, open questions and possible applications

“…Due to the flexible coordination sphere of lanthanides, the nature of the active species in biomimetics is often unknown. Recently, a combination of ion mobility spectrometry, mass spectrometry and quantum chemical calculations was used to gain a deeper insight into PQQ and lanthanide complexes formed with L2 in the gas phase (Sch€ afer et al, 2022).…”