Biological Pincer Complexes

Abstract

At least two types of pincer complexes are known to exist in biology. A metal‐pyrroloquinolone quinone (PQQ) cofactor was first identified in bacterial methanol dehydrogenase, and later also found in selected short‐chain alcohol dehydrogenases of other microorganisms. The PQQ‐associated metal can be calcium, magnesium, or a rare earth element depending on the enzyme sequence. Synthesis of this organic ligand requires a series of accessory proteins acting on a small peptide, PqqA. Binding of metal to PQQ yields an ONO‐type pincer complex. More recently, a nickel‐pincer nucleotide (NPN) cofactor was discovered in lactate racemase, LarA. This cofactor derives from nicotinic acid adenine dinucleotide via action of a carboxylase/hydrolase, sulfur transferase, and nickel insertase, resulting in an SCS‐type pincer complex. The NPN cofactor likely occurs in selected other racemases and epimerases of bacteria, archaea, and a few eukaryotes.

Cerium–quinone redox couples put under scrutiny

“…Quinones are multifunctional organic molecules exhibiting intriguing redox behaviour. 1,2 Of particular note is their importance in biological electron–transfer processes (photosynthesis, respiration) 3 and in industrial catalysis (anthraquinone process for hydrogen peroxide production). 4 Quinones can engage in one or two electron redox processes involving the formation of either semiquinolates or hydroquinolates.…”

The Earlier the Better: Structural Analysis and Separation of Lanthanides with Pyrroloquinoline Quinone

“…However, factors such as changing coordination numbers across the Ln‐series, lack of efficient activation and negative impact on redox cycling of PQQ by certain Ln in the active site have been proposed . PQQ is one of the few pincer ligands existing in nature and coordinates via a carboxylic acid moiety, a pyridyl nitrogen and a quinone oxygen atom . Similar binding motifs have been used in ligands employed in the separation of lanthanides and actinides.…”

Recent Advances in Chemistry of Unsymmetrical Phosphorus-Based Pincer Nickel Complexes: From Design to Catalytic Applications

“…The mechanisms of this transformation have been studied using DFT calculations. The most probable pathway for proton-coupled hydride transfer involves the intermediate formation of an unsymmetrical complex [ 146 ]. Inspired by the structures of the active site of lactate racemase, similar scorpion-like SCS nickel complexes were proposed, and their potentials for catalytic hydrogenation of CO 2 [ 147 ] and for the asymmetric transfer hydrogenation of 1-acetonaphthone (the phosphorus-based complexes 52a – 52d in Figure 9 ) [ 148 ] were computationally predicted.…”