Inorganics, Volume 8, Issue 8 (August 2020) – 2 articles

In this research, two 4-styrylpyridines carrying an acceptor –NO₂ (L1) or a donor –NMe₂ group (L2) were axially coordinated to A₄ Zn^II porphyrins displaying in 5,10,15,20 meso position aryl moieties with remarkable electron withdrawing properties (pentafluorophenyl (TFP)), and with moderate to strong electron donor properties (phenyl (TPP) < 3,5-di-tert-butylphenyl (TBP) < bis(4-tert-butylphenyl)aniline) (TNP)). The second order nonlinear optical (NLO) properties of the resulting complexes were measured in CHCl₃ solution by the Electric-Field-Induced Second Harmonic generation technique, and the quadratic hyperpolarizabilities β_λ were compared to the Density Functional Theory (DFT)-calculated scalar quantities β_||. Our combined experimental and theoretical approach shows that different interactions are involved in the NLO response of L1- and L2-substituted A₄ Zn^II porphyrins, suggesting a role of backdonation-type mechanisms in the determination of the negative sign of Electric-Field-Induced Second Harmonic generation (EFISH) β_λ, and a not negligible third order contribution for L1-carrying complexes. Full article

Tungsten is the heaviest element used in biological systems. It occurs in the active sites of several bacterial or archaeal enzymes and is ligated to an organic cofactor (metallopterin or metal binding pterin; MPT) which is referred to as tungsten cofactor (Wco). Wco-containing enzymes are found in the dimethyl sulfoxide reductase (DMSOR) and the aldehyde:ferredoxin oxidoreductase (AOR) families of MPT-containing enzymes. Some depend on Wco, such as aldehyde oxidoreductases (AORs), class II benzoyl-CoA reductases (BCRs) and acetylene hydratases (AHs), whereas others may incorporate either Wco or molybdenum cofactor (Moco), such as formate dehydrogenases, formylmethanofuran dehydrogenases or nitrate reductases. The obligately tungsten-dependent enzymes catalyze rather unusual reactions such as ones with extremely low-potential electron transfers (AOR, BCR) or an unusual hydration reaction (AH). In recent years, insights into the structure and function of many tungstoenzymes have been obtained. Though specific and unspecific ABC transporter uptake systems have been described for tungstate and molybdate, only little is known about further discriminative steps in Moco and Wco biosynthesis. In bacteria producing Moco- and Wco-containing enzymes simultaneously, paralogous isoforms of the metal insertase MoeA may be specifically involved in the molybdenum- and tungsten-insertion into MPT, and in targeting Moco or Wco to their respective apo-enzymes. Wco-containing enzymes are of emerging biotechnological interest for a number of applications such as the biocatalytic reduction of CO₂, carboxylic acids and aromatic compounds, or the conversion of acetylene to acetaldehyde. Full article