Boron in Prebiological Evolution.

Boron(III), as borate (or boric acid), mediates the synthesis of ribose, ribonucleosides, and ribonucleotides. These reactions are carried out under moderate temperatures (typically 70-95 °C) with organic molecules (or their derivatives) detected in interstellar space and inorganic ions found in minerals on Earth (and could occur during early stages of prebiotic evolution). Research in this century suggests that borate was a relevant prebiological reagent, thus reinforcing the RNA world hypothesis as an explanation for the origin of life. Herein, these developments on prebiological chemistry related to boron species are reviewed.

Nitrite Reduction in Aqueous Solution Mediated by Amavadin Homologues: N2 O Formation and Water Oxidation.

Reactions of two vanadium(IV) complex anions that are homologues of amavadin, [V(HIDPA)2 ]2- and [V(HIDA)2 ]2- (HIDPA=N-oxyiminodipropionate, HIDA=N-oxyiminodiacetate), with the nitrite ion (NO2- ) in aqueous solution were investigated by experimental (absorption spectroscopy in the visible range, through measurements of dioxygen formed in solution from water oxidation and identification of nitrogen oxide species of a gaseous atmosphere from nitrite reduction by using an IR analyser) and theoretical methods. Two reactions, mediated by the vanadium complexes, with environmental and biological significance, were observed in this system, namely, reduction of nitrite to N2 O and oxidation of water to molecular oxygen. The reduction of nitrite, as studied by DFT calculations, occurs through the formation of NO (ΔG≠ =14.3 kcal mol-1 ), which is strongly dependent on pH and slightly endergonic, and is then easily converted into N2 O, with an overall activation barrier of ΔG≠ =11.8 kcal mol-1 . The later process includes dimerisation of NO assisted by one molecule of the V complex, protonation and oxidation of the formed ONNO.- ligand by another amavadin molecule or by nitrite, and NO bond cleavage/proton transfer in the ONNOH- ligand. The results indicate that amavadin exhibits an unusual nitrite reductase type activity that could be involved in nitrogen metabolism of Amanita muscaria and other fungi containing this vanadium complex.

Amavadin and Homologues as Mediators of Water Oxidation.

The vanadium(IV) N-hydroxyiminodicarboxylate complexes [V(HIDPA)2](2-) and [V(HIDA)2](2-), close models of the amavadin (a natural product from Amanita fungi lacking the V=O group but exhibiting a rare NO-bound oxyiminate moiety), are shown to be the first recognized complexes of the early transition metals (up to periodic Group 7) that mediate the oxidation of water. The reactions were analyzed by visible spectrophotometry, mass spectrometry, and measurement of evolved dioxygen using Ce(4+) as sacrificial oxidant. A mechanism proposed on the basis of DFT calculations involves the reversible oxidation to the mononuclear V(V)-{ON<} center, where the redox active oxyimino group plays a key role and metal oxidation state variation is only one unit. The more similar model of the metallobiomolecule, [V(HIDPA)2](2-), displays a lower oxidation rate than [V(HIDA)2](2-) but does not undergo appreciable degradation, in contrast to the latter.