Borophosphates and silicophosphates as plausible contributors to the emergence of life

Boron as a Hypothetical Participant in the Prebiological Enantiomeric Enrichment

Boron, as borate (or boric acid), is known as a mediator of the synthesis of ribose, ribonucleosides, and ribonucleotides (precursors of RNA) under plausible prebiotic conditions. With regard to these phenomena, the potential participation of this chemical element (as a constituent of minerals or hydrogels) for the emergence of prebiological homochirality is considered. This hypothesis is based on characteristics of crystalline surfaces as well as solubility of some minerals of boron in water or specific features of hydrogels with ester bonds from reaction of ribonucleosides and borate.

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.

Synthesis of ribonucleotides from the corresponding ribonucleosides under plausible prebiotic conditions within self-assembled supramolecular structures

Abiotic synthesis of ribonucleotides, mainly at the 5′ position, from the corresponding ribonucleosides within guanosine:borate hydrogels in the temperature range of 70–90 °C, using urea and a phosphate source (K₂HPO₄ or hydroxyapatite).

Hydroxymethanesulfonate from Volcanic Sulfur Dioxide: A "Mineral" Reservoir for Formaldehyde and Other Simple Carbohydrates in Prebiotic Chemistry

While formaldehyde (HCHO) was likely generated in Earth's prebiotic atmosphere by ultraviolet light, electrical discharge, and/or volcano-created lightning, HCHO could not have accumulated in substantial amounts in prebiotic environments, including those needed for prebiotic processes that generate nucleosidic carbohydrates. HCHO at high concentrations in alkaline solutions self-reacts in the Cannizzaro reaction to give methanol and formate, neither having prebiotic value. Here, we explore the possibility that volcanic sulfur dioxide (SO2) might have generated a reservoir for Hadean HCHO by a reversible reaction with HCHO to give hydroxymethanesulfonate (HMS). We show that salts of HMS are stable as solids at 90°C and do not react with themselves in solution, even at high (>8 M) concentrations. This makes them effective stores of HCHO, since the reverse reaction slowly delivers HCHO back into an environment where it can participate in prebiotically useful reactions. Specifically, we show that in alkaline borate solutions, HCHO derived from HMS allows formation of borate-stabilized carbohydrates as effectively as free HCHO, without losing material to Cannizzaro products. Further, we show that SO2 can perform similar roles for glycolaldehyde and glyceraldehyde, two intrinsically unstable carbohydrates that are needed by various models as precursors for RNA building blocks. Zircons from the Hadean show that the Hadean mantle likely provided volcanic SO2 at rates at least as great as the rates of atmospheric HCHO generation, making the formation of Hadean HMS essentially unavoidable. Thus, hydroxymethylsulfonate adducts of formaldehyde, glycolaldehyde, and glyceraldehyde, including the less soluble barium, strontium, and calcium salts, are likely candidates for prebiotically useful organic minerals on early Earth.

More than boric acid: Increasing relevance of boron in medicine

Although boron has been a chemical element of interest since the ancient times, only a few boron-containing compounds (BCCs) had been used for medicinal purposes before the 21^st century. Among these, only boric acid has been explored in multiple therapeutic applications. Hence, it is common to extrapolate from boric acid to all BCCs, supposing a similar biological effect. However, boric acid is just one of dozens of BCCs in nature and thousands available from chemical synthesis. Nowadays, there is a boom in research on new BCCs as potential tools in the prevention, diagnosis and therapy of human disease. We herein discuss the new role of BCCs in drug development, with emphasis on the compounds for which a mechanism of action has been proposed or demonstrated. Because of data gathered in recent years, BCCs have expanded beyond the well-known fields of antimicrobial and antineoplastic agents, now being explored for their possible use as enzyme inhibitors, regulators of protein expression and modulators of the immune response, as well as in biomaterials. We suggest that translational medicine can accelerate the medicinal applications of BCCs, which is especially important for the human diseases that are generating a high global burden.

BORON PREPARATIONS IN PSYCHIATRY AND NEUROLOGY: THEIR RISE, FALL AND RENEWED INTEREST

The use of boron preparations (borax and boric acid) in medicine began long before their isolation in pure form. The mineral water of  boron-containing sources has been historically used to treat skin  diseases, to wash eyes, to disinfect wounds, etc. Also, what is of  interest in the context of this article, boron-containing waters were  used as calming, anti- anxiety, anticonvulsant and sleep-promoting  remedy. In 1777, boric acid was first isolated from the mineral water of a healing spring source in Florence. Historically, first name  of this compound was sal sedativum (“soothing salt”). However, the  discovery of boron toxicity led to the cessation of its internal use. In  recent decades, it has been found that boron is a microelement  necessary for many metabolic processes in the body. It affects  memory, cognitive functions, anxiety level, sleep, mood, regulates  calcium and magnesium exchange, metabolism of vitamin D and sex  steroids. It has been shown that some cases of treatment  resistance to standard therapy, for example in epilepsy, anxiety and depression, are related to boron deficiency. In this regard,  interest in the use of boron preparations in psychiatry and neurology, but in much smaller doses and on new scientific grounds, flared up again.

Crystal structure and spin-trimer magnetism of Rb2.3(H2O)0.8Mn3[B4P6O24(O,OH)2]

The novel borophosphate Rb_2.3(H₂O)_0.8Mn₃[B₄P₆O₂₄(O,OH)₂] was prepared under hydrothermal conditions at 553 K. Its crystal structure was determined using single-crystal X-ray diffraction data obtained from a non-merohedral twin and refined against F² to R = 0.057. The compound crystallizes in the orthorhombic space group Pbcn, with unit-cell parameters a = 20.076(2) Å, b = 9.151(1) Å, c = 12.257(1) Å, V = 2251.8(2) ų, and Z = 4. The title compound is the first example of a borophosphate with manganese ions adopting both octahedral and tetrahedral coordinations. Its unique crystal structure is formed by borophosphate slabs and chains of Mn²⁺-centered polyhedra sharing edges and vertices. These 2D and 1D fragments interconnect into a framework with open channels that accommodate Rb⁺ cations and water molecules. Topological relationships between borophosphates built from three-membered rings of two borate and one phosphate tetrahedra sharing oxygen vertices, amended by additional PO₄ and HPO₄ tetrahedra, are discussed. The temperature dependence of the magnetic susceptibility of Rb_2.3(H₂O)_0.8Mn₃[B₄P₆O₂₄(O,OH)₂] reveals predominant antiferromagnetic exchange interactions and the high-temperature effective magnetic moment corresponding to the high-spin S = 5/2 state of Mn²⁺ ions. At 12.5 K, a magnetic transition is evidenced by ac-susceptibility and specific heat measurements. A spin-trimer model with the leading exchange interaction J ∼ 3.2 K is derived from density-functional band-structure calculations and accounts for all experimental observations.

Evaporite Borate-Containing Mineral Ensembles Make Phosphate Available and Regiospecifically Phosphorylate Ribonucleosides: Borate as a Multifaceted Problem Solver in Prebiotic Chemistry

RNA is currently thought to have been the first biopolymer to support Darwinian natural selection on Earth. However, the phosphate esters in RNA and its precursors, and the many sites at which phosphorylation might occur in ribonucleosides under conditions that make it possible, challenge prebiotic chemists. Moreover, free inorganic phosphate may have been scarce on early Earth owing to its sequestration by calcium in the unreactive mineral hydroxyapatite. Herein, it is shown that these problems can be mitigated by a particular geological environment that contains borate, magnesium, sulfate, calcium, and phosphate in evaporite deposits. Actual geological environments, reproduced here, show that Mg²⁺ and borate sequester phosphate from calcium to form the mineral lüneburgite. Ribonucleosides stabilized by borate mobilize borate and phosphate from lüneburgite, and are then regiospecifically phosphorylated by the mineral. Thus, in addition to guiding carbohydrate pre-metabolism, borate minerals in evaporite geoorganic contexts offer a solution to the phosphate problem in the "RNA first" model for the origins of life.

Current data regarding the structure-toxicity relationship of boron-containing compounds

Boron is ubiquitous in nature, being an essential element of diverse cells. As a result, humans have had contact with boron containing compounds (BCCs) for a long time. During the 20th century, BCCs were developed as antiseptics, antibiotics, cosmetics and insecticides. Boric acid was freely used in the nosocomial environment as an antiseptic and sedative salt, leading to the death of patients and an important discovery about its critical toxicology for humans. Since then the many toxicological studies done in relation to BCCs have helped to establish the proper limits of their use. During the last 15 years, there has been a boom of research on the design and use of new, potent and efficient boron containing drugs, finding that the addition of boron to some known drugs increases their affinity and selectivity. This mini-review summarizes two aspects of BCCs: toxicological data found with experimental models, and the scarce but increasing data about the structure-activity relationship for toxicity and therapeutic use. As is the case with boron-free compounds, the biological activity of BCCs is related to their chemical structure. We discuss the use of new technology to discover potent and efficient BCCs for medicinal therapy by avoiding toxic effects.

Serpentinization and the Formation of H2 and CH4 on Celestial Bodies (Planets, Moons, Comets)

Serpentinization involves the hydrolysis and transformation of primary ferromagnesian minerals such as olivine ((Mg,Fe)2SiO4) and pyroxenes ((Mg,Fe)SiO3) to produce H2-rich fluids and a variety of secondary minerals over a wide range of environmental conditions. The continual and elevated production of H2 is capable of reducing carbon, thus initiating an inorganic pathway to produce organic compounds. The production of H2 and H2-dependent CH4 in serpentinization systems has received significant interdisciplinary interest, especially with regard to the abiotic synthesis of organic compounds and the origins and maintenance of life in Earth's lithosphere and elsewhere in the Universe. Here, serpentinization with an emphasis on the formation of H2 and CH4 are reviewed within the context of the mineralogy, temperature/pressure, and fluid/gas chemistry present in planetary environments. Whether deep in Earth's interior or in Kuiper Belt Objects in space, serpentinization is a feasible process to invoke as a means of producing astrobiologically indispensable H2 capable of reducing carbon to organic compounds.

Boric acid: a simple molecule of physiologic, therapeutic and prebiotic significance

Boric acid, H3BO3, is a weak acid and at physiological pH is in the form of an uncharged small molecule. Behaving as a Lewis acid, it forms complexes with amino- and hydroxy acids, carbohydrates, nucleotides and vitamins through electron donor-acceptor interactions. These interactions are believed to be beneficial for human health. Synthetic bis-chelate complexes of boric acid with organic biomolecules are therefore considered for nutritional and/or pharmaceutical applications. The use of boric acid for BNCT has gained attention due to the short biological half-life, solubility, plasma circulation and the non-selective soft tissue accumulation properties of this simple molecule. Complexation of boric acid with sugars is of particular importance in understanding the role of boron as a carrier for nucleotides and carbohydrates. A potential and catalytic role of boric acid in peptide and nucleic acid synthesis and in the stabilization of sugar molecules by acting as a complexing agent have been demonstrated. Its possible role as a phosphorylation chaperone in a prebiotic world has been recently suggested. This contribution reviews the highlights in the physiologic, therapeutic and prebiotic significance of boric acid in the last decade.

From the RNA world to the RNA/protein world: contribution of some riboswitch-binding species?

Some amino acids and their formal derivatives, currently riboswitch-binding species, could have interacted with polyribonucletides in prebiotic environments, leading to the peptide formation. If the resulting compounds had led to a sustainable polymerization of amino acids and the new structures had catalytic activity, such would have been an important contribution to the transition from the RNA world to the RNA/Protein world.