Ternary borate-nucleoside complex stabilization by ribonuclease A demonstrates phosphate mimicry

Synthesis of Boron-Containing Nucleoside Analogs

Over the last century, nucleoside-based therapeutics have demonstrated remarkable effectiveness in the treatment of a wide variety of diseases from cancer to HIV. In addition, boron-containing drugs have recently emerged as an exciting and fruitful avenue for medicinal therapies. However, borononucleosides have largely been unexplored in the context of medicinal applications. Herein, we report the synthesis, isolation, and characterization of two novel boron-containing nucleoside compound libraries which may find utility as therapeutic agents. Our synthetic strategy employs efficient one-step substitution reactions between a diverse variety of nucleoside scaffolds and an assortment of n-alkyl potassium trifluoroborate-containing electrophiles. We demonstrated that these alkylation reactions are compatible with cyclic and acyclic nucleoside substrates, as well as increasing alkyl chain lengths. Furthermore, regioselective control of product formation can be readily achieved through manipulation of base identity and reaction temperature conditions.

Elemental exchange: Bioisosteric replacement of phosphorus by boron in drug design

Continuous efforts are being directed toward the employment of boron in drug design due to its advantages and unique characteristics including a plethora of target engagement modes, lower metabolism, and synthetic accessibility, among others. Phosphates are components of multiple drug molecules as well as clinical candidates, since they play a vital role in various biochemical functions, being components of nucleotides, energy currency- ATP as well as several enzyme cofactors. This review discusses the unique chemistry of boron functionalities as phosphate bioisosteres - "the boron-phosphorus elemental exchange strategy" as well as the superiority of boron groups over other commonly employed phosphate bioisosteres. Boron phosphate-mimetics have been utilized for the development of enzyme inhibitors as well as novel borononucleotides. Both the boron functionalities described in this review-boronic acids and benzoxaboroles-contain a boron connected to two oxygens and one carbon atom. The boron atom of these functional groups coordinates with a water molecule in the enzyme site forming a tetrahedral molecule which mimics the phosphate structure. Although boron phosphate-mimetic molecules - FDA-approved Crisaborole and phase II/III clinical candidate Acoziborole are products of the boron-phosphorus bioisosteric elemental exchange strategy, this technique is still in its infancy. The review aims to promote the use of this strategy in future medicinal chemistry projects.

The role of monodentate tetrahedral borate complexes in boric acid binding to a soil organic matter analogue

Boron is an essential plant micronutrient responsible for several important functions. Boron availability in soils may be influenced by binding with soil organic matter (SOM), particularly with aromatic diol and polyphenol groups on SOM. The mechanism by which aromatic diols bind boron, however, remains unclear. The objective of this work is to further investigate interaction between boric acid and varying concentrations of an aromatic, polyphenolic SOM analogue (tannic acid at 5, 10 and 20 g L^-1) from pH = 5-9. UV/Visible spectroscopy showed boric acid enhanced tannic acid deprotonation at pH = 7.0 and 9.0, resulting in singly deprotonated tannic acid subunits. Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) showed boric acid/tannic acid binding for all concentrations at pH = 7 and 9, whereas binding at pH = 5.0 was observed only at 20 g L^-1 tannic acid. Uncomplexed boron species were not evident at pH = 9.0, but were detectable at pH = 7.0 at lower tannic acid concentrations and prevalent at pH = 5.0, qualitatively indicating binding affinity increases from pH = 5.0 to 9.0. ATR-FTIR results indicated tetrahedral coordination of boron upon complexation to tannic acid with a monodentate mechanism. These results collectively highlight a transition of solution planar boric acid to a tetrahedral, monodentate coordination with a single phenol group in tannic acid polyphenols. This contrasts with previous spectroscopic studies, which indicated bidentate tetrahedral or monodentate trigonal planar orientations prevail at aromatic diol sites. This work presents a previously unobserved boric acid coordination mechanism to an SOM analogue and, therefore, may better inform prediction and modeling of boron behavior in soils.

Comparative Study on Neutron Irradiation Sensitization Effects of Nucleotide Borate Esters and Several Other Boron Agents

More effective boron-containing compounds are needed for use in boron neutron capture therapy (BNCT). Here, borate esters were synthesized by heating and dehydrating nucleotides adenosine triphosphate (ATP), adenosine diphosphate (ADP), adenosine monophosphate (AMP), the nucleoside (inosine) or glycerol in the presence of boric acid (H₃BO₃). Borate ester products were compared to clinical boron agent boronophenylalanine (BPA) and several other borate esters for neutron-sensitization effects using the A549 cell line. Cells were incubated with boron agent solutions (2.3 mM) for 5 h, then washed, resuspended in fresh media, and irradiated with a neutron dose of 0.33 Sv followed by cell survival assessment using the CCK-8 method. Calculated radiosensitization values (control group cell survival rate/boron agent-treated experimental group cell survival rate) were 3.9 ± 0.2 (ATP borate ester), 2.4 ± 0.1 (BPA), 2.1 ± 0.1 (ADP borate ester), 1.9 ± 0.2 (AMP borate ester), 1.7 ± 0.3 (glycerin borate ester), 1.4 ± 0.1 (inosine borate ester), 1.3 ± 0.3 (triethanolamine borate ester) and 1.3 ± 0.5 (H₃BO₃). Borate esters derived from nucleotides ATP, ADP or AMP exhibited significantly higher sensitization values than did those derived from glycerol, inosine or triethanolamine. Notably, due to its relatively higher water solubility and degree of tumor cell enrichment, ATP borate ester exhibited the highest sensitization rate overall, significantly exceeding rates obtained for BPA and borate esters of ADP and AMP. Flow cytometric determinations of boron agent-treated cell survival at 24 h postirradiation revealed long-term apoptosis rates of 4.8-6.6 ± 0.2% (nucleotide borate ester groups) and 5.6 ± 0.3% (BPA group) compared to 3.9 ± 0.1% (irradiation control group without boron agent) and 2.6 ± 0.2% (blank control group). Significant differences between experimental and control groups demonstrated that nucleotide borate esters and BPA induced long-term radiosensitization effects. In particular, postirradiation percentages of ATP borate ester-treated cells progressing to DNA replication prophase (G₁ phase) increased significantly, while percentages of cells progressing to S phase significantly decreased, demonstrating cellular DNA replication inhibition. Meanwhile, boron content values of tumor tissue, measured using inductively coupled plasma mass spectrometry (ICP-MS) and expressed as tumor-to-normal tissue boron ratios (T/N), were not significantly different between nucleotide borate ester- and BPA-fed groups of tumor-bearing mice. However, tumor tissue boron concentrations of nucleotide borate ester-fed mice (0.81-0.88 ± 0.04 µg/g) significantly exceeded those of BPA-fed mice (0.52 ± 0.05 µg/g) and thus provided greater tumor tissue boron enrichment for achieving a stronger neutron radiation-sensitizing effect. In conclusion, nucleotide borate esters, especially ATP borate ester, exhibited superior neutron radiosensitization effects than did other representative borate ester compounds and significantly greater long-term radiation effects as well. Thus, nucleotide borate esters have several advantages over other borate esters for BNCT and therefore warrant further study.

Effects of boron-containing compounds on cardiovascular disease risk factors - A review

Boron is considered to be a biological trace element but there is substantial and growing support for it to be classified as an essential nutrient for animals and humans, depending on its speciation. Boron-containing compounds have been reported to play an important role in biological systems. Although the exact biochemical functions of boron-containing compounds have not yet been fully elucidated, previous studies suggest an active involvement of these molecules in the mediation of inflammation and oxidative stress. Chronic inflammation and oxidative stress are known to amplify the effects of the main cardiovascular risk factors: smoking, diet, obesity, arterial hypertension, dyslipidemia, type 2 diabetes (as modifiable risk factors), and hyperhomocysteinemia and age (as independent risk factors). However, the role of boron-containing compounds in cardiovascular systems and disease prevention has yet to be established. This paper is a review of boron-containing compounds' existence in nature and their possible functions in living organisms, with a special focus on certain cardiovascular risk factors that may be diminished by intake of these compounds, leading to a reduction of cardiovascular morbidity and/or mortality.

Dynamic boronic acid-mediated autoligation of DNA strands

The single common feature of all biological systems is the dependence on self-assembly of molecular units to be morphed into well-defined functional architectures. Thanks to a dynamic equilibrium process, incorrect structural units are rejected with high levels of fidelity. The development of synthetic systems displaying similar attributes is an emerging field with wide applications from biotechnology to medicine. In this context, we developed a stimuli-responsive nucleic acid-based system relying on the reversible formation of cyclic boronate internucleosidic linkages. The dynamic assembly of this new borono-based helix has been accomplished through a DNA- and an RNA-templated autoligation process featuring a 5'-ended boronic acid oligonucleotide connecting to a 3'-ended ribonucleosidic oligonucleotide partner.

Is boron a prebiotic element? A mini-review of the essentiality of boron for the appearance of life on earth

Boron is probably a prebiotic element with special importance in the so-called "sugars world". Boron is not present on Earth in its elemental form. It is found only in compounds, e.g., borax, boric acid, kernite, ulexite, colemanite and other borates. Volcanic spring waters sometimes contain boron-based acids (e.g., boric, metaboric, tetraboric and pyroboric acid). Borates influence the formation of ribofuranose from formaldehyde that feeds the "prebiotic metabolic cycle". The importance of boron in the living world is strongly related to its implications in the prebiotic origins of genetic material; consequently, we believe that throughout the evolution of life, the primary role of boron has been to provide thermal and chemical stability in hostile environments. The complexation of boric acid and borates with organic cis-diols remains the most probable chemical mechanism for the role of this element in the evolution of the living world. Because borates can stabilize ribose and form borate ester nucleotides, boron may have provided an essential contribution to the "pre-RNA world".