A theta-shaped amphiphilic cobaltabisdicarbollide anion: transition from monolayer vesicles to micelles

Compelling DNA intercalation through 'anion-anion' anti-coulombic interactions: boron cluster self-vehicles as promising anticancer agents

Anticancer drugs inhibit DNA replication by intercalating between DNA base pairs, forming covalent bonds with nucleotide bases, or binding to the DNA groove. To develop safer drugs, novel molecular structures with alternative binding mechanisms are essential. Stable boron hydrides offer a promising alternative for cancer therapy, opening up additional options like boron neutron capture therapy based on 10B and thermal neutron beams or proton boron fusion therapy using 11B and proton beams. These therapies are more efficient when the boron compound is ideally located inside cancer cells, particularly in the nucleus. Current cancer treatments often utilize small, polycyclic, aromatic, planar molecules that intercalate between ds-DNA base pairs, requiring only a spacing of approximately 0.34 nm. In this paper, we demonstrate another type of intercalation. Notably, [3,3'-Fe(1,2-C2B9H11)2]-, ([o-FESAN]-), a compact 3D molecule measuring 1.1 nm × 0.6 nm, can as well intercalate by strong non-bonding interactions preferentially with guanine. Unlike known intercalators, which are positive or neutral, [o-FESAN]- is a negative species and when an [o-FESAN]- molecule approaches the negatively charged DNA phosphate chain an anion-anion interaction consistently anti-electrostatic via Ccluster-H⋯O-P bonds occurs. Then, when more molecules approach, an elongated outstandingly self-assembled structure of [o-FESAN]--[o-FESAN]- forms moving anions towards the interthread region to interact with base pairs and form aggregates of four [o-FESAN]- anions per base pair. These aggregates, in this environment, are generated by Ccluster-H⋯O-C, N-H⋯H-B and Ccluster-H⋯H-B interactions. The ferrabis(dicarbollide) boron-rich small molecules not only effectively penetrate the nucleus but also intercalate with ds-DNA, making them promising for cancer treatment. This amphiphilic anionic molecule, used as a carrier-free drug, can enhance radiotherapy in a multimodal perspective, providing healthcare professionals with improved tools for cancer treatment. This work demonstrates these findings with a plethora of techniques.

Unveiling Non-Covalent Interactions in Novel Cooperative Photoredox Systems for Efficient Alkene Oxidation in Water

A new cooperative photoredox catalytic system, [RuII(trpy)(bpy)(H2O)][3,3'-Co(8,9,12-Cl3-1,2-C2B9H8)2]2, 5, has been synthesized and fully characterized for the first time. In this system, the photoredox catalyst [3,3'-Co(8,9,12-Cl3-1,2-C2B9H8)2]-[Cl6-1]-, a metallacarborane, and the oxidation catalyst [RuII(trpy)(bpy)(H2O)]2+, 2 are linked by non-covalent interactions. This compound, along with the one previously synthesized by us, [RuII(trpy)(bpy)(H2O)][(3,3'-Co(1,2-C2B9H11)2]2, 4, are the only examples of cooperative molecular photocatalysts in which the catalyst and photosensitizer are not linked by covalent bonds. Both cooperative systems have proven to be efficient photocatalysts for the oxidation of alkenes in water through Proton Coupled Electron Transfer processes (PCETs). Using 0.05 mol% of catalyst 4, total conversion values were achieved after 15 min with moderate selectivity for the corresponding epoxides, which decreases with reaction time, along with the TON values. However, with 0.005 mol% of catalyst, the conversion values are lower, but the selectivity and TON values are higher. This occurs simultaneously with an increase in the amount of the corresponding diol for most of the substrates studied. Photocatalyst 4 acts as a photocatalyst in both the epoxidation of alkenes and their hydroxylation in aqueous medium. The hybrid system 5 shows generally higher conversion values at low loads compared to those obtained with 4 for most of the substrates studied. However, the selectivity values for the corresponding epoxides are lower even after 15 min of reaction. This is likely due to the enhanced oxidizing capacity of CoIV in catalyst 5, resulting from the presence of more electron-withdrawing substituents on the metallacarborane platform.

Toward Distinguishing between the Superchaotropic and Hydrophobic Characters of Nanometric-Sized Ions in Interaction with PEGylated Surfaces

In this study, we explore the superchaotropic effect of various polyoxometalate or boron cluster nano-ions on hydrophilic neutral surfaces. Nano-ions, characterized by low charge densities, exhibit strong adsorption on non-ionic hydrophilic surfaces like PEGylated micelles. This adsorption phenomenon was attributed to the enthalpically favorable dehydration of nano-ions, the so-called superchaotropic effect. Here, we investigate the adsorption of three nano-ions, α-SiW12O404-, α-PW12O403-, and B12I122-, with decreasing charge density or increasing superchaotropicity (or hydrophobicity), on hydrophilic solid surfaces, PEGylated gold nanoparticles, and PEGylated gold-coated quartz crystal. Solid surfaces are devoid of hydrophobic regions, enabling the study of the subtle nuance between hydrophobic and superchaotropic effects. Unlike adsorption on PEGylated micelles, the adsorption constant decreases with a reduced charge density, aligning with the well-established principle that hydrophobic ions do not adsorb on hydrophilic surfaces. This research improves our understanding of the subtle difference between superchaotropic and hydrophobic effects in nano-ion adsorption phenomena.

The Rise of Boron-Containing Compounds: Advancements in Synthesis, Medicinal Chemistry, and Emerging Pharmacology

Boron-containing compounds (BCC) have emerged as important pharmacophores. To date, five BCC drugs (including boronic acids and boroles) have been approved by the FDA for the treatment of cancer, infections, and atopic dermatitis, while some natural BCC are included in dietary supplements. Boron's Lewis acidity facilitates a mechanism of action via formation of reversible covalent bonds within the active site of target proteins. Boron has also been employed in the development of fluorophores, such as BODIPY for imaging, and in carboranes that are potential neutron capture therapy agents as well as novel agents in diagnostics and therapy. The utility of natural and synthetic BCC has become multifaceted, and the breadth of their applications continues to expand. This review covers the many uses and targets of boron in medicinal chemistry.

Exploring the Role of Metal in the Biointeraction of Metallacarboranes with C. elegans Embryos

Cobaltabis(dicarbollides), ferrabis(dicarbollide), and their halogenated derivatives are the most studied metallacarboranes with great medical potential. These versatile compounds and their iodinated derivatives can be used in chemotherapy, radiotherapy, particle therapy, and bioimaging when isotopes are used. These metallacarboranes have been evaluated in vitro and recently in vivo with complex animal models. Lately, these studies have been complemented using the invertebrate Caenorhabditis elegans (C. elegans), a nematode largely used in toxicology. When evaluated at the L4 stage, cobaltabis(dicarbollides), ([o-COSAN]- and [8,8'-I2 -o-COSAN]- ), exhibited a higher mean lethal dose (LD50 ) than ferrabis(dicarbollides) ([o-FESAN]- and [8,8'-I2 -o-FESAN]- ). In this work, we used the C. elegans embryos since they are a complex biological barrier with concentric layers of polysaccharides and proteins that protect them from the environment. We assessed if the metal atom changes their biointeraction with the C. elegans embryos. First, we assessed the effects on embryo development for metallacarboranes and their di-iodinated derivatives. We observed changes in color and in their surface structure. An exhaustive physicochemical characterization was performed to understand better this interaction, revealing a stronger interaction of ferrabis(dicarbollide) compounds with C. elegans embryos than the cobaltabis(dicarbollide) molecules. Unveiling the biological interaction of these compounds is of great interest for their future biomedical applications.

How a few help all: cooperative crossing of lipid membranes by COSAN anions

Experimental results show that the presence of a concentration gradient of certain nano-ions (most notably cobaltabisdicarbollide ([o-COSAN]- anions), induce a current across intact artificial phospholipid bilayers in spite of the high Born free energy estimated for these ions. The mechanism underlying this observed translocation of nano-anions across membranes has yet to be determined. Here we show, using molecular dynamics simulations, that the permeation of [o-COSAN]- anions across a lipid bilayer proceeds in a cooperative manner. Single nano-ions can enter the bilayer but permeation is hampered by a free energy barrier of about 8kBT. The interaction between these nano-ions inside a leaflet induces a flip-flop translocation mechanism with the formation of transient, elongated structure inside the membrane. This cooperative flip-flop allows an efficient distribution of [o-COSAN]- anions in both leaflets of the bilayer. These results suggest the existence of a new mechanism for permeation of nano-ions across lipid membranes, relevant for those that have the appropriate self-assembly character.

Chemistry of Carbon-Substituted Derivatives of Cobalt Bis(dicarbollide)(1-) Ion and Recent Progress in Boron Substitution

The cobalt bis(dicarbollide)(1-) anion (1-), [(1,2-C2B9H11)2-3,3'-Co(III)](1-), plays an increasingly important role in material science and medicine due to its high chemical stability, 3D shape, aromaticity, diamagnetic character, ability to penetrate cells, and low cytotoxicity. A key factor enabling the incorporation of this ion into larger organic molecules, biomolecules, and materials, as well as its capacity for "tuning" interactions with therapeutic targets, is the availability of synthetic routes that enable easy modifications with a wide selection of functional groups. Regarding the modification of the dicarbollide cage, syntheses leading to substitutions on boron atoms are better established. These methods primarily involve ring cleavage of the ether rings in species containing an oxonium oxygen atom connected to the B(8) site. These pathways are accessible with a broad range of nucleophiles. In contrast, the chemistry on carbon vertices has remained less elaborated over the previous decades due to a lack of reliable methods that permit direct and straightforward cage modifications. In this review, we present a survey of methods based on metalation reactions on the acidic C-H vertices, followed by reactions with electrophiles, which have gained importance in only the last decade. These methods now represent the primary trends in the modifications of cage carbon atoms. We discuss the scope of currently available approaches, along with the stereochemistry of reactions, chirality of some products, available types of functional groups, and their applications in designing unconventional drugs. This content is complemented with a report of the progress in physicochemical and biological studies on the parent cobalt bis(dicarbollide) ion and also includes an overview of recent syntheses and emerging applications of boron-substituted compounds.

Single stop analysis of a protein surface using molecular probe electrochemistry

Visualization of a protein in its native form and environment without any interference has always been a challenging task. Contrary to the assumption that protein surfaces are smooth, they are in fact highly irregular with undulating surfaces. Hence, in this study, we have tackled this ambiguous nature of the 'surface' of a protein by considering the 'effective' protein surface (EPS) with respect to its interaction with the geometrically well-defined and structurally inert anionic molecule [3,3'-Co(1,2-C2B9H11)2]-, abbreviated as [o-COSAN]-, whose stability, propensity for amine residues, and self-assembling abilities are well reported. This study demonstrates the intricacies of protein surfaces exploiting simple electrochemical measurements using a 'small molecule' redox-active probe. This technique offers the advantage of not utilizing any harsh experimental conditions that could alter the native structure of the protein and hence the protein integrity is retained. Identification of the amino acid residues which are most involved in the interactions with [3,3'-Co(1,2-C2B9H11)2]- and how a protein's environment affects these interactions can help in gaining insights into how to modify proteins to optimize their interactions particularly in the fields of drug design and biotechnology. In this research, we have demonstrated that [3,3'-Co(1,2-C2B9H11)2]- anionic small molecules are excellent candidates for studying and visualizing protein surfaces in their natural environment and allow proteins to be classified according to the surface composition, which imparts their properties. [3,3'-Co(1,2-C2B9H11)2]- 'viewed' each protein surface differently and hence has the potential to act as a simple and easy to handle cantilever for measuring and picturing protein surfaces.

Metallacarborane Cluster Anions of the Cobalt Bisdicarbollide-Type as Chaotropic Carriers for Transmembrane and Intracellular Delivery of Cationic Peptides

Cobalt bisdicarbollides (COSANs) are inorganic boron-based anions that have been previously reported to permeate by themselves through lipid bilayer membranes, a propensity that is related to their superchaotropic character. We now introduce their use as selective and efficient molecular carriers of otherwise impermeable hydrophilic oligopeptides through both artificial and cellular membranes, without causing membrane lysis or poration at low micromolar carrier concentrations. COSANs transport not only arginine-rich but also lysine-rich peptides, whereas low-molecular-weight analytes such as amino acids as well as neutral and anionic cargos (phalloidin and BSA) are not transported. In addition to the unsubstituted isomers (known as ortho- and meta-COSAN), four derivatives bearing organic substituents or halogen atoms have been evaluated, and all six of them surpass established carriers such as pyrenebutyrate in terms of activity. U-tube experiments and black lipid membrane conductance measurements establish that the transport across model membranes is mediated by a molecular carrier mechanism. Transport experiments in living cells showed that a fluorescent peptide cargo, FITC-Arg8, is delivered into the cytosol.

Closo-dodecaborate-based dianionic surfactants with distorted classical morphology: Synthesis and atypical micellization in water

HYPOTHESIS

To challenge the classical concept of step-like micellization of ionic surfactants with singular critical micelle concentration, novel amphiphilic compounds with bulky dianionic head and the alkoxy tail connected via short linker, which can complex sodium cations, were synthesized in the form of disodium salts.

EXPERIMENT

The surfactants were synthesized by opening of a dioxanate ring attached to closo-dodecaborate by activated alcohol, which allows for attachment of alkyloxy tails of desired length to boron cluster dianion. The synthesis of the compounds with high cationic purity (sodium salt) is described. Self-assembly of the surfactant compound at air/water interface and in bulk water was studied by tensiometry, light and small angle X-ray scattering, electron microscopy, NMR spectroscopy, MD simulations and by isothermal titration calorimetry, ITC. The peculiarities in the micelle structure and formation were revealed by thermodynamic modelling and MD simulations of the micellization process.

FINDINGS

In an atypical process, the surfactants self-assemble in water to form relatively small micelles, where the aggregation number is decreasing with the surfactant concentration. The extensive counterion binding is a key characteristic of the micelles. The analysis strongly indicates complex compensation between the degree of bound sodium ions and the aggregation number. For the first time, a three-step thermodynamic model was used to estimate the thermodynamic parameters associated with micellization process. Diverse micelles differing in size and counterion binding can (co-)exist in the solution over the broad concentration and temperature range. Thus, the concept of step-like micellization was found inappropriate for these types of micelles.

Towards the Application of Purely Inorganic Icosahedral Boron Clusters in Emerging Nanomedicine

Traditionally, drugs were obtained by extraction from medicinal plants, but more recently also by organic synthesis. Today, medicinal chemistry continues to focus on organic compounds and the majority of commercially available drugs are organic molecules, which can incorporate nitrogen, oxygen, and halogens, as well as carbon and hydrogen. Aromatic organic compounds that play important roles in biochemistry find numerous applications ranging from drug delivery to nanotechnology or biomarkers. We achieved a major accomplishment by demonstrating experimentally/theoretically that boranes, carboranes, as well as metallabis(dicarbollides), exhibit global 3D aromaticity. Based on the stability-aromaticity relationship, as well as on the progress made in the synthesis of derivatized clusters, we have opened up new applications of boron icosahedral clusters as key components in the field of novel healthcare materials. In this brief review, we present the results obtained at the Laboratory of Inorganic Materials and Catalysis (LMI) of the Institut de Ciència de Materials de Barcelona (ICMAB-CSIC) with icosahedral boron clusters. These 3D geometric shape clusters, the semi-metallic nature of boron and the presence of exo-cluster hydrogen atoms that can interact with biomolecules through non-covalent hydrogen and dihydrogen bonds, play a key role in endowing these compounds with unique properties in largely unexplored (bio)materials.

Aqueous solubilization of hydrophobic compounds by inorganic nano-ions: An unconventional mechanism

HYPOTHESIS

Solubilization of hydrophobic compounds in water is commonly performed by using organic solubilizers such as hydrotropes, surfactants, co-solvents, and macrocycles to form host-guest complexes. 3,3'-commo-bis[closo-1,2-dicarba-3-cobaltadodecaborane] derivatives (COSANs) are fully inorganic and non-amphiphilic ionic boron clusters with nanometric size (nano-ions) showing superchaotropic properties as they strongly bind to neutral organic molecules. Therefore, we expect COSANs to act as solubilizers of sparingly water-soluble molecules, but with a mechanism different from all other organic solubilizers known so far.

EXPERIMENTS

The aqueous solubilization efficiency of COSANs towards butanol was evaluated by determining phase diagrams and comparing them to classical solubilizers. Nanostructuration of the mixture was studied using UV spectroscopy, small-angle X-ray, and neutron scattering with contrast variation.

FINDINGS

COSANs act as efficient aqueous solubilizers of medium-chain alcohols (0.6 < log P < 1.5). Unlike surfactants, COSAN is an efficient solubilizer in its monomeric state, at concentrations well below its critical aggregation concentration. Solubilization by COSAN takes place with a bi-dimensional anisotropic growth of COSAN/butanol co-assemblies, whereas solubilization by surfactant occurs via an isotropic swelling of micelles. Appealingly, COSANs/2-butanol co-assemblies efficiently solubilize more hydrophobic compounds with log P values up to around 6, offering new opportunities in many applied fields.

New Boron Delivery Agents

This proceeding article compiles current research on the development of boron delivery drugs for boron neutron capture therapy that was presented and discussed at the National Cancer Institute (NCI) Workshop on Neutron Capture Therapy that took place on April 20-22, 2022. The most used boron sources are icosahedral boron clusters attached to peptides, proteins (such as albumin), porphyrin derivatives, dendrimers, polymers, and nanoparticles, or encapsulated into liposomes. These boron clusters and/or carriers can be labeled with contrast agents allowing for the use of imaging techniques, such as PET, SPECT, and fluorescence, that enable quantification of tumor-localized boron and their use as theranostic agents.

Boron clusters (ferrabisdicarbollides) shaping the future as radiosensitizers for multimodal (chemo/radio/PBFR) therapy of glioblastoma

Glioblastoma multiforme (GBM) is the most common and fatal primary brain tumor, and is highly resistant to conventional radiotherapy and chemotherapy. Therefore, the development of multidrug resistance and tumor recurrence are frequent. Given the poor survival with the current treatments, new therapeutic strategies are urgently needed. Radiotherapy (RT) is a common cancer treatment modality for GBM. However, there is still a need to improve RT efficiency, while reducing the severe side effects. Radiosensitizers can enhance the killing effect on tumor cells with less side effects on healthy tissues. Herein, we present our pioneering study on the highly stable and amphiphilic metallacarboranes, ferrabis(dicarbollides) ([o-FESAN]^- and [8,8'-I2-o-FESAN]^-), as potential radiosensitizers for GBM radiotherapy. We propose radiation methodologies that utilize secondary radiation emissions from iodine and iron, using ferrabis(dicarbollides) as iodine/iron donors, aiming to achieve a greater therapeutic effect than that of a conventional radiotherapy. As a proof-of-concept, we show that using 2D and 3D models of U87 cells, the cellular viability and survival were reduced using this treatment approach. We also tested for the first time the proton boron fusion reaction (PBFR) with ferrabis(dicarbollides), taking advantage of their high boron (¹¹B) content. The results from the cellular damage response obtained suggest that proton boron fusion radiation therapy, when combined with boron-rich compounds, is a promising modality to fight against resistant tumors. Although these results are encouraging, more developments are needed to further explore ferrabis(dicarbollides) as radiosensitizers towards a positive impact on the therapeutic strategies for GBM.

History of Cobaltabis(dicarbollide) in Potentiometry, No Need for Ionophores to Get an Excellent Selectivity

This work is a mini-review highlighting the relevance of the θ metallabis(dicarbollide) [3,3'-Co(1,2-C₂B₉H₁₁)₂]^- with its peculiar and differentiating characteristics, among them the capacity to generate hydrogen and dihydrogen bonds, to generate micelles and vesicles, to be able to be dissolved in water or benzene, to have a wide range of redox reversible couples and many more, and to use these properties, in this case, for producing potentiometric membrane sensors to monitor amine-containing drugs or other nitrogen-containing molecules. Sensors have been produced with this monoanionic cluster [3,3'-Co(1,2-C₂B₉H₁₁)₂]^-. Other monoanionic boron clusters are also discussed, but they are much fewer. It is noteworthy that most of the electrochemical sensor species incorporate an ammonium cation and that this cation is the species to be detected. Alternatively, the detection of the borate anion itself has also been studied, but with significantly fewer examples. The functions of the borate anion in the membrane are different, even as a doping agent for polypyrrole which was the conductive ground on which the PVC membrane was deposited. Apart from these cases related to closo borates, the bulk of the work has been devoted to sensors in which the θ metallabis (dicarbollide) [3,3'-Co(1,2-C₂B₉H₁₁)₂]^- is the key element. The metallabis (dicarbollide) anion, [3,3'-Co(1,2-C₂B₉H₁₁)₂]^-, has many applications; one of these is as new material used to prepare an ion-pair complex with bioactive protonable nitrogen containing compounds, [YH]_x[3,3'-Co(1,2-C₂B₉H₁₁)₂]_y as an active part of PVC membrane potentiometric sensors. The developed electrodes have Nernstian responses for target analytes, i.e., antibiotics, amino acids, neurotransmitters, analgesics, for some decades of concentrations, with a short response time, around 5 s, a good stability of membrane over 45 days, and an optimal selectivity, even for optical isomers, to be used also for real sample analysis and environmental, clinical, pharmaceutical and food analysis.

Metallacarborane Derivatives as Innovative Anti-Candida albicans Agents

Infections caused by Candida species have increased significantly in the past decades and are among the leading causes of morbidity and mortality worldwide, resulting in serious public health problems. Currently, conventional antifungals are often ineffective as Candida spp. have developed growing resistance to systemic drugs. Since inorganic metallacarboranes are known to affect cellular events, new derivatives of these abiotic compounds were tested against Candida albicans. Compounds based on cobalt bis-dicarbollide [COSAN] were studied on Candida albicans strains, including a panel of 100 clinical isolates. The presented data prove that metallacarborane derivatives are effective against clinical isolates of Candida albicans, even those resistant to systemic drugs, and show synergistic potential in combination with amphotericin B, and low toxicity against human cells and Danio rerio embryos. This paper is a consequential step in the investigations of the broad spectrum and valuable future medical applications of metallacarboranes, especially in the fight against drug-resistant pathogens.

Visible-Light-Induced Palladium-Catalyzed Cross-Coupling of Iodocarboranes with (Hetero)Arenes

This work describes a general method for the efficient production of a class of cage B-centered carboranyl radicals at the B3, B4, and B9 sites via a visible-light-promoted palladium(0)/palladium(I) pathway using readily available iodo-o-carboranes as the starting materials. The electrophilicities of these hypervalent boron-centered radicals decrease in the following order: B3 > B4 > B9. They are useful intermediates for the preparation of a family of cage B-(hetero)arylated o-carboranes at ambient temperature.

Synthesis of Cobalt Bis(Dicarbollide)—Curcumin Conjugates for Potential Use in Boron Neutron Capture Therapy

A series of novel cobalt bis(dicarbollide)—curcumin conjugates were synthesized. Two conjugates were obtained through the nucleophilic ring-opening reaction of the 1,4-dioxane and tetrahydropyran derivatives of cobalt bis(dicarbollide) with the OH group of curcumin, and using two equiv. of the oxonium derivatives, two other conjugates containing two cobalt bis(dicarbollide) units per molecule were obtained. In contrast to curcumin, the conjugates obtained were found to be non-cytotoxic against both tumor and normal cell lines. The analysis of the intracellular accumulation of the conjugates by flow cytometry showed that all cobalt bis(dicarbollide)—curcumin conjugates entered HCT116 colorectal carcinoma cells in a time-dependent manner. New non-cytotoxic conjugates contain a large amount of boron atoms in the biomolecule and can potentially be used for further biological research into boron neutron capture therapy (BNCT).

Water soluble organometallic small molecules as promising antibacterial agents: synthesis, physical-chemical properties and biological evaluation to tackle bacterial infections

The Na[3,3'-Fe(8-I-1,2-C₂B₉H₁₀)₂] and Na[2,2'-M(1,7-C₂B₉H₁₁)] (M = Co³⁺, Fe³⁺) small molecules are synthesized and the X-ray structures of [(H₃O)(H₂O)₅][2,2'-Co(1,7-C₂B₉H₁₁)₂] and [Cs(MeCN)][8,8'-I₂-Fe(1,2 C₂B₉H₁₀)₂], both displaying a transoid conformation of the [M(C₂B₉)₂]^- framework, are reported. Importantly, the supramolecular structure of [(H₃O)(H₂O)₅][2,2'-Co(1,7-C₂B₉H₁₁)₂] presents 2D layers leading to a lamellar arrangement of the anions while the cation layers form polymeric water rings made of six- and four-membered rings of water molecules connected via OH⋯H hydrogen bonds; B-H⋯O contacts connect the cationic and anionic layers. Herein, we highlight the influence of the ligand isomers (ortho-/meta-), the metal effect (Co³⁺/Fe³⁺) on the same isomer, as well as the influence of the presence of the iodine atoms on the physical-chemical and biological properties of these molecules as antimicrobial agents to tackle antibiotic-resistant bacteria, which were tested with four Gram-positive bacteria, five Gram-negative bacteria, and three Candida albicans strains that have been responsible for human infections. We have demonstrated an antimicrobial effect against Candida species (MIC of 2 and 3 nM for Na[3,3'-Co(8-I-1,2-C₂B₉H₁₀)₂] and Na[2,2'-Co(1,7-C₂B₉H₁₁)₂], respectively), and against Gram-positive and Gram-negative bacteria, including multiresistant MRSA strains (MIC of 6 nM for Na[3,3'-Co(8-I-1,2-C₂B₉H₁₀)₂]). The selectivity index for antimicrobial activity of Na[3,3'-Co(1,2-C₂B₉H₁₁)₂] and Na[3,3'-Co(8-I-1,2-C₂B₉H₁₀)₂] compounds is very high (165 and 1180, respectively), which reveals that these small anionic metallacarborane molecules may be useful to tackle antibiotic-resistant bacteria. Moreover, we have demonstrated that the outer membrane of Gram-negative bacteria constitutes an impermeable barrier for the majority of these compounds. Nonetheless, the addition of two iodine groups in the structure of the parent Na[3,3'-Co(1,2-C₂B₉H₁₁)₂] had an improved effect (3-7 times) against Gram-negative bacteria. Possibly the changes in their physical-chemical properties make the meta-isomers and the ortho-di-iodinated small molecules more permeable for crossing this barrier. It should be emphasized that the most active metallabis(dicarbollide) small molecules are both transoid conformers in contrast to the ortho- [3,3'-Co(1,2-C₂B₉H₁₁)₂]^- that is cisoid. The fact that these small molecules cross the mammalian membrane and have antimicrobial properties but low toxicity for mammalian cells (high selectivity index, SI) represents a promising tool to treat infectious intracellular bacteria. Since there is an urgent need for antibiotic discovery and development, this study represents a relevant advance in the field.

Synthesis and Antibacterial Activity Studies of the Conjugates of Curcumin with closo-Dodecaborate and Cobalt Bis(Dicarbollide) Boron Clusters

A series of novel conjugates of cobalt bis(dicarbollide) and closo-dodecaborate with curcumin were synthesized by copper(I)-catalyzed azide-alkyne cycloaddition. These conjugates were tested for antibacterial activity. It was shown that all derivatives are active when exposed to Bacillus cereus ATCC 10702 and are not active against Gram-negative microorganisms and Candida albicans at the maximum studied concentration of 1000 mg/L. The conjugate of alkynyl-curcumin with azide synthesized from the tetrahydropyran derivative of cobalt bis(dicarbollide) exhibited activity against Gram-positive microorganisms: Staphylococcus aureus ATCC 29213, Enterococcus faecalis ATCC 29212 and the clinical isolate MRSA 17, that surpassed curcumin by 2–4 times.

Visible-Light-Promoted Nickel-Catalyzed Cross-Coupling of Iodocarboranes with (Hetero)Arenes via Boron-Centered Carboranyl Radicals

A general strategy for the generation of hypervalent boron-centered carboranyl radicals at the B(3), B(4), and B(9) positions has been developed for the first time via visible-light-promoted iodine atom abstraction from iodo-o-carboranes by low-valent nickel complex. These radicals react with various (hetero)arenes to afford a wide range of cage B-arylated carborane derivatives at room temperature in very good to excellent yields with a broad substrate scope. Their electrophilicities are dependent on the vertex charges of the cage and follow the order B(3) > B(4) > B(9). Both visible light and nickel catalyst are proved critical to the generation of boron-centered carboranyl radicals. The involvement of boron radicals is supported by control experiments. A reaction mechanism associated with these reactions is also proposed. This strategy offers a new protocol for the generation of boron-centered carboranyl radicals at the selected boron vertex, leading to a facile synthesis of a large class of cage boron substituted carborane molecules.

Carboranes as unique pharmacophores in antitumor medicinal chemistry

Carborane is a carbon-boron molecular cluster that can be viewed as a 3D analog of benzene. It features special physical and chemical properties, and thus has the potential to serve as a new type of pharmacophore for drug design and discovery. Based on the relative positions of two cage carbons, icosahedral closo-carboranes can be classified into three isomers, ortho-carborane (o-carborane, 1,2-C2B10H12), meta-carborane (m-carborane, 1,7-C2B10H12), and para-carborane (p-carborane, 1,12-C2B10H12), and all of them can be deboronated to generate their nido- forms. Cage compound carborane and its derivatives have been demonstrated as useful chemical entities in antitumor medicinal chemistry. The applications of carboranes and their derivatives in the field of antitumor research mainly include boron neutron capture therapy (BNCT), as BNCT/photodynamic therapy dual sensitizers, and as anticancer ligands. This review summarizes the research progress on carboranes achieved up to October 2021, with particular emphasis on signaling transduction pathways, chemical structures, and mechanistic considerations of using carboranes.

Potential application of metallacarboranes as internal reference: An electrochemical comparative study to ferrocene

Ferrocene and its derivatives have been extensively used as internal reference in electrochemical processes. Yet, they possess limitations such as solvent restrictions that require chemical modifications. In this regard, we...

Cobaltabis(dicarbollide) ([o-COSAN]-) as Multifunctional Chemotherapeutics: A Prospective Application in Boron Neutron Capture Therapy (BNCT) for Glioblastoma

PURPOSE

The aim of our study was to assess if the sodium salt of cobaltabis(dicarbollide) and its di-iodinated derivative (Na[o-COSAN] and Na[8,8'-I2-o-COSAN]) could be promising agents for dual anti-cancer treatment (chemotherapy + BNCT) for GBM.

METHODS

The biological activities of the small molecules were evaluated in vitro with glioblastoma cells lines U87 and T98G in 2D and 3D cell models and in vivo in the small model animal Caenorhabditis elegans (C. elegans) at the L4-stage and using the eggs.

RESULTS

Our studies indicated that only spheroids from the U87 cell line have impaired growth after treatment with both compounds, suggesting an increased resistance from T98G spheroids, contrary to what was observed in the monolayer culture, which highlights the need to employ 3D models for future GBM studies. In vitro tests in U87 and T98G cells conclude that the amount of 10B inside the cells is enough for BNCT irradiation. BNCT becomes more effective on T98G after their incubation with Na[8,8'-I2-o-COSAN], whereas no apparent cell-killing effect was observed for untreated cells.

CONCLUSIONS

These small molecules, particularly [8,8'-I2-o-COSAN]-, are serious candidates for BNCT now that the facilities of accelerator-based neutron sources are more accessible, providing an alternative treatment for resistant glioblastoma.

Rhodium-catalyzed sequential B(3)-, B(4)-, and B(5)-trifunctionalization of o-carboranes with three different substituents

A rhodium-catalyzed one-pot trifunctionalization of o-carboranes with three different substituents via a carboxy group directed sequential B(5)-alkenylation, B(4)-alkyne annulation and B(3)-acyloxylation has been developed for the first time, leading to the synthesis of a new class of B(3,4,5)-trisubstituted o-carborane derivatives. Treatment of 1-COOH-2-CH₃-o-C₂B₁₀H₁₀ with ArCCAr in the presence of a [Cp*RhCl₂]₂ catalyst and a Cu(OPiv)₂ oxidant gave 1,4-[COOC(Ar)C(Ar)]-2-Me-3-OPiv-5-[C(Ar)CH(Ar)-o-C₂B₁₀H₇ in good to high yields. This protocol represents a new strategy for the catalytic selective polyfunctionalization of carboranes with different substituents.

Light-Induced On/Off Switching of the Surfactant Character of the o-Cobaltabis(dicarbollide) Anion with No Covalent Bond Alteration

Cobaltabis(dicarbollide) anion ([o-COSAN]- ) is a well-known metallacarborane with multiple applications in a variety of fields. In aqueous solution, the cisoid rotamer is the most stable disposition in the ground state. The present work provides theoretical evidence on the possibility to photoinduce the rotation from the cisoid to the transoid rotamer, a conversion that can be reverted when the ground state is repopulated. The non-radiative decay mechanisms proposed in this work are coherent with the lack of fluorescence observed in 3D fluorescence mapping experiments performed on [o-COSAN]- and its derivatives. This phenomenon induced by light has the potential to destruct the vesicles and micelles cisoid [o-COSAN]- typically forms in aqueous solution, which could lead to promising applications, particularly in the field of nanomedicine.

Electrooxidative o-carborane chalcogenations without directing groups: cage activation by copper catalysis at room temperature

Copper-catalyzed electrochemical direct chalcogenations of o-carboranes was established at room temperature. Thereby, a series of cage C-sulfenylated and C-selenylated o-carboranes anchored with valuable functional groups was accessed with high levels of position- and chemo-selectivity control. The cupraelectrocatalysis provided efficient means to activate otherwise inert cage C-H bonds for the late-stage diversification of o-carboranes.

Synchrotron-Based Fourier-Transform Infrared Micro-Spectroscopy (SR-FTIRM) Fingerprint of the Small Anionic Molecule Cobaltabis(dicarbollide) Uptake in Glioma Stem Cells

The anionic cobaltabis (dicarbollide) [3,3'-Co(1,2-C2B9H11)2]-, [o-COSAN]-, is the most studied icosahedral metallacarborane. The sodium salts of [o-COSAN]- could be an ideal candidate for the anti-cancer treatment Boron Neutron Capture Therapy (BNCT) as it possesses the ability to readily cross biological membranes thereby producing cell cycle arrest in cancer cells. BNCT is a cancer therapy based on the potential of 10B atoms to produce α particles that cross tissues in which the 10B is accumulated without damaging the surrounding healthy tissues, after being irradiated with low energy thermal neutrons. Since Na[o-COSAN] displays a strong and characteristic ν(B-H) frequency in the infrared range 2.600-2.500 cm-1, we studied the uptake of Na[o-COSAN] followed by its interaction with biomolecules and its cellular biodistribution in two different glioma initiating cells (GICs), mesenchymal and proneural respectively, by using Synchrotron Radiation-Fourier Transform Infrared (FTIR) micro-spectroscopy (SR-FTIRM) facilities at the MIRAS Beamline of ALBA synchrotron light source. The spectroscopic data analysis from the bands in the regions of DNA, proteins, and lipids permitted to suggest that after its cellular uptake, Na[o-COSAN] strongly interacts with DNA strings, modifies proteins secondary structure and also leads to lipid saturation. The mapping suggests the nuclear localization of [o-COSAN]-, which according to reported Monte Carlo simulations may result in a more efficient cell-killing effect compared to that in a uniform distribution within the entire cell. In conclusion, we show pieces of evidence that at low doses, [o-COSAN]- translocates GIC cells' membranes and it alters the physiology of the cells, suggesting that Na[o-COSAN] is a promising agent to BNCT for glioblastoma cells.

Iridium-Catalyzed Annulation of o-Carboranyl Carboxylic Acids with Alkynes: Synthesis of Carborano-Isocoumarins

An efficient iridium-catalyzed formal [4+2] annulation of carboranyl carboxylic acids with alkynes is developed, resulting in the facile synthesis of a new class of carborano-isocoumarin derivatives. The carboxyl group not only serves as a directing group to control the regioselectivity but also ingeniously becomes a part of the final products. The reaction mechanism involves sequential carboxyl-directed B(4)-H metalation, alkyne insertion, and reductive elimination.

Preparing (Metalla)carboranes for Nanomedicine

"There's plenty of room at the bottom" (Richard Feynman, 1959): an invitation for (metalla)carboranes to enter the (new) field of nanomedicine. For two decades, the number of publications on boron cluster compounds designed for potential applications in medicine has been constantly increasing. Hundreds of compounds have been screened in vitro or in vivo for a variety of biological activities (chemotherapeutics, radiotherapeutics, antiviral, etc.), and some have shown rather promising potential for further development. However, until now, no boron cluster compounds have made it to the clinic, and even clinical trials have been very sparse. This review introduces a new perspective in the field of medicinal boron chemistry, namely that boron-based drugs should be regarded as nanomedicine platforms, due to their peculiar self-assembly behaviour in aqueous solutions, and treated as such. Examples for boron-based 12- and 11-vertex clusters and appropriate comparative studies from medicinal (in)organic chemistry and nanomedicine, highlighting similarities, differences and gaps in physicochemical and biological characterisation methods, are provided to encourage medicinal boron chemists to fill in the gaps between chemistry laboratory and real applications in living systems by employing bioanalytical and biophysical methods for characterising and controlling the aggregation behaviour of the clusters in solution.

Synthesis and Structure of Nido-Carboranyl Azide and Its "Click" Reactions

Novel zwitter-ionic nido-carboranyl azide 9-N3(CH2)3Me2N-nido-7,8-C2B9H11 was prepared by the reaction of 9-Cl(CH2)3Me2N-nido-7,8-C2B9H11 with NaN3. The solid-state molecular structure of nido-carboranyl azide was determined by single-crystal X-ray diffraction. 9-N3(CH2)3Me2N-nido-7,8-C2B9H11 was used for the copper(I)-catalyzed azide-alkyne cycloaddition with phenylacetylene, alkynyl-3β-cholesterol and cobalt/iron bis(dicarbollide) terminal alkynes to form the target 1,2,3-triazoles. The nido-carborane-cholesterol conjugate 9-3β-Chol-O(CH2)C-CH-N3(CH2)3Me2N-nido-7,8-C2B9H11 with charge-compensated group in a linker can be used as a precursor for preparation of liposomes for Boron Neutron Capture Therapy (BNCT). A series of novel zwitter-ionic boron-enriched cluster compounds bearing a 1,2,3-triazol-metallacarborane-carborane conjugated system was synthesized. Prepared conjugates contain a large amount of boron atom in the biomolecule and potentially can be used for BNCT.

Electrochemical B-H Nitrogenation: Access to Amino Acid and BODIPY-Labeled nido-Carboranes

Electrocatalyzed oxidative B-H nitrogenations of nido-carborane (nido-7,8-C2 B9 H12- ) with N-heterocycles have been established, enabling the preparation of various N-substituted nido-carboranes without chemical oxidants or metal catalyst under ambient conditions. The electrolysis manifold occurred with high levels of efficiency as well as chemo- and position- selectivity, employing sustainable electricity as the sole oxidant. The strategy set the stage for a user-friendly access to novel amino acid and fluorogenic boron-dipyrrin (BODIPY)-labeled nido-carborane hybrids.

Reversibly Switchable Fluorescent Molecular Systems Based on Metallacarborane-Perylenediimide Conjugates

Icosahedral metallacarboranes are θ-shaped anionic molecules in which two icosahedra share one vertex that is a metal center. The most remarkable of these compounds is the anionic cobalt-based metallacarborane [Co(C₂ B₉ H₁₁ )₂ ]^- , whose oxidation-reduction processes occur via an outer sphere electron process. This, along with its low density negative charge, makes [Co(C₂ B₉ H₁₁ )₂ ]^- very appealing to participate in electron-transfer processes. In this work, [Co(C₂ B₉ H₁₁ )₂ ]^- is tethered to a perylenediimide dye to produce the first examples of switchable luminescent molecules and materials based on metallacarboranes. In particular, the electronic communication of [Co(C₂ B₉ H₁₁ )₂ ]^- with the appended chromophore unit in these compounds can be regulated upon application of redox stimuli, which allows the reversible modulation of the emitted fluorescence. As such, they behave as electrochemically-controlled fluorescent molecular switches in solution, which surpass the performance of previous systems based on conjugates of perylendiimides with ferrocene. Remarkably, they can form gels by treatment with appropriate mixtures of organic solvents, which result from the self-assembly of the cobaltabisdicarbollide-perylendiimide conjugates into 1D nanostructures. The interplay between dye π-stacking and metallacarborane electronic and steric interactions ultimately governs the supramolecular arrangement in these materials, which for one of the compounds prepared allows preserving the luminescent behavior in the gel state.

Noncovalently Linked Metallacarboranes on Functionalized Magnetic Nanoparticles as Highly Efficient, Robust, and Reusable Photocatalysts in Aqueous Medium

A successful homogeneous photoredox catalyst has been fruitfully heterogenized on magnetic nanoparticles (MNPs) coated with a silica layer, keeping intact its homogeneous catalytic properties but gaining others due to the easy magnetic separation and recyclability. The amine-terminated magnetic silica nanoparticles linked noncovalently to H[3,3'-Co(1,2-C₂B₉H₁₁)₂]^- (H[1]), termed MSNPs-NH₂@H[1], are highly stable and do not produce any leakage of the photoredox catalyst H[1] in water. The magnetite MNPs were coated with SiO₂ to provide colloidal stability and silanol groups to be tethered to amine-containing units. These were the MSNPs-NH₂ on which was anchored, in water, the cobaltabis(dicarbollide) complex H[1] to obtain MSNPs-NH₂@H[1]. Both MSNPs-NH₂ and MSNPs-NH₂@H[1] were evaluated to study the morphology, characterization, and colloidal stability of the MNPs produced. The heterogeneous MSNP-NH₂@H[1] system was studied for the photooxidation of alcohols, such as 1-phenylethanol, 1-hexanol, 1,6-hexanediol, or cyclohexanol among others, using catalyst loads of 0.1 and 0.01 mol %. Surfactants were introduced to prevent the aggregation of MNPs, and cetyl trimethyl ammonium chloride was chosen as a surfactant. This provided adequate stability, without hampering quick magnetic separation. The results proved that the catalysis could be speeded up if aggregation was prevented. The recyclability of the catalytic system was demonstrated by performing 12 runs of the MSNPs-NH₂@H[1] system, each one without loss of selectivity and yield. The cobaltabis(dicarbollide) catalyst supported on silica-coated magnetite nanoparticles has proven to be a robust, efficient, and easily reusable system for the photooxidation of alcohols in water, resulting in a green and sustainable heterogeneous catalytic system.

Highlights on the Binding of Cobalta-Bis-(Dicarbollide) with Glucose Units

Metalla-bis-dicarbollides, such as the cobalta-bis-dicarbollide (COSAN) anion [Co(C₂ B₉ H₁₁ )₂ ]^- , have attracted much attention in biology but a deep understanding of their interactions with cell components is still missing. For this purpose, we studied the interactions of COSAN with the glucose moiety, which is ubiquitous at biological interfaces. Octyl-glucopyranoside surfactant (C8G1) was chosen as a model as it self-assembles in water and creates a hydrated glucose-covered interface. At low COSAN content and below the critical micellar concentration (CMC) of C8G1, COSAN binds to C8G1 monomers through the hydrophobic effect. Above the CMC of C8G1, COSAN adsorbs onto C8G1 micelles through the superchaotropic effect. At high COSAN concentrations, COSAN disrupts C8G1 micelles and the assemblies become similar to COSAN micelles but with a small amount of solubilized C8G1. Therefore, COSAN binds in a versatile way to C8G1 through either the hydrophobic or superchaotropic effect depending on their relative concentrations.

Rational Control of Self-Recognition of Macroionic γ-Cyclodextrin by Host-Guest Interaction with Super-Chaotropic Borate Cluster Ions

We report a feasible method to control self-recognition during the self-assembly of a hydrophilic macroion, phosphate-functionalized γ-cyclodextrin (γ-CD-P), though host-guest interactions. We confirmed that γ-CD-P can form a host-guest complex with a super-chaotropic anion, namely the B₁₂ F₁₂ ^2- borate cluster, by using NMR spectroscopy and isothermal titration calorimetry. The loaded γ-CD-P, which has a higher charge density, can be distinguished from the uncomplexed γ-CD-P, leading to self-sorting behavior during the self-assembly process, confirmed by the formation of two types of individual supramolecular structures (R_h of ca. 57 nm and 18 nm, determined by light scattering) instead of hybrid structures in mixed dilute solution. This self-recognition behavior is accounted for by the difference in intermolecular electrostatic interactions arising from the loading.

Boron-Containing Lipids and Liposomes: New Conjugates of Cholesterol with Polyhedral Boron Hydrides

A series of boron-containing lipids were prepared by reactions of cyclic oxonium derivatives of polyhedron boranes and metallacarboranes (closo-dodecaborate anion, cobalt and iron bis(dicarbollides)) with amine and carboxylic acids which are derived from cholesterol. Stable liposomal formulations, on the basis of synthesized boron-containing lipids, hydrogenated soybean l-α-phosphatidylcholine and (HSPC) 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000] (DSPE-PEG) as excipients, were prepared and then characterized by dynamic light scattering (DLS) that revealed the formation of particles to be smaller than 200 nm in diameter. The resulting liposomal formulations showed moderate to excellent loading and entrapment efficiency, thus justifying the design of the compounds to fit in the lipid bilayer and ensuring ease of in vivo use for future application. The liposomal formulations based on cobalt and iron bis(dicarbollide)-based lipids were found to be nontoxic against both human breast normal epithelial cells MCF-10A and human breast cancer cells MCF-7.

Factors affecting the morphology of some organic and inorganic nanostructures for drug delivery: characterization, modifications, and toxicological perspectives

Introduction: In this review, we aim to highlight the impact of various processes and formulation variables influencing the characteristics of certain surfactant-based nanoconstructs for drug delivery. Areas covered: The review includes the discussion on processing parameters for the preparation of nanoconstructs, especially those made up of surfactants. Articles published in last 15 years (437) were reviewed, 381 articles were selected for data review and most appropriate articles (215) were included in article. Effect of variables such as surfactant concentration and type, membrane additives, temperature, and pH-dependent transitions on morphology has been highlighted along with effect of shape on nanoparticle uptake by cells. Various characterization techniques explored for these nanostructures with respect to size, morphology, lamellarity, distribution, etc., and a separate section on polymeric vesicles and the influence of block copolymers, type of block copolymer, control of block length, interaction of multiple block copolymers on the structure of polymersomes and chimeric nanostructures have been discussed. Finally, applications, modification, degradation, and toxicological aspects of these drug delivery systems have been highlighted. Expert opinion: Parameters influencing the morphology of micelles and vesicles can directly or indirectly affect the efficacy of small molecule cellular internalization as well as uptake in the case of biologicals.[Figure: see text].