Clusters of Hydroxyl-Functionalized Cations Stabilized by Cooperative Hydrogen Bonds: The Role of Polarizability and Alkyl Chain Length

Use of Silica Nanoparticles for Drug Delivery in Cardiovascular Disease

PURPOSE

Cardiovascular disease (CVD) is the leading cause of death worldwide. The current CVD therapeutic drugs require long-term treatment with high doses, which increases the risk of adverse effects while offering only marginal treatment efficacy. Silica nanoparticles (SNPs) have been proven to be an efficient drug delivery vehicle for numerous diseases, including CVD. This article reviews recent progress and advancement in targeted delivery for drugs and diagnostic and theranostic agents using silica nanoparticles to achieve therapeutic efficacy and improved detection of CVD in clinical and preclinical settings.

METHODS

A search of PubMed, Scopus, and Google Scholar databases from 1990 to 2023 was conducted. Current clinical trials on silica nanoparticles were identified through ClinicalTrials.gov. Search terms include silica nanoparticles, cardiovascular diseases, drug delivery, and therapy.

FINDINGS

Silica nanoparticles exhibit biocompatibility in biological systems, and their shape, size, surface area, and surface functionalization can be customized for the safe transport and protection of drugs in blood circulation. These properties also enable effective drug uptake in specific tissues and controlled drug release after systemic, localized, or oral delivery. A range of silica nanoparticles have been used as nanocarrier for drug delivery to treat conditions such as atherosclerosis, hypertension, ischemia, thrombosis, and myocardial infarction.

IMPLICATIONS

The use of silica nanoparticles for drug delivery and their ongoing development has emerged as a promising strategy to improve the effectiveness of drugs, imaging agents, and theranostics with the potential to revolutionize the treatment of CVD.

How Like-Charge Attraction Influences the Mobility of Cations in Hydroxyl-Functionalized Ionic Liquids

Attractive interactions between ions of like charge remain an elusive concept. Observing and quantifying this type of interaction in liquids and solutions is still a major challenge. Recently, we have shown that cation-cation interactions are present in hydroxyl-functionalized ionic liquids and that they can be controlled by the shape, charge distribution and functionality of the ions. In the present study, we demonstrate that cationic cluster formation does not only change the local structures of the ionic liquids but also influences the dynamics of the cations in a characteristic way. We show that solid-state ²H NMR spectroscopy is well suited for the study of molecular motion, even if the hydrogen bonded species of interest are indistinguishable due to fast deuteron exchange. We also provide valuable information about the applicability of well-accepted relaxation models.

Quantification and Distribution of Three Types of Hydrogen Bonds in Mixtures of an Ionic Liquid with the Hydrogen-Bond-Accepting Molecular Solvent DMSO Explored by Neutron Diffraction and Molecular Dynamics Simulations

The concept of hydrogen bonding is celebrating its 100th birthday. Hydrogen bonds (H-bonds) play a key role in the structure and function of biological molecules, the strength of materials, and molecular binding. Herein, we study H-bonding in mixtures of a hydroxyl-functionalized ionic liquid with the neutral, H-bond-accepting molecular liquid dimethylsulfoxide (DMSO) using neutron diffraction experiments and molecular dynamics simulations. We report the geometry, strength, and distribution of three different types of H-bond OH···O, formed between the hydroxyl group of the cation and either the oxygen atom of another cation, the counteranion, or the neutral molecule. Such a variety of different strengths and distributions of H-bonds in one single mixture could hold the promise of providing solvents with potential applications in H-bond-related chemistry, for example, to alter the natural selectivity patterns of catalytic reactions or the conformation of catalysts.

Recent advances in NMR spectroscopy of ionic liquids

This review presents recent developments in the application of NMR spectroscopic techniques in the study of ionic liquids. NMR has been the primary tool not only for the structural characterization of ionic liquids, but also for the study of dynamics. The presence of a host of NMR active nuclei in ionic liquids permits widespread use of multinuclear NMR experiments. Chemical shifts and multinuclear coupling constants are used routinely for the structure elucidation of ionic liquids and of products formed by their covalent interactions with other materials. Also, the availability of a multitude of NMR techniques has facilitated the study of dynamical processes in them. These include the use of NOESY to study inter-ionic interactions, pulsed-field gradient techniques for probing transport properties, and relaxation measurements to elucidate rotational dynamics. This review will focus on the application of each of these techniques to investigate ionic liquids.

Insights into Ionic Liquids: From Z-Bonds to Quasi-Liquids

Ionic liquids (ILs) hold great promise in the fields of green chemistry, environmental science, and sustainable technology due to their unique properties, such as a tailorable structure, the various types available, and their environmentally friendly features. On the basis of multiscale simulations and experimental characterizations, two unique features of ILs are as follows: (1) strong coupling interactions between the electrostatic forces and hydrogen bonds, namely in the Z-bond, and (2) the unique semiordered structure and properties of ultrathin films, specifically regarding the quasi-liquid. In accordance with the aforementioned theoretical findings, many cutting-edge applications have been proposed: for example, CO₂ capture and conversion, biomass conversion and utilization, and energy storage materials. Although substantial progress has been made recently in the field of ILs, considerable challenges remain in understanding the nature of and devising applications for ILs, especially in terms of e.g. in situ/real-time observation and highly precise multiscale simulations of the Z-bond and quasi-liquid. In this Perspective, we review recent developments and challenges for the IL research community and provide insights into the nature and function of ILs, which will facilitate future applications.

Anti-Electrostatic Pi-Hole Bonding: How Covalency Conquers Coulombics

Intermolecular bonding attraction at π-bonded centers is often described as "electrostatically driven" and given quasi-classical rationalization in terms of a "pi hole" depletion region in the electrostatic potential. However, we demonstrate here that such bonding attraction also occurs between closed-shell ions of like charge, thereby yielding locally stable complexes that sharply violate classical electrostatic expectations. Standard DFT and MP2 computational methods are employed to investigate complexation of simple pi-bonded diatomic anions (BO-, CN-) with simple atomic anions (H-, F-) or with one another. Such "anti-electrostatic" anion-anion attractions are shown to lead to robust metastable binding wells (ranging up to 20-30 kcal/mol at DFT level, or still deeper at dynamically correlated MP2 level) that are shielded by broad predissociation barriers (ranging up to 1.5 Å width) from long-range ionic dissociation. Like-charge attraction at pi-centers thereby provides additional evidence for the dominance of 3-center/4-electron (3c/4e) nD-π*AX interactions that are fully analogous to the nD-σ*AH interactions of H-bonding. Using standard keyword options of natural bond orbital (NBO) analysis, we demonstrate that both n-σ* (sigma hole) and n-π* (pi hole) interactions represent simple variants of the essential resonance-type donor-acceptor (Bürgi-Dunitz-type) attraction that apparently underlies all intermolecular association phenomena of chemical interest. We further demonstrate that "deletion" of such π*-based donor-acceptor interaction obliterates the characteristic Bürgi-Dunitz signatures of pi-hole interactions, thereby establishing the unique cause/effect relationship to short-range covalency ("charge transfer") rather than envisioned Coulombic properties of unperturbed monomers.

Thermodynamically Stable Cationic Dimers in Carboxyl-Functionalized Ionic Liquids: The Paradoxical Case of "Anti-Electrostatic" Hydrogen Bonding

We show that carboxyl-functionalized ionic liquids (ILs) form doubly hydrogen-bonded cationic dimers (c⁺=c⁺) despite the repulsive forces between ions of like charge and competing hydrogen bonds between cation and anion (c⁺–a⁻). This structural motif as known for formic acid, the archetype of double hydrogen bridges, is present in the solid state of the IL 1−(carboxymethyl)pyridinium bis(trifluoromethylsulfonyl)imide [HOOC−CH₂−py][NTf₂]. By means of quantum chemical calculations, we explored different hydrogen-bonded isomers of neutral (HOOC–(CH₂)_n–py⁺)₂(NTf₂⁻)₂, single-charged (HOOC–(CH₂)_n–py⁺)₂(NTf₂⁻), and double-charged (HOOC– (CH₂)_n−py⁺)₂ complexes for demonstrating the paradoxical case of “anti-electrostatic” hydrogen bonding (AEHB) between ions of like charge. For the pure doubly hydrogen-bonded cationic dimers (HOOC– (CH₂)_n−py⁺)₂, we report robust kinetic stability for n = 1–4. At n = 5, hydrogen bonding and dispersion fully compensate for the repulsive Coulomb forces between the cations, allowing for the quantification of the two equivalent hydrogen bonds and dispersion interaction in the order of 58.5 and 11 kJmol⁻¹, respectively. For n = 6–8, we calculated negative free energies for temperatures below 47, 80, and 114 K, respectively. Quantum cluster equilibrium (QCE) theory predicts the equilibria between cationic monomers and dimers by considering the intermolecular interaction between the species, leading to thermodynamic stability at even higher temperatures. We rationalize the H-bond characteristics of the cationic dimers by the natural bond orbital (NBO) approach, emphasizing the strong correlation between NBO-based and spectroscopic descriptors, such as NMR chemical shifts and vibrational frequencies.