Adamantane-type clusters: compounds with a ubiquitous architecture but a wide variety of compositions and unexpected materials properties

The research into adamantane-type compounds has gained momentum in recent years, yielding remarkable new applications for this class of materials. In particular, organic adamantane derivatives (AdR4) or inorganic adamantane-type compounds of the general formula [(RT)4E6] (R: organic substituent; T: group 14 atom C, Si, Ge, Sn; E: chalcogenide atom S, Se, Te, or CH2) were shown to exhibit strong nonlinear optical (NLO) properties, either second-harmonic generation (SHG) or an unprecedented type of highly-directed white-light generation (WLG) - depending on their respective crystalline or amorphous nature. The (missing) crystallinity, as well as the maximum wavelengths of the optical transitions, are controlled by the clusters' elemental composition and by the nature of the organic groups R. Very recently, it has been additionally shown that cluster cores with increased inhomogeneity, like the one in compounds [RSi{CH2Sn(E)R'}3], not only affect the chemical properties, such as increased robustness and reversible melting behaviour, but that such 'cluster glasses' form a conceptually new basis for their use in light conversion devices. These findings are likely only the tip of the iceberg, as beside elemental combinations including group 14 and group 16 elements, many more adamantane-type clusters (on the one hand) and related architectures representing extensions of adamantane-type clusters (on the other hand) are known, but have not yet been addressed in terms of their opto-electronic properties. In this review, we therefore present a survey of all known classes of adanmantane-type compounds and their respective synthetic access as well as their optical properties, if reported.

One-Stage Catalytic Oxidation of Adamantane to Tri-, Tetra-, and Penta-Ols

Tertiary tetraols of adamantane (C10H16, Tricyclo[3.3.1.1(3,7)]decan) have been widely used for the synthesis of highly symmetric compounds with unique physical and chemical properties. The methods for one-stage simultaneously selective, deep, and cheap oxidation of adamantane to tetraols of different structures have not yet been developed. In this research, chemically simple, cheap, and environmentally friendly reagents are used and that is the first step in this direction. The conditions, under which the impact of a hydrogen peroxide water solution on adamantane dissolved in acetonitrile results in full conversion of adamantane and formation of a total 72% mixture of its tri-, tetra-, and penta-oxygenated products, predominantly poliols, have been found. Conversion and adamantane oxidation depth are shown to depend on the ratio of components of the water-acetonitrile solution and the method of oxidizer solution introduction when using the dimer form of 1:1 dimethylglyoxime and copper dichloride complex as a catalyst. Under the conditions of mass-spectrometry ionization by electrons (70 eV), fragmentation across three C–C bonds of the molecular ions framework of adamantane tertiary alcohols Ad(OH)n in the range n = 0–4 increases linearly with the rise of n.

Hybridization Networks of mRNA and Branched RNA Hybrids

Messenger RNA (mRNA) is emerging as an attractive biopolymer for therapy and vaccination. To become suitable for vaccination, mRNA is usually converted to a biomaterial, using cationic peptides, polymers or lipids. An alternative form of converting mRNA into a material is demonstrated that uses branched oligoribonucleotide hybrids with the ability to hybridize with one or more regions of the mRNA sequence. Two such hybrids with hexamer arms and adamantane tetraol as branching element were prepared by solution-phase synthesis. When a rabies mRNA was treated with the branched hybrids at 1 M NaCl concentration, biomaterials formed that contained both of the nucleic acids. These results show that branched oligoribonucleotides are an alternative to the often toxic reagents commonly used to formulate mRNA for medical applications.

Scalable One-Pot-Liquid-Phase Oligonucleotide Synthesis for Model Network Hydrogels

Solid-phase oligonucleotide synthesis (SPOS) based on phosphoramidite chemistry is currently the most widespread technique for DNA and RNA synthesis but suffers from scalability limitations and high reagent consumption. Liquid-phase oligonucleotide synthesis (LPOS) uses soluble polymer supports and has the potential of being scalable. However, at present, LPOS requires 3 separate reaction steps and 4-5 precipitation steps per nucleotide addition. Moreover, long acid exposure times during the deprotection step degrade sequences with high A content (adenine) due to depurination and chain cleavage. In this work, we present the first one-pot liquid-phase DNA synthesis technique which allows the addition of one nucleotide in a one-pot reaction of sequential coupling, oxidation, and deprotection followed by a single precipitation step. Furthermore, we demonstrate how to suppress depurination during the addition of adenine nucleotides. We showcase the potential of this technique to prepare high-purity 4-arm PEG-T20 (T = thymine) and 4-arm PEG-A20 building blocks in multigram scale. Such complementary 4-arm PEG-DNA building blocks reversibly self-assemble into supramolecular model network hydrogels and facilitate the elucidation of bond lifetimes. These model network hydrogels exhibit new levels of mechanical properties (storage modulus, bond lifetimes) in DNA bonds at room temperature (melting at 44 °C) and thus open up pathways to next-generation DNA materials programmable through sequence recognition and available for macroscale applications.

Non-porous organic crystals and their interaction with guest molecules from the gas phase

Some organic molecules encapsulate solvents upon crystallization. One class of compounds that shows a high propensity to form such crystalline solvates are tetraaryladamantanes (TAAs). Recently, tetrakis(dialkoxyphenyl)-adamantanes have been shown to encapsulate a wide range of guest molecules in their crystals, and to stabilize the guest molecules against undesired reactions. The term ‘encapsulating organic crystals’ (EnOCs) has been coined for these species. In this work, we studied the behavior of three TAAs upon exposition to different guest molecules by means of sorption technique. We firstly measured the vapor adsorption/desorption isotherms with water, tetrahydrofuran and toluene, and secondly, we studied the uptake of methane on dry and wet TAAs. Uptake of methane beyond one molar equivalent was detected for wet crystals, even though the materials showed a lack of porosity. Thus far, such behavior, which we ascribe to methane hydrate formation, had been described for porous non-crystalline materials or crystals with detectable porosity, not for non-porous organic crystals. Our results show that TAA crystals have interesting properties beyond the formation of conventional solvates. Gas-containing organic crystals may find application as reservoirs for gases that are difficult to encapsulate or are slow to form crystalline hydrates in the absence of a host compound.

Wet tetraaryladamantane crystals take up methane in form of methane hydrate structure I, even though they appear non-porous to argon.

Turning DNA Binding Motifs into a Material for Flow Cells

Nanoscale assemblies of DNA strands are readily designed and can be generated in a wide range of shapes and sizes. Turning them into solids that bind biomolecules reversibly, so that they can act as active material in flow cells, is a challenge. Among the biomolecular ligands, cofactors are of particular interest because they are often the most expensive reagents of biochemical transformations, for which controlled release and recycling are desirable. We have recently described DNA triplex motifs that bind adenine-containing cofactors, such as NAD, FAD and ATP, reversibly with low micromolar affinity. We sought ways to convert the soluble DNA motifs into a macroporous solid for cofactor binding. While assemblies of linear and branched DNA motifs produced hydrogels with undesirable properties, long DNA triplexes treated with protamine gave materials suitable for flow cells. Using exchangeable cells in a flow system, thermally controlled loading and discharge were demonstrated. Employing a flow cell loaded with ATP, bioluminescence was induced through thermal release of the cofactor. The results show that materials generated from functional DNA structures can be successfully employed in macroscopic devices.

Building expanded structures from tetrahedral DNA branching elements, RNA and TMV protein

By combining both chemical and enzymatic ligation with procedures guiding the self-assembly of nanotubular tobacco mosaic virus (TMV)-like particles (TLPs), novel nucleoprotein structures based on DNA-terminated branching elements, RNA scaffolds and TMV coat protein (CP) are made accessible. Tetrahedral tetrakis(hydroxybiphenyl)adamantane cores with four 5'-phosphorylated dinucleotide arms were coupled to DNA linkers by chemical ligation. The resulting three-dimensional (3D) branching elements were enzymatically ligated to the 3' termini of RNA scaffolds either prior to or after the RNAs' incorporation into TLPs. Thus, architectures with interconnected nanotube domains in two different length classes were generated, each containing 70 CP subunits per 10 nm length. Short TMV origin-of-assembly-containing RNA scaffolds ligated to the DNA allowed the growth of protein-coated 34 nm tubes on the terminal RNA strands in situ. Alternatively, 290 nm pre-fabricated tubes with accessible RNA 3' termini, attained by DNA blocking elements hybridized to the RNAs, were ligated with the branched cores. Both approaches resulted in four-armed nanoobjects, demonstrating a so far unique combination of organic synthesis of branching elements, enzymatic modifications, nucleic acid-based scaffolding and RNA-guided and DNA-controlled assembly of tubular RNA-encapsidating protein domains, yielding a novel class of 3D nucleoprotein architectures with polyvalent protein elements. In the long term, the production route might give rise to supramolecular systems with complex functionalities, installed via the orthogonal coupling of effector molecules to TLP domains.

Encapsulating Active Pharmaceutical Ingredients in Self-Assembling Adamantanes with Short DNA Zippers

Formulating pharmaceutically active ingredients for drug delivery is a challenge. There is a need for new drug delivery systems that take up therapeutic molecules and release them into biological systems. We propose a novel mode of encapsulation that involves matrices formed through co-assembly of drugs with adamantane hybrids that feature four CG dimers as sticky ends. Such adamantanes are accessible via inexpensive solution-phase syntheses, and the resulting materials show attractive properties for controlled release. This is demonstrated for two different hybrids and a series of drugs, including anticancer drugs, antibiotics, and cyclosporin. Up to 20 molar equivalents of active pharmaceutical ingredients (APIs) are encapsulated in hybrid materials. Encapsulation is demonstrated for DNA-binding and several non-DNA binding compounds. Nanoparticles were detected that range in size from 114-835 nm average diameter, and ζ potentials were found to be between -29 and +28 mV. Release of doxorubicin into serum at near-constant rates for 10 days was shown, demonstrating the potential for slow release. The encapsulation and release in self-assembling matrices of dinucleotide-bearing adamantanes appears to be broadly applicable and may thus lead to new drug delivery systems for APIs.