Solvent-Free Mechanosynthesis of Oligopeptides by Coupling Peptide Segments of Different Lengths - Elucidating the Role of Cesium Carbonate in Ball Mill Processes

We report an idea for the synthesis of oligopeptides using a solvent-free ball milling approach. Our concept is inspired by block play, in which it is possible to construct different objects using segments (blocks) of different sizes and lengths. We prove that by having a library of short peptides and employing the ball mill mechanosynthesis (BMMS) method, peptides can be easily coupled to form different oligopeptides with the desired functional and biological properties. Optimizing the BMMS process we found that the best yields we obtained when TBTU and cesium carbonate were used as reagents. The role of Cs2CO3 in the coupling mechanism was followed on each stage of synthesis by 1H, 13C and 133Cs NMR employing Magic Angle Spinning (MAS) techniques. It was found that cesium carbonate acts not only as a base but is also responsible for the activation of substrates and intermediates. The unique information about the BMMS mechanism is based on the analysis of 2D NMR data. The power of BMMS is proved by the example of different peptide combinations, 2+2, 3+2, 4+2, 5+2 and 4+4. The tetra-, penta-, hexa-, hepta- and octapeptides obtained under this project were fully characterized by MS and NMR techniques.

Complementary mechanochemical and biphasic approaches for the synthesis of organic thiocyanates using hexacyanoferrates as non-toxic cyanide sources

Herein, we describe two complementary approaches towards various organic thiocyanates that are affordable, reliable and follow the principles of sustainable chemistry, starting from commercially available thiols or disulfides. Additionally, the application of this mild method to the first total synthesis of psammaplin B is demonstrated. Non-toxic and inexpensive ferricyanide is used as the cyanide source, which can be activated either in a mechanochemical, solvent-free approach, or in a biphasic solvent system allowing easier work-up. A total of 27 examples is demonstrated, with up to quantitative yields.

Frontiers in Prebiotic Chemistry and Early Earth Environments

The Prebiotic Chemistry and Early Earth Environments (PCE₃) Consortium is a community of researchers seeking to understand the origins of life on Earth and in the universe. PCE₃ is one of five Research Coordination Networks (RCNs) within NASA’s Astrobiology Program. Here we report on the inaugural PCE₃ workshop, intended to cross-pollinate, transfer information, promote cooperation, break down disciplinary barriers, identify new directions, and foster collaborations. This workshop, entitled, “Building a New Foundation”, was designed to propagate current knowledge, identify possibilities for multidisciplinary collaboration, and ultimately define paths for future collaborations. Presentations addressed the likely conditions on early Earth in ways that could be incorporated into prebiotic chemistry experiments and conceptual models to improve their plausibility and accuracy. Additionally, the discussions that followed among workshop participants helped to identify within each subdiscipline particularly impactful new research directions. At its core, the foundational knowledge base presented in this workshop should underpin future workshops and enable collaborations that bridge the many disciplines that are part of PCE₃.

Chirality in Organic and Mineral Systems: A Review of Reactivity and Alteration Processes Relevant to Prebiotic Chemistry and Life Detection Missions

Chirality is a central feature in the evolution of biological systems, but the reason for biology’s strong preference for specific chiralities of amino acids, sugars, and other molecules remains a controversial and unanswered question in origins of life research. Biological polymers tend toward homochiral systems, which favor the incorporation of a single enantiomer (molecules with a specific chiral configuration) over the other. There have been numerous investigations into the processes that preferentially enrich one enantiomer to understand the evolution of an early, racemic, prebiotic organic world. Chirality can also be a property of minerals; their interaction with chiral organics is important for assessing how post-depositional alteration processes could affect the stereochemical configuration of simple and complex organic molecules. In this paper, we review the properties of organic compounds and minerals as well as the physical, chemical, and geological processes that affect organic and mineral chirality during the preservation and detection of organic compounds. We provide perspectives and discussions on the reactions and analytical techniques that can be performed in the laboratory, and comment on the state of knowledge of flight-capable technologies in current and future planetary missions, with a focus on organics analysis and life detection.

Potassium at the Origins of Life: Did Biology Emerge from Biotite in Micaceous Clay?

Intracellular potassium concentrations, [K+], are high in all types of living cells, but the origins of this K+ are unknown. The simplest hypothesis is that life emerged in an environment that was high in K+. One such environment is the spaces between the sheets of the clay mineral mica. The best mica for life's origins is the black mica, biotite, because it has a high content of Mg++ and because it has iron in various oxidation states. Life also has many of the characteristics of the environment between mica sheets, giving further support for the possibility that mica was the substrate on and within which life emerged. Here, a scenario for life's origins is presented, in which the necessary processes and components for life arise in niches between mica sheets; vesicle membranes encapsulate these processes and components; the resulting vesicles fuse, forming protocells; and eventually, all of the necessary components and processes are encapsulated within individual cells, some of which survive to seed the early Earth with life. This paper presents three new foci for the hypothesis of life's origins between mica sheets: (1) that potassium is essential for life's origins on Earth; (2) that biotite mica has advantages over muscovite mica; and (3) that micaceous clay is a better environment than isolated mica for life's origins.

Mechanochemical Solvent-Free Suzuki-Miyaura Cross-Coupling of Amides via Highly Chemoselective N-C Cleavage

Although cross-coupling reactions of amides by selective N-C cleavage are one of the most powerful and burgeoning areas in organic synthesis due to the ubiquity of amide bonds, the development of mechanochemical, solid-state methods remains a major challenge. Herein, we report the first mechanochemical strategy for highly chemoselective, solvent-free palladium-catalyzed cross-coupling of amides by N-C bond activation. The method is conducted in the absence of external heating, for short reaction time and shows excellent chemoselectivity for σ N-C bond activation. The reaction shows excellent functional group tolerance and can be applied to late-stage functionalization of complex APIs and sequential orthogonal cross-couplings exploiting double solventless solid-state methods. The results extend mechanochemical reaction environments to advance the chemical repertoire of N-C bond interconversions to solid-state environmentally friendly mechanochemical methods.

Direct Amidation of Esters by Ball Milling*

The direct mechanochemical amidation of esters by ball milling is described. The operationally simple procedure requires an ester, an amine, and substoichiometric KOtBu and was used to prepare a large and diverse library of 78 amide structures with modest to excellent efficiency. Heteroaromatic and heterocyclic components are specifically shown to be amenable to this mechanochemical protocol. This direct synthesis platform has been applied to the synthesis of active pharmaceutical ingredients (APIs) and agrochemicals as well as the gram-scale synthesis of an active pharmaceutical, all in the absence of a reaction solvent.

Sustainability Assessment of Mechanochemistry by Using the Twelve Principles of Green Chemistry

In recent years, mechanochemistry has been growing into a widely accepted alternative for chemical synthesis. In addition to their efficiency and practicality, mechanochemical reactions are also recognized for their sustainability. The association between mechanochemistry and Green Chemistry often originates from the solvent-free nature of most mechanochemical protocols, which can reduce waste production. However, mechanochemistry satisfies more than one of the Principles of Green Chemistry. In this Review we will present a series of examples that will clearly illustrate how mechanochemistry can significantly contribute to the fulfillment of Green Chemistry in a more holistic manner.

Mechanochemical Prebiotic Peptide Bond Formation*

The presence of amino acids on the prebiotic Earth, either stemming from endogenous chemical routes or delivered by meteorites, is consensually accepted. Prebiotically plausible pathways to peptides from inactivated amino acids are still unclear as most oligomerization approaches rely on thermodynamically disfavored reactions in solution. Now, a combination of prebiotically plausible minerals and mechanochemical activation enables the oligomerization of glycine at ambient temperature in the absence of water. Raising the reaction temperature increases the degree of oligomerization concomitantly with the formation of a commonly unwanted cyclic glycine dimer (DKP). However, DKP is a productive intermediate in the mechanochemical oligomerization of glycine. The findings of this research show that mechanochemical peptide bond formation is a dynamic process that provides alternative routes towards oligopeptides and establishes new synthetic approaches for prebiotic chemistry.

Petasis vs. Strecker Amino Acid Synthesis: Convergence, Divergence and Opportunities in Organic Synthesis

α-Amino acids find widespread applications in various areas of life and physical sciences. Their syntheses are carried out by a multitude of protocols, of which Petasis and Strecker reactions have emerged as the most straightforward and most widely used. Both reactions are three-component reactions using the same starting materials, except the nucleophilic species. The differences and similarities between these two important reactions are highlighted in this review.

Electro-Mechanochemical Atom Transfer Radical Cyclizations using Piezoelectric BaTiO3

The formation and regeneration of active CuI species is a fundamental mechanistic step in copper-catalyzed atom transfer radical cyclizations (ATRC). Typically, the presence of the catalytically active CuI species in the reaction mixture is secured by using high CuI catalyst loadings or the addition of complementary reducing agents. In this study it is demonstrated how the piezoelectric properties of barium titanate (BaTiO3 ) can be harnessed by mechanical ball milling to induce electrical polarization in the strained piezomaterial. This strategy enables the conversion of mechanical energy into electrical energy, leading to the reduction of a CuII precatalyst into the active CuI species in copper-catalyzed mechanochemical solvent-free ATRC reactions.

Mechanochemical Synthesis of Short DNA Fragments

We demonstrate the first mechanochemical synthesis of DNA fragments by ball milling, enabling the synthesis of oligomers of controllable sequence and length using multi-step, one-pot reactions, without bulk solvent or the need to isolate intermediates. Mechanochemistry allowed for coupling of phosphoramidite monomers to the 5'-hydroxyl group of nucleosides, iodine/water oxidation of the resulting phosphite triester linkage, and removal of the 5'-dimethoxytrityl (DMTr) protecting group in situ in good yields (up to 60 % over three steps) to produce DNA dimers in a one-pot manner. H-Phosphonate chemistry under milling conditions enabled coupling and protection of the H-phosphonate linkage, as well as removal of the 5'-DMTr protecting group in situ, enabling a one-pot process with good yields (up to 65 % over three steps, or ca. 87 % per step). Sulfurization of the internucleotide linkage was possible using elemental sulfur (S8) or sulfur transfer reagents, yielding the target DNA phosphorothioate dimers in good yield (up to 80 % over two steps). This work opens the door to creation of solvent-free synthesis methodologies for DNA and RNA therapeutics.

Mechanochemical Palladium-Catalyzed Carbonylative Reactions Using Mo(CO)6

Esters and amides were mechanochemically prepared by palladium-catalyzed carbonylative reactions of aryl iodides by using molybdenum hexacarbonyl as a convenient solid carbonyl source and avoiding a direct handling of gaseous carbon monoxide. Real-time monitoring of the mechanochemical reaction by in situ pressure sensing revealed that CO is rapidly transferred from Mo(CO)6 to the active catalytic system without significant release of molecular carbon monoxide.

A Mechanochemical-Assisted Synthesis of Boron, Nitrogen Co-Doped Porous Carbons as Metal-Free Catalysts

A green and convenient solid-state method assisted by mechanical energy is employed for the synthesis of boron (B) and nitrogen (N) co-doped porous carbons (B,N-Cs). Glutamic acid (Glu) and boric acid (H₃ BO₃ ) are used as the N-containing carbon precursor and boron source, respectively. This method is easy to perform and proved to be efficient towards co-doping B and N into the carbon matrix with high contents of B (7 atom %) and N (10 atom %). By adjusting the molar ratio of H₃ BO₃ to Glu, the surface chemical states of B and N could be readily modulated. When increasing H₃ BO₃ dosage, the pore size of B,N-Cs could be tuned ranging from micropores to mesopores with a Brunauer-Emmett-Teller (BET) surface area up to 940 m²  g^-1 . Finally, the B,N-Cs were applied as metal-free catalysts for the cycloaddition of CO₂ to epoxides, which outperform the N-doped carbon catalyst (NC-900) and the physically mixed catalyst of NC-900/B₄ C. The enhanced activity is attributed to the cooperative effect between B and N sites. X-ray photoelectron spectroscopy (XPS) analysis reveals that BN₃ in the B,N-Cs serves as a critical active site for the cooperative catalysis.

Preparation, physicochemical and pharmacological study of curcumin solid dispersion with an arabinogalactan complexation agent

Pharmaceutical solid dispersions (SD) of curcumin (Cur) with macromolecule polysaccharide arabinogalactan (AG) from wood of Larix sibirica were prepared by mechanical ball milling. The physical properties of the dispersed curcumin mixture in solid state were characterized by scanning electron microscope, differential scanning calorimetry and powder X-ray diffraction studies. These methods showed a strong decrease in the degree of crystallinity of Cur and its transformation to amorphization state, accompanied by the formation of the guest-host type complexes. The behavior of the samples in solutions was characterized by reverse phase HPLC, ¹H NMR spectroscopy, UV-Visible spectroscopy and gel permeation chromatography (GPC). Mechanochemically prepared complexes demonstrated the increased solubility of Cur up to ~10.5 times in contrast to pure curcumin. The rapid storage test showed high chemical stability of Cur, which depended on mass relations of Cur-AG. Besides, improved membrane permeability of Cur-AG SD was tested by parallel artificial membrane permeability assay. Pharmacokinetic study of Cur-AG SD formulation in rat demonstrated a significant~8-fold enhancement of bioavailability in comparison to pure curcumin. In MTT tests, Cur-AG SD showed moderate cytotoxicity against human glioblastoma cells and immortalized human fibroblasts. Therefore, Cur-AG solid dispersion was a more promising and efficacious formulation for application in oral drug delivery.

Switching Chemoselectivity: Using Mechanochemistry to Alter Reaction Kinetics

A reaction manifold has been discovered in which the chemoselectivity can be altered by switching between neat milling and liquid assisted grinding (LAG) with polar additives. After investigation of the reaction mechanism, it has been established that this switching in reaction pathway is due to the neat mechanochemical conditions exhibiting different kinetics for a key step in the transformation. This proof of concept study demonstrates that mechanochemistry can be used to trap the kinetic product of a reaction. It is envisaged that, if this concept can be successfully applied to other transformations, novel synthetic processes could be discovered and known reaction pathways perturbed or diverted.

Photoredox-Catalyzed Cα-H Cyanation of Unactivated Secondary and Tertiary Aliphatic Amines: Late-Stage Functionalization and Mechanistic Studies

This paper describes the development and mechanistic studies of a general, high-yielding amine C_α-H cyanation protocol via photoredox catalysis. Inexpensive NaCN is employed as the cyanide source and air is the external oxidant, resulting in mild and highly functional group tolerant conditions. Notably, efficient C_α-H cyanations of secondary and tertiary aliphatic amines and of complex, biologically active compounds (drugs) can be performed using the established methodology. Mechanistic studies suggest that the carboxylic acid additive has three effects: formation of a stabilizing hemiaminal intermediate, prevention of catalyst decomposition by protonating the substrate, and modulation of fluorescence quenching of the photoexcited catalyst species.

From Synthesis of Amino Acids and Peptides to Enzymatic Catalysis: A Bottom-Up Approach in Mechanochemistry

Recently, chemical reactions induced or facilitated by mechanical energy have gained recognition in diverse areas of chemical synthesis. In particular, mechanosyntheses of amino acids and short peptides, along with their applications in catalysis, have revealed the high degree of stability of peptide bonds in environments of harsh mechanical stress. These observations quickly led to the recent interest in developing mechanochemical enzymatic reactions. Experimentally, manual grinding, ball-milling techniques, and twin-screw extrusion technology have proven valuable to convey mechanical forces into a chemical synthesis. These practices have enabled the establishment of more sustainable alternatives for chemical synthesis by reducing the use of organic solvents and waste production, thereby having a direct impact on the E-factor of the chemical process. In this Minireview, the series of events that allowed the development of mechanochemical enzymatic reactions are described from a bottom-up perspective.