Synthesis and Anticancer and Antiviral Activities of C-2'-Branched Arabinonucleosides.

d-Arabinofuranosyl-pyrimidine and -purine nucleoside analogues containing alkylthio-, acetylthio- or 1-thiosugar substituents at the C2' position were prepared from the corresponding 3',5'-O-silylene acetal-protected nucleoside 2'-exomethylenes by photoinitiated, radical-mediated hydrothiolation reactions. Although the stereochemical outcome of the hydrothiolation depended on the structure of both the thiol and the furanoside aglycone, in general, high d-arabino selectivity was obtained. The cytotoxic effect of the arabinonucleosides was studied on tumorous SCC (mouse squamous cell) and immortalized control HaCaT (human keratinocyte) cell lines by MTT assay. Three pyrimidine nucleosides containing C2'-butylsulfanylmethyl or -acetylthiomethyl groups showed promising cytotoxicity at low micromolar concentrations with good selectivity towards tumor cells. SAR analysis using a methyl ß-d-arabinofuranoside reference compound showed that the silyl-protecting group, the nucleobase and the corresponding C2' substituent are crucial for the cell growth inhibitory activity. The effects of the three most active nucleoside analogues on parameters indicative of cytotoxicity, such as cell size, division time and cell generation time, were investigated by near-infrared live cell imaging, which showed that the 2'-acetylthiomethyluridine derivative induced the most significant functional and morphological changes. Some nucleoside analogues also exerted anti-SARS-CoV-2 and/or anti-HCoV-229E activity with low micromolar EC50 values; however, the antiviral activity was always accompanied by significant cytotoxicity.

Nano-3D-Printed Photochromic Micro-Objects.

Molecular photoswitches that can reversibly change color upon irradiation are promising materials for applications in molecular actuation and photoresponsive materials. However, the fabrication of photochromic devices is limited to conventional approaches such as mold casting and spin-coating, which cannot fabricate complex structures. Reported here is the first photoresist for direct laser writing of photochromic 3D micro-objects via two-photon polymerization. The integration of photochromism into thiol-ene photo-clickable resins enables rapid two-photon laser processing of highly complex microstructures and facile postmodification using a series of donor-acceptor Stenhouse adduct (DASA) photoswitches with different excitation wavelengths. The versatility of thiol-ene photo-click reactions allows fine-tuning of the network structure and physical properties as well as the type and concentration of DASA. When exposed to visible light, these microstructures exhibit excellent photoresponsiveness and undergo reversible color-changing via photoisomerization. It is demonstrated that the fluorescence variations of DASAs can be used as a reporter of photoswitching and thermal recovery, allowing the reading of DASA-containing sub-micrometric structures in 3D. This work delivers a new approach for custom microfabrication of 3D photochromic objects with molecularly engineered color and responsiveness.

Hybridization of Step-/Chain-Growth and Radical/Cationic Polymerizations Using Thioacetals as Key Components for Triblock, Periodic and Random Multiblock Copolymers with Thermoresponsiveness.

A novel strategy for synthesizing a series of multiblock copolymers is developed by combining radical/cationic step-growth polymerizations of dithiols and divinyl ethers and chain-growth cationic degenerative chain-transfer (DT) polymerizations of vinyl ethers using thioacetals as key components. The combination of radical step-growth polymerization and a cationic thiol-ene reaction or cationic step-growth polymerization enables the synthesis of a series of macro chain-transfer agents (CTAs) composed of poly(thioether) and thioacetal groups at different positions. The resulting products are 1) bifunctional macro CTAs with thioacetal groups at both chain ends, 2) periodic macro CTAs periodically having thioacetal groups in the main chain, and 3) random macro CTAs randomly having thioacetal groups in the main chain. Subsequently, the obtained macro CTAs are used for chain-growth cationic DT polymerization of methoxyethyl vinyl ether (MOVE) to result in 1) triblock, 2) periodic, and 3) random multiblock copolymers consisting of poly(thioether) and poly(MOVE) segments. All these triblock and multiblock copolymers composed of hydrophobic poly(thioether) and hydrophilic poly(MOVE) segments show an amphiphilic tendency to form characteristic micelles in aqueous solutions. In addition, due to the thermoresponsive poly(MOVE) segments, the obtained copolymers exhibit lower critical solution temperatures that depend on the segment sequences and lengths.

Luminescent and Robust Perovskite-Silicone Elastomers Prepared by Light Induced Thiol-Ene Reaction.

The preparation of a series of luminescent perovskite-silicone elastomer (PSE) composites by embedding inorganic lead halide perovskite nanocrystals (CsPbBr3 NCs) into networks constructed by trimethylolpropane tris(2-mercaptoacetate) and sulfone-containing silicone copolymers with vinyl side groups (PSMVS) is reported herein. The networks are obtained by an environmentally friendly thiol-ene cross-linking reaction under 30 W household LED light. The conducted analysis shows that the prepared PSEs display strong green fluorescence due to encapsulation of CsPbBr3 NCs, which constitute a luminescent center in sulfone-containing silicone networks. Using PSMVS as basic polymers instead of commercial polysiloxanes endows PSEs with enhanced mechanical strength and excellent luminescent stability at high temperatures. The PSEs show robust tensile stress and >650% elongation. Additionally, the construction of colorful ultraviolet light-emitting diodes (UV-LEDs) by an in situ cross-linking process is described.

The Photocatalyzed Thiol-ene reaction: A New Tag to Yield Fast, Selective and reversible Paramagnetic Tagging of Proteins.

Paramagnetic restraints have been used in biomolecular NMR for the last three decades to elucidate and refine biomolecular structures, but also to characterize protein-ligand interactions. A common technique to generate such restraints in proteins, which do not naturally contain a (paramagnetic) metal, consists in the attachment to the protein of a lanthanide-binding-tag (LBT). In order to design such LBTs, it is important to consider the efficiency and stability of the conjugation, the geometry of the complex (conformational exchanges and coordination) and the chemical inertness of the ligand. Here we describe a photo-catalyzed thiol-ene reaction for the cysteine-selective paramagnetic tagging of proteins. As a model, we designed an LBT with a vinyl-pyridine moiety which was used to attach our tag to the protein GB1 in fast and irreversible fashion. Our tag T1 yields magnetic susceptibility tensors of significant size with different lanthanides and has been characterized using NMR and relaxometry measurements.

Enhancement of the Insulation Properties of Poly(2-oxazoline)-co-Polyester Networks by the Addition of Nanofillers.

Copoly(2-nonyl-2-oxazoline)-stat-poly(2-dec-9'enyl-2-oxazoline)s can be crosslinked by the thiol-ene reaction with glycol dimercaptoacetate. The copoly(2-oxazoline)-stat-copolyester is tested as dielectric for high-voltage applications, either as unfilled resin or as composite with nanoscaled fillers of silica, alumina, and hexagonal boron nitride. During AC voltage tests, all materials have an average breakdown strength of 45-50 kV mm-1 . For DC voltage tests, samples with SiO2 (hBN) have an average breakdown strength of ≈100 (80) kV mm-1 , while the unfilled copoly(2-oxazoline) has an average breakdown strength of ≈60 kV mm-1 . Permittivity measurements at 20 °C and 50 Hz reveal that all nanocomposites are dielectrics (D = 0.06-0.08), while the unfilled copoly(2-oxazoline)s has a high loss factor of D = 8.43. This phenomenon can be retraced to the phase separation in the crosslinked copolymer, the M-OH functionality of silica and alumina particles, and models of polymer-particle interactions such as the Tanaka model, revealing that the nanofillers reduce the interfacial and dipolar polarizability.

In situ-forming hyaluronic acid hydrogel through visible light-induced thiol-ene reaction.

Here we present hyaluronic acid (HA) hydrogels crosslinked via thiol-ene reaction initiated by visible blue light exposure in the presence of riboflavin phosphate (RFP). The gelation procedure is rapid and proceeds as effectively with exposure to blue light as it does with UV light. We successfully initiated the thiol-ene reaction by RFP with blue light, which triggered gelation that proceeds over about 5 min at 36 °C after an initial small change in modulus upon light exposure. Gel transparency was also evaluated, and the HA gel exhibited over 80% transmittance in the visible spectrum. The degradation and protein release kinetics of the photo-crosslinked HA hydrogel are also presented. The capacity of blue light to initiate thiol-ene reaction was equal to or more effective than UV light of the same energy. The cytocompatibility of hydrogels was evaluated using corneal fibroblasts, and the light-induced fabrication procedure and resultant gel materials did not affect cell viability. The results indicate that an RFP-based, BL-initiated photo-reaction to gelate HA may be an effective and promising modality for applications where in situ gelation is desired.

Rapid Preparation of Silsesquioxane-Based Ionic Liquids.

Three new hybrid ionic liquids (ILs) based on cage silsesquioxane (SQ) were rapidly prepared in high yields from octa(mercaptopropyl)silsesquioxane and 1-allyl-3-methylimidazolium salts (Br(-), BF4(-), PF6(-)) through the photochemical thiol-ene reaction. These SQ-based ILs exhibited low glass transition temperatures and good thermal stability. The unique amphiphilic nature of these hybrid ILs cause them to self-assemble into perfect vesicles with "yolk-shell" structures, in which cages formed the "yolk" due to their aggregation and outer anions formed the "shell".

Constructing thioether-tethered cyclic peptides via on-resin intra-molecular thiol-ene reaction.

Thiol-ene reactions have been used in a variety of applications that mostly involve an inter-molecular pathway. Herein, we report a facile method to construct thioether-tethered cyclic peptides via an intra-molecular thiol-ene reaction. This reaction is efficient, selective, and has good residue compatibility. Short peptides with thioether tethers were constructed and were used to construct longer cyclic peptides. This synthetic method may be useful for constructing bioactive peptides. Copyright © 2016 European Peptide Society and John Wiley & Sons, Ltd.

4D Multimaterial Printing of Soft Actuators with Spatial and Temporal Control.

Soft actuators (SAs) are devices which can interact with delicate objects in a manner not achievable with traditional robotics. While it is possible to design a SA whose actuation is triggered via an external stimulus, the use of a single stimulus creates challenges in the spatial and temporal control of the actuation. Herein, a 4D printed multimaterial soft actuator design (MMSA) whose actuation is only initiated by a combination of triggers (i.e., pH and temperature) is presented. Using 3D printing, a multilayered soft actuator with a hydrophilic pH-sensitive layer, and a hydrophobic magnetic and temperature-responsive shape-memory polymer layer, is designed. The hydrogel responds to environmental pH conditions by swelling or shrinking, while the shape-memory polymer can resist the shape deformation of the hydrogel until triggered by temperature or light. The combination of these stimuli-responsive layers allows for a high level of spatiotemporal control of the actuation. The utility of the 4D MMSA is demonstrated via a series of cargo capture and release experiments, validating its ability to demonstrate active spatiotemporal control. The MMSA concept provides a promising research direction to develop multifunctional soft devices with potential applications in biomedical engineering and environmental engineering.

Direct peptide lipidation through thiol-ene coupling enables rapid synthesis and evaluation of self-adjuvanting vaccine candidates.

A radical lipidation: Application of a novel thiol-ene lipidation enables the one-step synthesis of self-adjuvanting antigenic peptides as vaccine candidates. The resultant monoacyl lipopeptides are shown to activate monocytes in a robust manner.

Thiol-ene conjugation of VEGF peptide to electrospun scaffolds as potential application for angiogenesis.

Vascular endothelial growth factor (VEGF) plays a vital role in promoting attachment and proliferation of endothelial cells, and induces angiogenesis. In recent years, much research has been conducted on functionalization of tissue engineering scaffolds with VEGF or VEGF-mimetic peptide to promote angiogenesis. However, most chemical reactions are nonspecific and require organic solvents, which can compromise control over functionalization and alter peptide/protein activity. An attractive alternative is the fabrication of functionalizable electrospun fibers, which can overcome these hurdles. In this study, we used thiol-ene chemistry for the conjugation of a VEGF-mimetic peptide to the surface of poly (ε-caprolactone) (PCL) fibrous scaffolds with varying amounts of a functional PCL-diacrylate (PCL-DA) polymer. 30% PCL-DA was selected due to homogeneous fiber morphology. A VEGF-mimetic peptide was then immobilized on PCL-DA fibrous scaffolds by a light-initiated thiol-ene reaction. 7-Mercapto-4-methylcoumarin, RGD-FITC peptide and VEGF-TAMRA mimetic peptide were used to validate the thiol-ene reaction with fibrous scaffolds. Tensile strength and elastic modulus of 30% PCL-DA fibrous scaffolds were significantly increased after the reaction. Conjugation of 30% PCL-DA fibrous scaffolds with VEGF peptide increased the surface water wettability of the scaffolds. Patterned structures could be obtained after using a photomask on the fibrous film. Moreover, in vitro studies indicated that scaffolds functionalized with the VEGF-mimetic peptide were able to induce phosphorylation of VEGF receptor and enhanced HUVECs survival, proliferation and adhesion. A chick chorioallantoic membrane (CAM) assay further indicated that the VEGF peptide functionalized scaffolds are able to promote angiogenesis in vivo. These results show that scaffold functionalization can be controlled via a simple polymer mixing approach, and that the functionalized VEGF peptide-scaffolds have potential for vascular tissue regeneration.

Green Chemistry for Crosslinking Biopolymers: Recent Advances in Riboflavin-Mediated Photochemistry.

Riboflavin (RF), which is also known as vitamin B2, is a water-soluble vitamin. RF is a nontoxic and biocompatible natural substance. It absorbs light (at wavelengths of 380 and 450 nm) in the presence of oxygen to form reactive singlet oxygen (1O2). The generated singlet oxygen acts as a photoinitiator to induce the oxidation of biomolecules, such as amino acids, proteins, and nucleotides, or to initiate chemical reactions, such as the thiol-ene reaction and crosslinking of tyramine and furfuryl groups. In this review, we focus on the chemical mechanism and utilization of the photochemistry of RF, such as protein crosslinking and hydrogel formation. Currently, the crosslinking method using RF as a photoinitiator is actively employed in ophthalmic clinics. However, a significant broadening is expected in its range of applications, such as in tissue engineering and drug delivery.

Combined bead polymerization and Cinchona organocatalyst immobilization by thiol-ene addition.

In this work, we report an unusually concise immobilization of Cinchona organocatalysts using thiol-ene chemistry, in which catalyst immobilization and bead polymerization is combined in a single step. A solution of azo initiator, polyfunctional thiol, polyfunctional alkene and an unmodified Cinchona-derived organocatalyst in a solvent is suspended in water and copolymerized on heating by thiol-ene additions. The resultant spherical and gel-type polymer beads have been evaluated as organocatalysts in catalytic asymmetric transformations.

Sulfur-Containing Silsesquioxane Derivatives Obtained by the Thiol-ene Reaction: Synthesis and Thermal Degradation.

This article presents the synthesis of new derivatives of octa(3-thiopropyl)silsesquioxane (SSQ-8SH) via thiol-ene reaction with simple olefins bearing alkyl groups as well as methoxysilyl, substituted aryl or fluoroalkyl groups. All products were characterized by 1 H NMR, 13 C NMR, 29 Si NMR, FTIR, GPC, MALDI-TOF-MS, as well as thermal analysis (TGA) to confirm their structures, purity and thermal stability, and finally give some insight into their thermal degradation pathway. In some of the structures obtained, the T5% values take place at high temperatures and are close to 230 °C. The thiol-ene reaction allowed to obtain 7 new compounds with high yield (>92 %), no by-products and in a relatively short time (24 h). All products are characterized by high conversion >99 %. New derivatives can find potential use as modifiers of plastics to improve their certain properties for example hydrophobicity or thermo-oxidative stability.

Synergistic reinforcing and cross-linking effect of thiol-ene-modified cellulose nanofibrils on natural rubber.

To achieve synergistic reinforcing and cross-linking effect across interface between hydrophilic nanocellulose and hydrophobic rubber, active thiol groups were introduced at reducing end of CNF while retaining hydroxyl groups on the surface, thus forming a percolation network in nanocomposites. The nanocomposites were obtained by casting/evaporating a mixture of dispersed modified CNF and NR in latex form, in which covalent cross-links were formed between thiol groups and double bonds of NR via photochemically initiated thiol-ene reaction. Strong interfacial interaction between NR matrix and end-modified CNF was characterized by Fourier-transform infrared spectroscopy. The structural and mechanical properties of the nanocomposites were evaluated by scanning electron microscopy, dynamic mechanical analysis and tensile tests. Compared to neat NR, the nanocomposite reinforced with 10 wt% modified CNF showed significantly higher values of tensile strength (0.33 to 5.83 MPa), Young's modulus (0.48 to 45.25 MPa) and toughness (2.63 to 22.24 MJ m-3).

A novel chitosan oligosaccharide derivative: Synthesis, antioxidant and antibacterial properties.

A novel thioether chitosan oligosaccharide (COS-All-Tio) was prepared by the reaction of chitosan oligosaccharide (COS) with 3-bromopropene, followed by the coupling with tiopronin (Tio) using a thiol-ene reaction. The degree of substitution of COS-All-Tio reached 1.48. The structure of COS-All-Tio was identified by IR, NMR spectra. It was found that COS-All-Tio possessed more potent antioxidant activities than COS. The IC50 values of COS-All-Tio for scavenging DPPH, ABTS+ and OH were 0.31, 0.39 and 0.73 mg/mL, respectively, while the corresponding values for COS were 0.66, 2.89 and 1.41 mg/mL, respectively. COS-All-Tio was also found to possess much stronger antibacterial effect than COS against five bacteria strains (Staphylococcus aureus, Bacillus subtilis, Listeria monocytogenes, Escherichia coli and Pseudomonas aeruginosa). Further, COS-All-Tio was found to be non-toxic to RAW264.7 macrophages and MRC-5 human lung cells. This work provides a convenient way to improve the antioxidant and antibacterial activities of COS.

Hyaluronic acid hydrogels crosslinked via blue light-induced thiol-ene reaction for the treatment of rat corneal alkali burn.

To assess corneal inflammation from alkali chemical burns, we examined the therapeutic effects of in situ-forming hyaluronic acid (HA) hydrogels crosslinked via blue light-induced thiol-ene reaction on a rat corneal alkali burn model. Animals were divided into three groups (n = 7 rats per group): untreated, treated with 0.1% HA eye drops, and treated with crosslinked HA hydrogels. Crosslinking of HA hydrogel followed by the administration of HA eye drops and crosslinked HA hydrogels were carried out once a day from days 0-4. Corneal re-epithelialization, opacity, neovascularization, thickness, and histology were evaluated to compare the therapeutic effects of the three groups. Further investigation was conducted on the transparency of HA hydrogels to acquire the practical capabilities of hydrogel as a reservoir for drug delivery. Compared to untreated animals, animals treated with crosslinked HA hydrogels exhibited greater corneal re-epithelialization on days 1, 2, 4, and 7 post-injury (p = 0.004, p = 0.007, p = 0.008, and p = 0.034, respectively) and the least corneal neovascularization (p = 0.008). Histological analysis revealed lower infiltration of stromal inflammatory cells and compact collagen structure in crosslinked HA hydrogel-treated animals than in untreated animals. These findings corresponded with immunohistochemical analyses indicating that the expression of inflammatory markers such as α-SMA, MMP9, and IL1-ß was lower in animals treated with crosslinked HA hydrogels than untreated animals and animals treated only with 0.1% HA eye drops. With beneficial pharmacological effects such as re-epithelization and anti-inflammation, in situ-forming hyaluronic acid (HA) hydrogels may be a promising approach to effective drug delivery in cases of corneal burn injuries.

Radical Mediated Rapid In Vitro Formation of c-Type Cytochrome.

A cytochrome c552 mutant from Thermus thermophilus HB8 (rC552 C14A) was reported, where the polypeptide with replaced Cys14 by alanine, overexpressed in the cytosol of E. coli. The apo-form of the C14A mutant (apo-C14A) without the original prosthetic group was obtained by simple chemical treatments that retained compact conformation amenable to reconstitution with heme b and zinc(II)-protoporphyrin(IX), gradually followed by spontaneous formation of a covalent bond between the polypeptide and porphyrin ring in the reconstituted apo-C14A. Further analysis suggested that the residual Cys11 and vinyl group of the porphyrin ring linked through the thiol-ene reaction promoted by light under ambient conditions. In this study, we describe the kinetic improvement of the covalent bond formation in accordance with the mechanism of the photoinduced thiol-ene reaction, which involves a thiyl radical as a reaction intermediate. Adding a radical generator to the reconstituted C14A mutant with either heme-b or zinc(II) porphyrin accelerated the bond-forming reaction, which supported the involvement of a radical species in the reaction. Partial observation of the reconstituted C14A in a dimer form and detection of sulfuryl radical by EPR spectroscopy indicated a thiyl radical on Cys11, a unique cysteinyl residue in rC552 C14A. The covalent bond forming mediated by the radical generator was also adaptable to the reconstituted apo-C14A with manganese(II)-protoporphyrin(IX), which also exhibits light-mediated covalent linkage formation. Therefore, the radical generator extends the versatility of producing c-type-like cytochrome starting from a metallo-protoporphyrin(IX) and the apo-C14A instantaneously.

N-Acetylcysteine is an effective analog of glutathione in reactions with reactive oxygen species.

The kinetic characteristics of the interaction of N-acetylcysteine (ASH) with reactive oxygen species (ROS), peroxyl radicals and hydrogen peroxide were determined. It was found that in terms of activity ASH in these reactions is similar to glutathione GSH, the main endogenous bioantioxidant. The kinetics of heat release in the interaction of GSH and ASH with H2O2 was studied for the first time by isothermal calorimetry. It is shown that the kinetic curves of heat release and changes in specific heat release rates practically coincide for both thiols taken in the stoichiometric ratio in the known reaction 2 TSH + H2O2 → TSST + 2 H2O. This indicates the relative autonomy of the S-H and S-S bonds in thiols and disulfides, which are not affected by other groups in the molecule. At pH<7, ASH, like GSH, interacts with H2O2 to form thiyl radicals, which initiate thiol-ene reactions with unsaturated phenol resveratrol. Under the same conditions, ASH ensures nearly the same radical initiation rates as GSH, and thiyl radicals from ASH are close in activity to GS• in chain propagation reactions.