Effects of Thiol Substitution on the Kinetics and Efficiency of Thiol-Michael Reactions and Polymerizations

Onium Photocages for Visible-Light-Activated Poly(thiourethane) Synthesis and 3D Printing

The lack of chemical diversity in light-driven reactions for 3D printing poses challenges in the production of structures with long-term ambient stability, recyclability, and breadth in properties (mechanical, optical, etc.). Herein we expand the scope of photochemistries compatible with 3D printing by introducing onium photocages for the rapid formation of poly(thiourethanes) (PTUs). Efficient nonsensitized visible-light photolysis releases organophosphine and -amine derivatives that catalyze thiol-isocyanate polyaddition reactions with excellent temporal control. Two resin formulations comprising commercial isocyanates and thiols were developed for digital light processing (DLP) 3D printing to showcase the fast production of high-resolution PTU objects with disparate mechanical properties. Onium photocages represent valuable tools to advance light-driven manufacturing of next-generation high-performance sustainable materials.

Development of a chromatographic method for optimizing the thiol-maleimide coupling of polyoxometalate-polymer hybrids

The covalent attachment of polyoxometalates (POMs) to polymers has been developed as a strategic approach for the advancement of POM-based hybrid materials with versatile applications. In this study, we utilized thiol-maleimide Michael addition to investigate the kinetics and efficacy of the "one-to-one" conjugation between Keggin type POM and polystyrene. We explored the effects of solvent polarity, catalyst, molecular weight of PS and synthetic strategies on the reaction kinetics and efficiency, by means of reverse-phase high-performance liquid chromatography (RP-HPLC). A series of comparative analysis affirmed the superior efficiency of the one-pot method, particularly when facilitated by the addition of a high-polarity solvent and an excess of maleimide. These findings offer valuable insights into the intricate interplay between reaction conditions, kinetics, and selectivity in thiol-maleimide reactions of POMs and polymers. They hold profound implications for advancing the study of POM-based multifunctional materials and the synthesis of complex hybrid molecules.

Oxime metathesis: tuneable and versatile chemistry for dynamic networks

Oxime chemistry has emerged as a versatile tool for use in a wide range of applications. In particular, the combination of oximes with esters and urethanes has enabled the realisation of Covalent Adaptable Networks (CANs) with improved and tunable dynamic properties. Nevertheless, an exclusively oxime-based chemistry has not yet been explored in the fabrication of CANs. In this work, we investigate the mechanism of the acid-catalysed dynamic exchange of oximes. We propose a metathesis mechanism that is well supported by both experimental and computational studies, which highlight the importance of the substituent effect on the exchange reaction kinetics. Then, as a proof of concept, we incorporate oxime groups into a cross-linked polymeric material and demonstrate the ability of oxime-based polymers to be reprocessed under acid catalysis while maintaining their structural integrity.

Interactions between Damaged Hair Keratin and Juglone as a Possible Restoring Agent: A Vibrational and Scanning Electron Microscopy Study

Juglone, a quinonic compound present in walnut extracts, was proposed as a restoring agent for hair keratin treated with permanent or discoloration processes. The proposed mechanism of restoration by juglone involves the formation of a Michael adduct between the quinone and the thiol moieties of cysteine residues. To this purpose, the first part of the present paper involved the spectroscopic study of the product of the reaction between juglone and N-acetyl-L-cysteine as a model compound. IR spectroscopy and Scanning Electron Microscopy (SEM) monitored the chemical and morphological variations induced by applying juglone to hair keratin. In order to simulate the most common hair treatments (i.e., permanent and discoloration), juglone was applied to hair that had been previously treated with a reducing agent, i.e., methyl thioglycolate (MT) or with bleaching agents (based on hydrogen peroxide and persulfates) followed by sodium hydrogen sulfite. IR spectroscopy allowed us to monitor the formation of Michael adducts between juglone and cysteine residues: the Michael adducts' content was related to the cysteine content of the samples. In fact, MT and sodium hydrogen sulfite favored the reduction of the disulfide bonds and increased the content of free cysteine residues, which can react with juglone. SEM analyses confirmed the trend observed by IR spectroscopy since hair samples treated with juglone adopted a more regular hair surface and more imbricated scales, thus supporting the possible use of juglone as a restoring agent for damaged hair keratins.

Ring-Opening Polymerization of Amino Acid N-Carboxyanhydrides with Unprotected/Reactive Side Groups. I. d-Penicillamine N-Carboxyanhydride

The ring-opening (co)polymerization (ROP) of N-carboxyanhydride (NCA) monomers bearing unprotected/reactive side groups is rare and challenging. Here, we report the ROP of a d-penicillamine NCA (Pen-NCA) monomer for the synthesis of tertiary thiol-functionalized (co)polypeptides. Through judicious selection of reaction solvents and the use of benzoic acid as an additive in the ROP, the intramolecular isomerization side reactions of Pen-NCA are suppressed, generating homo- and copolypeptides with improved yield, high molecular weight, and narrow molecular weight distributions. Successful postpolymerization modifications of the d-Pen-containing copolypeptides on the tertiary thiols are achieved with high efficiency through thiol-Michael, S_N2, and nitrosylation reactions. This work provides an efficient protection-free approach to generating functional polypeptides and creates a fundamental understanding for Pen-NCA chemistry.

Thia-Michael Reaction: The Route to Promising Covalent Adaptable Networks

While the Michael addition has been employed for more than 130 years for the synthesis of a vast diversity of compounds, the reversibility of this reaction when heteronucleophiles are involved has been generally less considered. First applied to medicinal chemistry, the reversible character of the hetero-Michael reactions has recently been explored for the synthesis of Covalent Adaptable Networks (CANs), in particular the thia-Michael reaction and more recently the aza-Michael reaction. In these cross-linked networks, exchange reactions take place between two Michael adducts by successive dissociation and association steps. In order to understand and precisely control the exchange in these CANs, it is necessary to get an insight into the critical parameters influencing the Michael addition and the dissociation rates of Michael adducts by reconsidering previous studies on these matters. This review presents the progress in the understanding of the thia-Michael reaction over the years as well as the latest developments and plausible future directions to prepare CANs based on this reaction. The potential of aza-Michael reaction for CANs application is highlighted in a specific section with comparison with thia-Michael-based CANs.

Synthesis and Characterization of Multifunctional Secondary Thiol Hardeners Using 3-Mercaptobutanoic Acid and Their Thiol-Epoxy Curing Behavior

3-Mercaptobutanoic acid (3-MBA) was synthesized by the less odorous Michael addition pathway using an isothiouronium salt intermediate. Using the synthesized 3-MBA, multifunctional secondary thiol (sec-thiol) compounds were obtained and applied to thiol-epoxy curing systems as hardeners. As the functionality of the sec-thiol hardeners increased, the purity of the product obtained after distillation decreased. The equivalent epoxy mixtures with multifunctional sec-thiol hardeners were evaluated based on their impact on the curing behavior in thiol-epoxy click reactions by differential scanning calorimetry. The thermal features of sec-thiol-epoxy click reactions in the presence of a base catalyst were assessed according to the functionality of the sec-thiol hardeners. Our results showed that sec-thiol hardeners with less reactivity to the epoxy group provide long-term storage stability for the formulated epoxy resin, promising for industrial applications.

Photopolymerizable Ionogel with Healable Properties Based on Dioxaborolane Vitrimer Chemistry

Ionogels are solid polymer gel networks loaded with ionic liquid (IL) percolating throughout each other, giving rise to ionically conducting solid electrolytes. They combine the mechanical properties of polymer networks with the ionic conductivity, non-volatility, and non-flammability of ILs. In the frame of their applications in electrochemical-based flexible electronics, ionogels are usually subjected to repeated deformation, making them susceptible to damage. It appears critical to devise a simple and effective strategy to improve their durability and lifespan by imparting them with healing ability through vitrimer chemistry. In this work, we report the original in situ synthesis of polythioether (PTE)-based vitrimer ionogels using fast photopolymerization through thiol-acrylate Michael addition. PTE-based vitrimer was prepared with a constant amount of the trithiol crosslinker and varied proportions of static dithiol spacers and dynamic chain extender BDB containing dynamic exchangeable boronic ester groups. The dynamic ionogels were prepared using 50 wt% of either 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide or 1-Ethyl-3-methylimidazolium trifluoromethanesulfonate, both of which were selected for their high ionic conductivity. They are completely amorphous (T_g below -30 °C), suggesting they can be used at low temperatures. They are stretchable with an elongation at break around 60%, soft with Young's modulus between 0.4 and 0.6 MPa, and they have high ionic conductivities for solid state electrolytes in the order of 10^-4 S·cm^-1 at room temperature. They display dynamic properties typical of the vitrimer network, such as stress relaxation and healing, retained despite the large quantity of IL. The design concept illustrated in this work further enlarges the library of vitrimer ionogels and could potentially open a new path for the development of more sustainable, flexible electrochemical-based electronics with extended service life through repair or reprocessing.

Catch, Modify and Analyze: Methods of Chemoselective Modification of Cysteine-Containing Peptides

One effective solution in the analysis of complex mixtures, including protein or cell hydrolysates, is based on chemoselective derivatization of a selected group of compounds by using selective tags to facilitate detection. Another method is based on the capture of the desired compounds by properly designed solid supports, resulting in sample enrichment. Cysteine is one of the rarest amino acids, but at least one cysteine residue is present in more than 91% of human proteins, which clearly confirms its important role in biological systems. Some cysteine-containing peptides may serve as significant molecular biomarkers, which may emerge as key indices in the management of patients with particular diseases. In the current review, we describe recent advances in the development of cysteine-containing peptide modification techniques based on solution and solid phase derivatization and enrichment strategies.