An Ionic Liquid Medium Enables Development of a Phosphine-Mediated Amine-Azide Bioconjugation Method

Catalytic Serine Labeling in Nonaqueous, Acidic Media

Chemoselective modification of alkyl alcohols (e. g., serine residues) on proteins has been a daunting challenge especially in aqueous media. Herein, we report chemical modification of alkyl alcohols in protein and cell lysate samples using carboxylic acid-based bioconjugation media. The acidic medium is not only useful to suppress reactivity of other nucleophiles in proteins, but the medium also serves as a potentially biomolecule-compatible solvent. The acid-catalyzed acylation strategy has a unique selectivity paradigm compared to the common active-serine-targeted method and would act as a new strategy for studying biological roles of serine residues.

Redox-neutral, metal-free tryptophan labeling of polypeptides in hexafluoroisopropanol (HFIP)

Despite the unmet needs for chemical tools to study biological roles of tryptophan in living systems, there has been a lack of chemical modification methods for tryptophan residues that can be used in cellular environments. Driven by a preliminary computational study of our previous research, this work experimentally examined our hypotheses to translate the metal-catalyzed tryptophan modification method in hexafluoroisopropanol (HFIP) into a metal-free process. While one of the hypotheses merely confirmed the superiority of the thiophene-ethanol reagent developed in the previous report, the second hypothesis resulted in the identification of a trifluoroborate salt and an acidic ionic liquid as alternatives for the catalysis. Labeling of lysates of a human cell line was achieved with the acidic ionic liquid catalyst, where negative impacts of the tryptophan labeling and HFIP medium on the cellular samples were apparently insignificant. Because the labeling process does not require any redox mediators and is a formal redox-neutral reaction, the metal-free approach would be of use for tryptophan biology research potentially related to their various redox roles.

Fluoroalcohols for chemical modification of biomolecules

While their broad utility in various chemistry fields were well recognized for decades, fluoroalcohols have recently emerged as a unique solvent system for bioconjugation development. This review describes examples and roles of fluoroalcohols such as trifluoroethanol (TFE) and hexafluoroisopropanol (HFIP) for chemical modification of biomolecules such as polypeptides, nucleic acids, and saccharides. Many chemical modification processes were facilitated by notable functions of those fluoroalcohols such as a proton shuttle, reversible adduct formation with reactive species, and compatibility with electrochemistry/photochemistry. The usefulness of the fluoroalcohol solvents can be even promoted by its combination with a different solvent system for reaction enhancement and protein stabilization. The collection of the various chemical transformations in this review is an indication of the rapid growth of the solvent-assisted bioconjugation field.

Selective Macrocyclization of Unprotected Peptides with an Ex Situ Gaseous Linchpin Reagent

Peptide cyclization has dramatic effects on a variety of important properties, enhancing metabolic stability, limiting conformational flexibility, and altering cellular entry and intracellular localization. The hydrophilic, polyfunctional nature of peptides creates chemoselectivity challenges in macrocyclization, especially for natural sequences without biorthogonal handles. Herein, we describe a gaseous sulfonyl chloride derived reagent that achieves amine-amine, amine-phenol, and amine-aniline crosslinking through a minimalist linchpin strategy that affords macrocyclic urea or carbamate products. The cyclization reaction is metal-mediated and involves a novel application of sulfine species that remains unexplored in aqueous or biological contexts. The aqueous method delivers unique cyclic or bicyclic topologies directly from a variety of natural bioactive peptides without the need for protecting-group strategies.

Advanced Insulin Synthesis by One-pot/Stepwise Disulfide Bond Formation Enabled by S-Protected Cysteine Sulfoxide

An advanced insulin synthesis is presented that utilizes one-pot/stepwise disulfide bond formation enabled by acid-activated S-protected cysteine sulfoxides in the presence of chloride anion. S-chlorocysteine generated from cysteine sulfoxides reacts with an S-protected cysteine to afford S-sulfenylsulfonium cation, which then furnishes the disulfide or reversely returns to the starting materials depending on the S-protection employed and the reaction conditions. Use of S-acetamidomethyl cysteine (Cys(Acm)) and its sulfoxide (Cys(Acm)(O)) selectively give the disulfide under weak acid conditions in the presence of MgCl2 even if S-p-methoxybenzyl cysteine (Cys(MBzl)) and its sulfoxide (Cys(MBzl)(O)) are also present. In contrast, the S-MBzl pair yields the disulfide under more acidic conditions in the presence of a chloride anion source. These reaction conditions allowed a one-pot insulin synthesis. Additionally, lipidated insulin was prepared by a one-pot disulfide-bonding/lipidation sequence.

Dissecting the stability of Atezolizumab with renewable amino acid-based ionic liquids: Colloidal stability and anticancer activity under thermal stress

Monoclonal antibodies (mAbs) have revolutionised the biopharmaceutical market. Being proteinaceous, mAbs are prone to chemical and physical instabilities. Various approaches were attempted to stabilise proteins against degradation factors. Ionic liquids (ILs) and deep eutectic solvents (DESs) have been established as green solvents for ever-increasing pharmaceutical and biopharmaceutical applications. Hence, amino acid (AA)-based ILs, were used for the first time, for mAb stabilisation. Choline (Ch)-based DESs were also utilised for comparison purposes. The prepared ILs and DESs were utilised to stabilise Atezolizumab (Amab, anti-PDL-1 mAb). The formulations of Amab in ILs and DESs were incubated at room temperature, 45 or 55 °C. Following this, the structural stability of Amab was appraised. Interestingly, Ch-Valine retained favourable structural stability of Amab with minimal detected aggregation or degradation as confirmed by UV-visible spectroscopy and protein Mass Spectroscopy. The measured hydrodynamic diameter of Amab in Ch-Valine ranged from 10.40 to 11.65 nm. More interestingly, the anticancer activity of Amab was evaluated, and Ch-Valine was found to be optimum in retaining the activity of Amab when compared to other formulations, including the control Amab sample. Collectively, this study has spotlighted the advantages of adopting the Ch-AA ILs for the structural and functional stabilising of mAbs.

Hexafluoroisopropanol as a Bioconjugation Medium of Ultrafast, Tryptophan-Selective Catalysis

The past decade has seen a remarkable growth in the number of bioconjugation techniques in chemistry, biology, material science, and biomedical fields. A core design element in bioconjugation technology is a chemical reaction that can form a covalent bond between the protein of interest and the labeling reagent. Achieving chemoselective protein bioconjugation in aqueous media is challenging, especially for generally less reactive amino acid residues, such as tryptophan. We present here the development of tryptophan-selective bioconjugation methods through ultrafast Lewis acid-catalyzed reactions in hexafluoroisopropanol (HFIP). Structure-reactivity relationship studies have revealed a combination of thiophene and ethanol moieties to give a suitable labeling reagent for this bioconjugation process, which enables modification of peptides and proteins in an extremely rapid reaction unencumbered by noticeable side reactions. The capability of the labeling method also facilitated radiofluorination application as well as antibody functionalization. Enhancement of an α-helix by HFIP leads to its compatibility with a certain protein, and this report also demonstrates a further stabilization strategy achieved by the addition of an ionic liquid to the HFIP medium. The nonaqueous bioconjugation approaches allow access to numerous chemical reactions that are unavailable in traditional aqueous processes and will further advance the chemistry of proteins.

Iron-sensitive protein conjugates formed with a Wittig reaction precursor in ionic liquid

In this report, formation of protein conjugates with an iron-sensitive enamine linkage is demonstrated through the ionic liquid-based bioconjugation method.

Site-specific DNA functionalization through the tetrazene-forming reaction in ionic liquids

Site-specific chemical modification of unprotected DNAs through a phosphine-mediated amine–azide coupling reaction in ionic liquid.

Phosphine-mediated three-component bioconjugation of amino- and azidosaccharides in ionic liquids

Bioconjugation of carbohydrates has been a challenging task because of their chemical, functional, and structural diversities, and no single chemical modification tool can be universally applicable to all the target substrates in different environments.