Site-Specific Chemical Modification of Peptide and Protein by Thiazolidinediones

Unexplored Vinylic-Substituted 5-Benzylidenethiazolidine-2,4-diones: Synthesis and DFT/NMR Stereochemical Assignment

By exploring an efficient and versatile method for the 6-functionalization of its scaffold, we investigated the opening of a new chemical space around benzylidenethiazolidine-2,4-dione (BTZD). The 6-chloro- and 6-formyl BTZD obtained in two steps starting from 5-lithioTZD were selected as key intermediates and involved in a Pd-catalyzed cross-coupling or Wittig olefination. A variety of aryl, heteroaryl, or alkenyl substituents was successfully introduced on the vinylic position of BTZD, and particular attention was paid to elucidate the stereochemistry of the benzylidene derivatives by using a combined DFT/NMR study.

Diastereoselective reversible C-C bond exchange of oxindole-thiazolidienediones for dynamic combinatorial chemistry

We herein describe a diastereoselective aldol exchange involving isatins and thiazolidinediones, providing oxindolyl-thiazolidienediones in aqueous media at pH 6. This equilibrium can also be achieved with oxindole exchange as well as cross-exchange within reasonable timescales. These metal and organic catalyst free reversible reactions provide a unique opportunity for the evolution of dynamic combinatorial libraries (DCLs) for target directed dynamic combinatorial chemistry (DCC) and system chemistry.

Chemical Modification for the "off-/on" Regulation of Enzyme Activity

Enzymes with excellent catalytic performance play important roles in living organisms. Advances in strategies for enzyme chemical modification have enabled powerful strategies for exploring and manipulating enzyme functions and activities. Based on the development of chemical enzyme modifications, incorporating external stimuli-responsive features-for example, responsivity to light, voltage, magnetic force, pH, temperature, redox activity, and small molecules-into a target enzyme to turn "on" and "off" its activity has attracted much attention. The ability to precisely control enzyme activity using different approaches would greatly expand the chemical biology toolbox for clarification and detection of signal transduction and in vivo enzyme function and significantly promote enzyme-based disease therapy. This review summarizes the methods available for chemical enzyme modification mainly for the off-/on control of enzyme activity and particularly highlights the recent progress regarding the applications of this strategy. This article is protected by copyright. All rights reserved.

An expeditious route to sterically encumbered nonproteinogenic α-amino acid precursors using allylboronic acids

A diastereoselective allylation of N-tert-butane sulfinyl α-iminoesters using allylboronic acids is developed to obtain optically active non-proteinogenic α-amino acid precursors in good yields and diastereoselectivities. Gram-scale synthesis, broad tolerance of functional groups, excellent stereodivergence, post-synthetic modifications, and easy removal of the chiral auxiliary are some of the key highlights. The protocol is applicable to various amino acids and short peptides, resulting in the incorporation of these precursors at the N-terminal position.

A diastereoselective allylation of N-tert-butane sulfinyl α-iminoesters using allylboronic acids is developed to obtain optically active non-proteinogenic α-amino acid precursors in good yield and diastereoselectivities.

The Wittig bioconjugation of maleimide derived, water soluble phosphonium ylides to aldehyde-tagged proteins

Herein we disclose the transformation of maleimides into water-soluble tris(2-carboxyethyl)phosphonium ylides and their subsequent application in the bioconjugation of protein- and peptide-linked aldehydes. The new entry into Wittig bioconjugate chemistry proceeds under mild conditions and relies on highly water soluble reagents, which are likely already part of most biochemists' inventory.

Bifunctional Reagents for Formylglycine Conjugation: Pitfalls and Breakthroughs

Formylglycine-generating enzymes specifically oxidize cysteine within the consensus sequence CxPxR to Cα -formylglycine (FGly). This noncanonical electrophilic amino acid can subsequently be addressed selectively by bioorthogonal hydrazino-iso-Pictet-Spengler (HIPS) or Knoevenagel ligation to attach payloads like fluorophores or drugs to proteins to obtain a defined payload-to-protein ratio. However, the disadvantages of these conjugation techniques include the need for a large excess of conjugation building block, comparably low reaction rates and limited stability of FGly-containing proteins. Therefore, functionalized clickable HIPS and tandem Knoevenagel building blocks were synthesized, conjugated to small proteins (DARPins) and subsequently linked to strained alkyne-containing payloads for protein labeling. This procedure allowed the selective bioconjugation of one or two DBCO-carrying payloads with nearly stoichiometric amounts at low concentrations. Furthermore, an azide-modified tandem Knoevenagel building block enabled the synthesis of branched PEG linkers and the conjugation of two fluorophores, resulting in an improved signal-to-noise ratio in live-cell fluorescence-imaging experiments targeting the EGF receptor.

Design and application of near-infrared fluorophore based on a novel thiazolidinedione-functionalized dicyanoisophorone

A novel dicyanoisophorone (DCI)-based NIR fluorophore employing 2, 4-thiazolidinediones as the modification site was designed for fluorescence imaging. The fluorophore was assessed as a switchable reporter for H₂O₂ and the probe exhibited lysosomes-targeted, a large turn-on fluorescence signal at 720 nm with a large stokes shift (150 nm) and can be used in biological systems. The ability of the novel fluorophore to emit NIR fluorescence through a "turn-on" activation mechanism makes it a promising fluorophore for in vivo imaging applications. The strategy of introducing the thiazolidinediones with the easy modification site into the fluorophore has a good application prospect to expand the application of the NIR fluorophore.

Application of the N-Dibenzyl Protective Group in the Preparation of β-Lactam Pseudopeptides

Despite the great importance of β-lactam antibiotics, there is still a limited number of synthetic approaches for the formation of β-lactam⁻containing dipeptides. In this study, we report upon the stereoselective preparation of β-lactam⁻containing pseudopeptides, where different reaction conditions and NH₂ protective groups were tested to obtain compounds that contain 3-amino-azetidin-2-one. We demonstrate that the protective group is essential for the outcome of the reaction. Successful implementation of dibenzyl-protected serine-containing dipeptides through the Mitsunobu reaction can provide the desired products at high yields and stereoselectivity.

Efficient Pictet-Spengler Bioconjugation with N-Substituted Pyrrolyl Alanine Derivatives

We discovered N-pyrrolyl alanine derivatives as efficient reagents for the fast and selective Pictet-Spengler reaction with aldehyde-containing biomolecules. Other aldehyde-labeling methods described so far have several drawbacks, like hydrolytic instability, slow reaction kinetics or not readily available labeling reagents. Pictet-Spengler cyclizations of pyrrolyl 2-ethylamine substituted at the pyrrole nitrogen are significantly faster than with analogues substituted at the α- and β- position. Functionalized N-pyrrolyl alanine derivatives can be synthesized in only 2-3 steps from commercially available materials. The small size of the reagent, the high reaction rate, and the easy synthesis make pyrrolyl alanine Pictet-Spengler (PAPS) an attractive choice for bioconjugation reactions. PAPS was shown as an efficient strategy for the site-selective biotinylation of an antibody as well as for the condensation of nucleic-acid derivatives, demonstrating the versatility of this reagent.

The one-pot nonhydrolysis Staudinger reaction and Staudinger or SPAAC ligation

The one-pot nonhydrolysis Staudinger reaction and Staudinger or SPAAC ligation were used for producing a FRET-based dyad in living cells as a proof-of-concept study.

Thiazolidin-5-imine Formation as a Catalyst-Free Bioorthogonal Reaction for Protein and Live Cell Labeling

A previously undescribed reaction involving the formation of a thiazolidin-5-imine linkage was developed for bioconjugation. Being highly specific and operating in aqueous media, this simple condensation reaction is used to chemoselectively label peptides, proteins, and living cells under physiological conditions without the need to use toxic catalysts or reducing reagents.

Amino-Acid-Catalyzed Direct Aldol Bioconjugation

A site-specific bioconjugation was developed based on direct aldol coupling using amino-acid-derived organocatalysts. The functionalization exhibits fast kinetics and occurs under mild, biocompatible conditions (viz., aqueous media, moderate temperature, and neutral pH). The resulting bioconjugates were found to be stable toward abundant aldolase enzymes, as well as acidic and basic pH. The methodology was demonstrated through conjugation of a variety of small molecules, dyes, and peptides to proteins, including a single-domain antibody, which was then used for cellular imaging.

Target-guided screening of fragments (TGSOF) in the discovery of inhibitors against EV-A71 3C protease

Target-guided screening of fragments (TGSOF) was developed and employed in the identification of EV-A71 3C protease (3Cpro) inhibitors. We identified 4-acetylpyridine and 3-acetylpyridine as effective P3 fragments of an inhibitor and obtained the corresponding irreversible inhibitors 12c and 12fvia this method. Furthermore, based on 12c and 12f, we have obtained reversible inhibitors 17c and 17f. These results demonstrated that TGSOF is a useful strategy for identifying suitable fragments in developing leads in drug discovery.

Thiazolidine-Masked α-Oxo Aldehyde Functionality for Peptide and Protein Modification

α-Oxo aldehyde-based bioconjugation chemistry has been widely explored in peptide and protein modifications for various applications in biomedical research during the past decades. The generation of α-oxo aldehyde via sodium periodate oxidation is usually limited to the N-terminus of a target protein. Internal-site functionalization of proteins with the α-oxo aldehyde handle has not been achieved yet. Herein we report a novel method for site-specific peptide and protein modification using synthetically or genetically incorporated thiazolidine-protected α-oxo aldehyde. Efficient unmasking of the aldehyde was achieved by silver ion-mediated hydrolysis of thiazolidine under mild conditions for the first time. A model peptide and a recombinant protein were used to demonstrate the utility of this new method, which were site-specifically modified by oxime ligation with an oxyamine-functionalized peptide labeling reagent. Therefore, our current method has enriched the α-oxo aldehyde synthetic tool box in peptide and protein bioconjugation chemistry and holds great potential to be explored in novel applications in the future.

Site-selective incorporation and ligation of protein aldehydes

The incorporation of aldehyde handles into proteins, and subsequent chemical reactions thereof, is rapidly proving to be an effective way of generating homogeneous, covalently linked protein constructs that can display a vast array of functionality.

Protein tetrazinylation via diazonium coupling for covalent and catalyst-free bioconjugation

This work reports an efficient reagent 1 for direct and covalent introduction of tetrazines onto the surface of proteins and viruses under mild conditions.