An overview of chemo- and site-selectivity aspects in the chemical conjugation of proteins

The bioconjugation of proteins-that is, the creation of a covalent link between a protein and any other molecule-has been studied for decades, partly because of the numerous applications of protein conjugates, but also due to the technical challenge it represents. Indeed, proteins possess inner physico-chemical properties-they are sensitive and polynucleophilic macromolecules-that make them complex substrates in conjugation reactions. This complexity arises from the mild conditions imposed by their sensitivity but also from selectivity issues, viz the precise control of the conjugation site on the protein. After decades of research, strategies and reagents have been developed to address two aspects of this selectivity: chemoselectivity-harnessing the reacting chemical functionality-and site-selectivity-controlling the reacting amino acid residue-most notably thanks to the participation of synthetic chemistry in this effort. This review offers an overview of these chemical bioconjugation strategies, insisting on those employing native proteins as substrates, and shows that the field is active and exciting, especially for synthetic chemists seeking new challenges.

Perfluoroaryl and Perfluoroheteroaryl Reagents as Emerging New Tools for Peptide Synthesis, Modification and Bioconjugation

Peptides and proteins are becoming increasingly valuable as medicines, diagnostic agents and as tools for biomedical sciences. Much of this has been underpinned by the emergence of new methods for the manipulation and augmentation of native biomolecules. Perfluoroaromatic reagents are perhaps one of the most diverse and exciting tools with which to modify peptides and proteins, due principally to their nucleophilic substitution chemistry, high electron deficiency and the ability for their reactivity to be tuned towards specific nucleophiles. As discussed in this minireview, in recent years, perfluoroaromatic reagents have found applications as protecting groups or activating groups in peptide synthesis and as orthogonal handles for peptide modification. Furthermore, they have applications in chemoselective ‘tagging’, stapling and bioconjugation of peptides and proteins, as well as tuning of ‘drug‐like’ properties. This review will also explore possible future applications of these reagents in biological chemistry.

Visible-Light-Driven Dehydrogenative Coupling of Primary Alcohols with Phenols Forming Aryl Carboxylates

A preparative method for obtaining aryl esters from aliphatic primary alcohols and phenols was developed. The reaction proceeds under the irradiation of visible light at ambient temperature, dispensing with any oxidant or hydrogen acceptor. Primary alcohols having a variety of functional groups are successfully esterified with phenols. The produced esters can be utilized as the precursor of various carbonyl compounds.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2011-2012

This review is the seventh update of the original article published in 1999 on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2012. General aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, and fragmentation are covered in the first part of the review and applications to various structural types constitute the remainder. The main groups of compound are oligo- and poly-saccharides, glycoproteins, glycolipids, glycosides, and biopharmaceuticals. Much of this material is presented in tabular form. Also discussed are medical and industrial applications of the technique, studies of enzyme reactions, and applications to chemical synthesis. © 2015 Wiley Periodicals, Inc. Mass Spec Rev 36:255-422, 2017.

A series of ditopic receptors for succinic acid binding

Three ditopic-abiotic receptors (R1, R2 and R3) have been designed and synthesised. The receptors have been applied for the recognition of dicarboxylic acids viz. malonic, succinic, glutaric and adipic acids. Among them, succinic acid shows the highest binding efficiency to all receptors. Recognition of succinic acid is very important due to its various adverse effects on human health upon prolonged exposure.

Synthesis and properties of differently charged chemiluminescent acridinium ester labels

Chemiluminescent acridinium dimethylphenyl esters containing N-sulfopropyl groups in the acridinium ring are highly sensitive, hydrophilic labels that are used in automated immunoassays for clinical diagnostics. Light emission from these labels is triggered with alkaline peroxide in the presence of a cationic surfactant. At physiological pH, N-sulfopropyl acridinium esters exist as water adducts that are commonly referred to as pseudobases. Pseudobase formation, which results from addition of water to the zwitterionic N-sulfopropyl acridinium ring, neutralizes the positive charge on the acridinium nitrogen and imparts a net negative charge to the label due to the sulfonate moiety. As a consequence, N-sulfopropyl acridinium ester conjugates of small molecule haptens as well as large molecules such as proteins gain negative charges at neutral pH. In the current study, we describe the synthesis and properties of two new hydrophilic acridinium dimethylphenyl ester labels where the net charge in the labels was altered. In one label, the structure of the hydrophilic N-alkyl group attached to the acridinium ring was changed so that the pseudobase of the label contains no net charge. In the second acridinium ester, two additional negative charges in the form of sulfopropyl groups were added to the acridinium ring to make this label's pseudobase strongly anionic. Chemiluminescence measurements of these labels, as well as their conjugates of an antibody with a neutral pI, indicate that acridinium ester charge while having a modest effect on emission kinetics has little influence on light output. However, our results demonstrate that acridinium ester charge can affect protein pI, apparent chemiluminescence stability and non-specific binding of protein conjugates to microparticles. These results emphasize the need for careful consideration of acridinium ester charge in order to optimize reagent stability and performance in immunoassays. In the current study, we observed that for a neutral protein, an acridinium ester with a hydrophilic but charge-neutral N-alkyl group afforded faster light emission, lower non-specific binding and better chemiluminescence stability than an analogous label with an anionic N-alkyl group.

Efficient formation of heterodimers from peptides and proteins using unsymmetrical polyfluorophenyl esters of dicarboxylic acids

An efficient method for the heteroconjugation of biomolecules carrying free amino groups was reported previously, where mixed polyfluorophenyl diesters of dicarboxylic acids with varied aliphatic chain length were shown to be efficient reagents for the conjugation of a variety of model biomolecules. The concept was based on the differential reactivity of the esters towards amines. The concept has now been further optimized, and a 2,6-difluorophenyl-pentafluorophenyl diester combination has been demonstrated to be the most efficient, both with respect to selectivity and to reaction rate. A pentafluorophenyl ester reacts faster with an amino group and requires a weaker base than a 2,6-difluorophenyl ester that requires a stronger base and longer reaction time. With the use of this combination of esters, we obtained considerably shortened reaction times compared with those reported previously, yet still retaining the desired selectivity in heteroconjugation. The increased reactivity of the bifunctional reagent allowed the construction of sophisticated peptide heteroconjugates from peptides, carbohydrates and proteins, showing a wide scope of applicability in the field of assembling functional bioconjugates.