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

Abstract

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.

Friedel–Crafts reactions for biomolecular chemistry

“…The development of methods with unique media could potentially be an alternative approach, as discussed above (Scheme 8E). 135 Even if intact small molecule reagents would not be usable for cellular contexts, there are other unique chemical strategies such as genetically encodable amino acids and peptides as bioorthogonal tags 144 as well as encapsulation methods of reactive molecules in aqueous solutions. 145 Biomolecular Friedel–Crafts reactions have rapidly grown in multiple ways, and alongside technological and strategic advances in chemical methods in various fields, the full potential of alkylation and acylation chemistry in biological systems may not have been seen yet.…”

“…To this end, an alternative approach using nonaqueous media that are potentially compatible with both Friedel–Crafts alkylation and protein substrates has been devised (Scheme 8E). 135 Tryptophan-selective labeling of peptides and proteins was achieved by using hexafluoroisopropanol (HFIP) as a solvent that is known to stabilize the carbocation and enhance the peptide secondary structure ( i.e. , α-helices).…”

Late-stage peptide modification and macrocyclization enabled by tertiary amine catalyzed tryptophan allylation

“…20,21 Take the last 3 years for instance, transition-metal-catalyzed C–H activation provides distinct peptide late-stage modification with good chemoselectivity, which has been explored by several groups. 22–30 However, the reported methods require harsh reaction conditions, such as high temperatures, oxidant additives, auxiliary directing groups, and complex reaction conditions, which restrict their application prospects in bioactive peptide research. With the development of photochemistry in recent years, radical-medicated C2 amination, 31–33 fluoroalkylation, 34,35 alkylation, 36,37 and thiation 38,39 on the indole ring of tryptophan have been explored for the functionalization of peptides.…”