Efficient and Selective Chemical Labeling of Electrochemically Generated Peptides Based on Spirolactone Chemistry

Electrochemistry-enabled residue-specific modification of peptides and proteins

Selective chemical reactions at precise amino acid residues of peptides and proteins have become an exploding field of research in the last few decades. With the emerging utility of bioconjugated peptides and proteins as drug leads and therapeutic agents, the design of smart protocols to modulate and conjugate biomolecules has become necessary. During this modification, the most important concern of biochemists is to keep intact the structural integrity of the biomolecules. Hence, a soft and selective biocompatible reaction environment is necessary. Electrochemistry, a mild and elegant tunable reaction platform to synthesize complex molecules while avoiding harsh and toxic chemicals, can provide such a reaction condition. However, this strategy is yet to be fully exploited in the field of selective modification of polypeptides. With this possibility, the use of electrochemistry as a reaction toolbox in peptide and protein chemistry is flourishing day by day. Unfortunately, there is no suitable review article summarizing the residue-specific modification of biomolecules. The present review provides a comprehensive summary of the latest manifested electrochemical approaches for the modulation of five redox-active amino acid residues, namely cysteine, tyrosine, tryptophan, histidine and methionine, found in peptides and proteins. The article also highlights the incredible potential of electrochemistry for the regio- as well as chemoselective bioconjugation strategy of biomolecules.

Electrochemistry for the Chemoselective Modification of Peptides and Proteins

Although electrochemical strategies for small-molecule synthesis are flourishing, this technology has yet to be fully exploited for the mild and chemoselective modification of peptides and proteins. With the growing number of diverse peptide natural products being identified and the emergence of modified proteins as therapeutic and diagnostic agents, methods for electrochemical modification stand as alluring prospects for harnessing the reactivity of polypeptides to build molecular complexity. As a mild and inherently tunable reaction platform, electrochemistry is arguably well-suited to overcome the chemo- and regioselectivity issues which limit existing bioconjugation strategies. This Perspective will showcase recently developed electrochemical approaches to peptide and protein modification. The article also highlights the wealth of untapped opportunities for the production of homogeneously modified biomolecules, with an eye toward realizing the enormous potential of electrochemistry for chemoselective bioconjugation chemistry.

Specific Affinity Enrichment of Electrochemically Cleaved Peptides Based on Cu(II)-Mediated Spirolactone Tagging

Specific digestion of proteins is an essential step for mass spectrometry-based proteomics, and the chemical labeling of the resulting peptides is often used for peptide enrichment or the introduction of desirable tags. Electrochemical oxidation yielding specific cleavage C-terminal to tyrosine (Tyr) and tryptophan (Trp) residues provides a potential alternative to enzymatic digestion and a possibility for further chemical labeling by introducing reactive spirolactone moieties. However, spirolactone-containing peptides suffer from low stability due to hydrolysis and intramolecular side reactions. We found that Cu(II) ions stabilize the spirolactone and prevent intramolecular side reactions during chemical labeling, allowing efficient chemical tagging with a reduced excess of labeling reagent without intramolecular side reactions. On the basis of this reaction, we developed an analytical procedure combining electrochemical digestion, Cu(II)-mediated spirolactone biotinylation, and enrichment by avidin affinity chromatography with mass spectrometry. The method was optimized with the tripeptide LWL and subsequently applied to chicken egg white lysozyme, in which one biotinylated electrochemistry (EC)-cleaved peptide was identified after affinity enrichment. This proof-of-principle shows that specific enrichment of electrochemically cleaved spirolactone-containing peptides can be used for protein identification and notably that inclusion of Cu(II) ions is essential for stabilizing spirolactones for subsequent biotinylation.