Interactive Bioconjugation at <i>N</i>‐Terminal Cysteines by Using <i>O</i>‐Salicylaldehyde Esters towards Dual Site‐Selective Functionalization
Maria J. S. A. Silva, Rafaela A. N. Cavadas, Hélio Faustino, Luı́s F. Veiros, Pedro M. P. Góis
Abstract
N-terminal Cys modification has been intensively studied to produce homogeneous bioconjugates essentially through two modes of reaction: reversible modification with the equilibrium shifted towards the formation of the desired conjugate or stable and irreversible conjugates. Herein, we report a new method of N-terminal cysteine modification using O-salicylaldehyde esters (OSAEs) through fast conjugation and irreversible deconjugation. These reagents can rapidly react with N-terminal Cys at low-micromolar concentration to form thiazolidines with subsequent hydrolysis of the ester moiety to the phenolic derivative. These phenolic thiazolidines can be hydrolyzed at acidic pH (≈4.5) to recover the intact N-terminal Cys. Bioconjugation reactions using OSAEs offer controlled reversibility to as act as a protecting group for N-terminal cysteines, allowing the modification of in-chain residues without perturbing the N-terminal Cys, which can then be deprotected and used as a conjugation site.