Achieving cysteine-selective peptide/protein bioconjugation via tunable triazine-pyridine chemistry

The development of cysteine (Cys)–selective bioconjugation reagents with enhanced stability remains a critical challenge for therapeutic applications such as antibody-drug conjugates (ADCs). Leveraging the modular 1,3,5-triazine scaffold, we report the design and optimization of triazine-pyridinium chemistry (TPC) reagents for selective Cys labeling. Through systematic structural modifications and computational studies, we identified reagent 9b , featuring a para - N , N -dimethylaminopyridinium leaving group and ethoxy substitution, as the most efficient and selective candidate for selective protein labeling. 9b demonstrated near-quantitative Cys labeling (>95% yield) under physiological conditions (pH 7.4) while suppressing tyrosine reactivity, a limitation of earlier TPC probes. The reagent demonstrated excellent compatibility with various peptides and proteins, including therapeutic antibodies like trastuzumab, showcasing its potential for constructing ADCs. The optimized labeling ensured robust stability of the conjugates in biological environments, highlighting the practical applicability of this methodology. Our findings underscore the promise of triazine-pyridinium chemistry in developing stable, site-specific bioconjugates for targeted therapeutic applications.