Efficiency and specificity evaluation of imidazole derivatization reactions for histidine-containing peptides.

BACKGROUND: Histidine residues are crucial for protein structure and function, playing key roles in metal coordination, catalysis, and post translational modifications. Comprehensive profiling of functional histidine sites remains technically challenging because of the insufficient chemoselectivity of labeling reagents, and the lack of robust enrichment methods compatible with complex proteomes. METHODS: Seven histidine-targeting probes representing three distinct reaction mechanisms were systematically evaluated using a peptide-centric workflow. Reaction conditions were optimized at the peptide level via mass spectrometry, and large-scale assessments of labeling efficiency, site selectivity, and sequence preferences were performed using tryptic digests of HeLa cell lysates. RESULTS: Nucleophilic substitution-based probes preferentially modify histidine in charged microenvironments, whereas nucleophilic addition reagents exhibit broad sequence tolerance and achieve the highest labeling coverage. In contrast, singlet oxygen-mediated probes display increased site-selectivity for C-terminal and dynamically accessible histidine sites, yielding highly confident labeling products with improved biocompatibility. Among all evaluated reagents, acrolein achieved the highest labeling efficiency, while 1-methyl-4-phenyl-1,2,4-triazoline-3,5-dione exhibited superior histidine specificity and compatibility with downstream enrichment strategies. CONCLUSION: This work provides a comparative framework for histidine-targeted chemical proteomics, establishes optimized labeling and analysis pipelines, and highlights complementary probe reactivities that can be combined with click chemistry for comprehensive histidine profiling in complex biological systems. Nevertheless, the peptide-level study may not fully reflect the reaction in intact proteins due to steric hindrance.