13C and 15N spectral editing inside histidine imidazole ring through solid-state NMR spectroscopy

Abstract

Histidine usually exists in three different forms (including biprotonated species, neutral τ and π tautomers) at physiological pH in biological systems. The different protonation and tautomerization states of histidine can be characteristically determined by 13C and 15N chemical shifts of imidazole ring. In this work, solid-state NMR techniques were developed for spectral editing of 13C and 15N sites in histidine imidazole ring, which provides a benchmark to distinguish the existing forms of histidine. The selections of 13Cγ, 13Cδ2, 15Nδ1, and 15Nε2 sites were successfully achieved based on one-bond homo- and hetero-nuclear dipole interactions. Moreover, it was demonstrated that 1H, 13C, and 15 chemical shifts were roughly linearly correlated with the corresponding atomic charge in histidine imidazole ring by theoretical calculations. Accordingly, the 1H, 13C and 15N chemical shifts variation in different protonation and tautomerization states could be ascribed to the atomic charge change due to proton transfer in biological process.