A New Raman Spectroscopic Probe of Both the Protonation State and Noncovalent Interactions of Histidine Residues

The amino acid histidine (His) has a number of unique roles that can dictate function in proteins, and these roles are typically conferred through noncovalent interactions that depend on the protonation state of His’s 4-substituted imidazole ring. His’s protonation state can vary near physiological pH, and a probe of His’s variable protonation state and its resulting noncovalent interactions that has both high time resolution and no sample limitations could find wide use in determining the role of particular His residues in proteins. Here we use a classic deuterium exchange reaction to replace the C2–H hydrogen atom of the His imidazole ring with deuterium, leading to a unique aromatic C2–D stretching vibration whose frequency is sensitive to environmental changes across the entire imidazole ring. Using nonresonant Raman spectroscopy, we demonstrate using model compounds that the frequency of this C2–D vibration shifts by 35 cm^–1 upon changes in the His protonation state. The C2–D band is a very weak infrared absorber, so this vibration is not expected to be useful in infrared transmission experiments for proteins. Solvent-dependent Raman experiments indicate that the C2–D band of the neutral imidazole ring is sensitive to H-bonding interaction with donors and acceptors of varying strengths, suggesting that the C2–D frequency can be used to identify H-bonding partners of specific His residues. Raman spectra at varying concentrations of Cu²⁺ also show the C2–D band’s sensitivity to metal coordination, with differences due to changes in the coordination environment. The strong Raman signal of this band and the sampling flexibility of Raman spectroscopy suggest that this vibration could be very useful in documenting the local role of His residues in many His-containing proteins and protein assemblies