2-Deuterated Histidine is a Raman Reporter of Histidine's Protonation State, Hydrogen Bonding, and Metal Coordination

Abstract

The C-2 proton on the imidazole side chain of the amino acid Histidine (His) can be isotopically exchanged for deuterium under mild reaction conditions, thus creating a unique C-D stretching band that can serve as a site-specific reporter of the environment and role of specific His residues in proteins. This band is a very weak infrared band but is clearly visible in nonresonant Raman spectra. His residues can participate in a number of different interactions unique to the His imidazole ring. Model compound work has shown that the peak frequency and spectral properties of this novel C-D vibrational band is dependent on imidazole protonation state, hydrogen bond donor and acceptor strength, and metal coordination, while salt bridge dependence in the model protein T4 lysozyme is currently being studied. Based on protein work that confirms the model compound protonation dependence, larger and more complex protein systems with multiple, distinct His residues, like superoxide dismutase, are currently targeted to report the environment and interactions of multiple specific His residues within the same protein.