Raman bands of N‐deuterated histidinium as markers of conformation and hydrogen bonding

Abstract

Raman spectra were measured for five crystals containing N‐deuterated histidinium and their C₂‐deuterated analogues. The spectra showed strong bands assignable to the C₄C₅ stretching mode, ν(C₄C₅), and the N_τC₂N_π symmetric stretching mode, ν(N_τC₂N_π), characteristic of the N‐deuterated imidazolium ring. By comparison with the crystal structures, the ν(C₄C₅) and ν(N_τC₂N_π) frequencies are found to increase linearly with increase of the absolute value of the C_αC_βC₄C₅ torsional angle (χ^{2, 1}), which determines the ring orientation relative to the amino acid backbone. Exceptionally, the ν(N_τC₂N_π) frequency of the C₂‐deuterated imidazolium ring does not depend on the conformation but shows a correlation with the ND stretching frequency of the ring, indicating that ν(N_τC₂N_π) becomes a marker of hydrogen‐bonding strength when the imidazolium ring is deuterated at C₂. These marker bands may be useful in studying the conformation and hydrogen‐bonding states of histidine residues in proteins and peptides.