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

Raman spectra were measured for five crystals containing N ‐deuterated histidinium and their C 2 ‐deuterated analogues. The spectra showed strong bands assignable to the C 4 C 5 stretching mode, ν(C 4 C 5 ), and the N τ C 2 N π symmetric stretching mode, ν(N τ C 2 N π ), characteristic of the N ‐deuterated imidazolium ring. By comparison with the crystal structures, the ν(C 4 C 5 ) and ν(N τ C 2 N π ) frequencies are found to increase linearly with increase of the absolute value of the C α C β C 4 C 5 torsional angle (χ 2, 1 ), which determines the ring orientation relative to the amino acid backbone. Exceptionally, the ν(N τ C 2 N π ) frequency of the C 2 ‐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 τ C 2 N π ) becomes a marker of hydrogen‐bonding strength when the imidazolium ring is deuterated at C 2 . These marker bands may be useful in studying the conformation and hydrogen‐bonding states of histidine residues in proteins and peptides.