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

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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 NτC2Nπ symmetric stretching mode, ν(NτC2Nπ), characteristic of the N‐deuterated imidazolium ring. By comparison with the crystal structures, the ν(C4C5) and ν(NτC2Nπ) frequencies are found to increase linearly with increase of the absolute value of the CαCβC4C5 torsional angle (χ2, 1), which determines the ring orientation relative to the amino acid backbone. Exceptionally, the ν(NτC2Nπ) 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(NτC2Nπ) 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.