Analytical Chemistry and Spectroscopy
Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
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.
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