Raman spectroscopic study of the interaction between sulfate anion and an imidazolium ring in ribonuclease A

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Abstract
Raman spectra of [bovine pancreas] RNase A in D2O solution at various pD values were studied with special attention to the N-deuterated imidazolium ring vibration at 1408 cm-1, the SO42- symmetric stretching vibration at 984 cm-1, the amide I'' band, and the tyrosine doublet. Concomitant decrease and increase in the intensities of the 1408- and 984-cm-1 bands in the pD range between 5 and 7 indicate that a sulfate anion is actually H-bonded to an imidazolium ring of a histidine residue located in the interior of the molecule. The mechanism of the sulfate desorption was compared with that on heat denaturation.