Laser Raman spectroscopy of adrenal iron-sulfur apoprotein: The anomalous tyrosine residue at position 82

Abstract

The Raman spectrum of adrenodoxin was measured, with particular emphasis on the Raman doublet due to 82Tyr, a residue which emits anomalous fluorescence. The tyrosine doublet is located at 860 and 832 cm−1 and the doublet relative intensity ratio is approximately 2 to 10. The very high frequency of the 860 cm−1 band and the very low relative intensity ratio I860/I832 suggest that the single tyrosine residue in adrenodoxin is unusually strongly hydrogen-bonded to a neighboring carboxylic acid residue. Comparison of the adrenodoxin Raman doublet with those for ionized tyrosine both in aqueous solution and in the anhydrous potassium salt leads us to conclude that the tyrosine is not ionized, though there is probably a very great electron density on the tyrosyl hydroxyl group. The present results together with the Chou-Fasman calculation of predicted secondary structure provide evidence that the 82Tyr hydroxyl group is very strongly hydrogen-bonded to a nearby glutamic acid residue.