Confirmation of the Assignment of the Iron-Histidine Stretching Mode in Myoglobin

Resonance Raman spectra of myoglobins reconstituted with hemes isotopically substituted at the central iron atom or the pyrrole nitrogen atoms have been recorded to address the issue of whether the strong line at ∼220 cm⁻¹ is the iron-histidine stretching mode or the iron-pyrrole nitrogen stretching mode. The frequency of the line at 220 cm⁻¹ is 1.7 cm⁻¹ lower in myoglobin reconstituted with the ⁵⁷Fe heme than it is in the ⁵⁴Fe-substituted heme. No large shifts were detected in any other Raman lines. When myoglobin reconstituted with ¹⁵N-substituted pyrrole nitrogens in the heme is compared to the unsubstituted myoglobin no large change is detected in the line at 220 cm⁻¹, but the frequency of the line at 243 cm⁻¹ is 1.5 cm⁻¹ lower. In comparing myoglobin buffered in D₂O to that buffered in H₂O only the line at 220 cm⁻¹ changes frequency (1.4 cm⁻¹). From these isotopic substitution studies, we conclude that the line at ∼220 cm⁻¹ in myoglobin is the iron-histidine stretching mode. The mode at ~243 cm⁻¹ has a significant contribution from the pyrrole nitrogens, and it is likely an out-of-plane pyrrole tilting mode. The ⁵⁴Fe-⁵⁷Fe isotope shift of 1.7 cm⁻¹ in the 220-cm⁻¹ line is smaller than predicted for a diatom oscillator of the iron and the histidine. We conclude that the iron-histidine stretching mode is either mixed with an internal mode of the histidine and/or mixed with skeletal modes of the porphyrin macrocycle.