Resonance Raman examination of axial ligand bonding and spin-state equilibria in metmyoglobin hydroxide and other heme derivatives

Abstract

Resonance Raman spectra and excitation profiles have been obtained within the 5700-6300-A absorption band of purified sperm whale metmyoglobin hydroxide (MbIIIOH) solutions. A large enhancement occurs for a Raman peak at 490 cm-1 which is shown by isotopic substitution of 18O for 16O to be almost purely an Fe-O stretch. The Fe-O vibration in MbIIIOH occurs 5 cm-1 to lower energy than the corresponding vibration at 495 cm-1 in human methemoglobin hydroxide (HbIIIOH) [Asher, S., Vickery, L., Schuster, T., & Sauer, K. (1977) Biochemistry 16, 5849], reflecting differences in ligand bonding between Mb(III) and Hb(III). A larger frequency difference (10 cm-1) exists between MbIIIF and HbIIIF for the Fe-F stretch. We do not observe separate Fe-O or Fe-F stretches from the alpha and beta chains of either HbIIIOH or HbIIIF. Excitation profile measurements for MbIIOH indicate that the 5700-6300-A absorption band is composed of two separate absorption bands which result from a high- and a low-spin form of MbIIIOH. The spin-state-sensitive Raman band at 1608 cm-1 reflects the high-spin species and has an excitation profile maximum at about 6000 A while the low-spin Raman band occurs at 1644 cm-1 and shows an excitation profile maximum at 5800 A. The Fe-O stretch at 490 cm-1 has an excitation profile maximum at about 6000 A. The differences in frequency and Raman cross section between the Fe-X vibrations in MbIIIX and HbIIIX (X = OH-, F-) can be related to increases in the out-of-plane iron distance for the high-spin species of MbIIIX. The shift in the 1644-cm-1 MbIIIOH low-spin state Raman band indicative of the heme core size to 1636 cm-1 in HbIIIOH indicates a larger heme core size in HbIIIOH. Raman frequency shifts are used to estimate differences in bond strain energies between MbIIIX and HbIIIX (X = OH-, F-). Previous resonance Raman excitation profile data can be interpreted in terms of separate contributions from different spin-state species.