Differences in iron-fluoride bonding between the isolated subunits of human methemoglobin fluoride and sperm whale metmyoglobin fluoride as measured by resonance Raman spectroscopy

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Abstract
The heme geometries of the isolated .alpha.- and .beta.-subunits of human methemoglobin fluoride (HbIIIF) and sperm whale metmyoglobin fluoride (MbIIIF) were examined by exiciting their Raman spectra within their .apprx. 6000-.ANG. charge-transfer absorption bands. The Fe.sbd.F stretching vibration at 471 cm-1 in the .beta.-subunits shifts to 466 cm-1 in the .alpha.-subunits and to 461 cm-1 in MbIIIF. The Fe.sbd.F bond is estimated to elongate by 0.02 .ANG. in the .alpha.-subunits and 0.03 .ANG. in MbIIIF compared with that in the .beta.-subunits. This bond elongation is interpreted to result from an increased Fe displacement toward the proximal histidine side of the heme in the series MbIIIF > .alpha. > .beta.. A comparison of the isolated subunit spectra with that of tetrameric HbIIIF indicates that little change occurs in isolated subunit heme geometry upon association into tetrameric HbIIIF. A correlation is found between the .lambda.max of the 6000-.ANG. charge-transfer absorption band and the Fe.sbd.F bond length. Elongation of the Fe.sbd.F bond is associated with a shift of the absorption spectral maximum to a longer wavelength. The absorption spectra shift induced by the inositol hexaphosphate induced R .fwdarw. T conversion does not result from a change in the Fe.sbd.F stretching frequency (.+-. 0.5 cm-1). Frequency shifts are observed for heme macrocyclic vibrational modes. The data are interpreted to indicate that the effect of the R .fwdarw. T conversion in HbIIIF is to perturb heme macrocycle conformation without altering the heme out-of-plane Fe distance or the Fe.sbd.F bond length.