Resonance Raman studies of carbon monoxide binding to iron "picket fence" porphyrin with unhindered and hindered axial bases. An inverse relationship between binding affinity and the strength of iron-carbon bond

Abstract

The stretching frequency of the iron-carbon bond, v(Fe-CO), is a direct measure of the iron-carbon bond strength when there is no change in the Fe-C-O geometry. Here we report resonance Raman detection of v(Fe-CO) frequencies in the CO complexes of iron (II) alpha, alpha, alpha, alpha-mesotetrakis(o-pivalamidophenyl)porphyrin, FeII(TpivPP), with trans ligands of varying strength: N-methylimidazole (N-MeIm), 1,2-dimethylimidazole (1,2-Me2Im), pyridine (py), and tetrahydrofuran (THF). It was found that the weaker the iron-trans ligand bond, the stronger the iron-carbon bond. Comparisons of sterically hindered (1,2-Me2Im) and unhindered (N-MeIm) bases are of particular interest because of their implication in the phenomenon of hemoglobin cooperativity and the mechanisms of protein control of heme reactivity. While the CO binding affinity of FeII(TpivPP)(1,2-MeIm) is approximately 400 times lower than that of FeII(TpivPP)(N-MeIm), the v(Fe-CO) frequency for the former (at 496 cm-1) is higher than that for the latter (at 489 cm-1). This example shows that the CO binding affinity cannot be directly correlated with the strength of the iron-carbon bond. Comparison of the CO binding to FeII(TpivPP)(THF) and FeII(TpivPP)(N-MeIm) reveals a similar relationship; the v(Fe-CO) frequency (at 527 cm-1) in FeII(TpivPP)(THF)(CO) is 38 cm-1 higher than that in FeII(TpivPP)(N-MeIm)(CO), but the CO binding affinity is lower for the THF complex.

Cytochrome a 3 Structure in Carbon Monoxide-Bound Cytochrome Oxidase

“…The line width (<10 cm-') of the 520cm-1 line is smaller than in model compounds and other proteins (9,12,13) and is consistent with the narrow width of the C-O stretching mode at 1963.5 cm-' detected by infrared absorption (14). The narrow width of the Fe-CO stretching mode implies a homogeneous, ordered environment for the ligand and a welldefined conformation, in accord with results of infrared studies showing that the CO is in a homogeneous, hydrophobic pocket (14).…”

A New Way to Degrade Heme

“…3. In the high frequency region, an isotope-sensitive signal is seen at 1928 cm Ϫ1 for the 12 Fe-C and C-O stretching modes normally show an inverse correlation that is sensitive to nature of the heme axial ligand (24). The data point for monoheme-MhuD is close to a correlation line for hemes with an imidazole ligand (Fig.…”

Vibrational dynamics of biological molecules: Multi-quantum contributions

“…2) suggests that the calculation provides a reliable guide to the character of the individual modes. This includes observed features at 507 cm -1 , 561 cm -1 , and 586 cm -1 ,associated with Fe-CO stretching and FeCO bending, as previously seen in numerous heme proteins [50][51][52][53][54][55][56][57][58][59][60]. A more detailed description of these modes, together with measurements on related compounds, will appear elsewhere.…”