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Resonance Raman Studies of Co—O2 and O—O Stretching Vibrations in Oxy-Cobalt Hemes

By Helen C. Mackin, Motonari Tsubaki and Nai-Teng Yu


Strong evidence suggests that the stretching vibration of the bound oxygen can be perturbed by an accidentally degenerate porphyrin ring mode, resulting in two split frequencies. In the Co(II)(TpivPP) (pyridine) 18O2 complex, we demonstrate that the ν(18O—18O) mode, after being shifted from its ν(16O—16O) value at 1,156 cm-1, undergoes a resonance interaction with the 1,080 cm-1 porphyrin mode, giving rise to two lines at 1,067 and 1,089 cm-1. In the O2 complex of Co(II) mesoporphyrin IX-substituted sperm whale myoglobin, we observed a dramatic intensity increase at 1,132 cm-1 upon 16O2 → 18O2 substitution, which is due to the reappearance of the 1,132-cm-1 porphyrin mode after the removal of resonance conditions. A decrease in O2 binding affinity, caused by the proximal base tension, corresponds to an increase in the Co—O2 stretching frequency. The ν(Co—O2) at 527 cm-1 for the low affinity Co(II)(TpivPP)(1,2-Me2Im) O2 complex is 11 cm-1 higher than the 516-cm-1 value for the high affinity complex (with N-MeIm replacing 1,2-Me2Im). However, in the corresponding iron complexes the reverse behavior is observed, i.e., the ν(Fe—O2) decreases for the (1,2-Me2Im) complex. There is a 24-cm-1 difference in the Co—O2 stretching frequencies between Co(II)(TpivPP)(N-MeIm)O2 (at 516 cm-1) and oxy meso CoMb (at 540 cm-1), suggesting a protein induced distortion of the Co—O—O linkage. However, the values for ν(Fe—O2) are nearly identical between Fe(II)(TpivPP)(N-MeIm)O2 (at 571 cm-1) and oxy Mb (at 573 cm-1), indicating that O2 binds to myoglobin in the same manner as in the sterically unhindered “picket fence” complex. Evidence is presented that suggests the presence of two dioxygen stretching frequencies due to two different conformers in each of the N-MeIm and 1,2-Me2Im complex of oxy Co(II)(TpivPP)

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