The oscillator strengths of hemoproteins. Their relation to the coordination structure and magnetic susceptibility.

Abstract

The oscillator strengths of hemoproteins in the light frequency range of 1.11 X 10(4) to 3.23 X 10(4) cm-1 (wavelength range of 900 to 310 nm) were measured by means of computer-assisted spectrophotometry. The obtained values of oscillator strength per molar heme ranged from about 1.4 to 2.2. By comparing the oscillator strength values of the ferric and ferric cyanide-bound forms of hemoproteins and also the values of low molecular weight ferric heme complexes, it was found that the oscillator strength was lower for those hemoproteins whose heme was coordinated with strong field ligands. It was also found that the hemoproteins showing a smaller pH-dependent change in the carbon monoxide-difference spectrum had lower oscillator strengths. The following linear relation was observed, with various ligand complexes of bovine methemoglobin, horse metmyoglobin, and ferric horseradish peroxidase, between the oscillator strength (f) determined in the present study and the respective magnetic susceptibility (10(6) X chi 20 degrees M) values in the literature: f = A (10(6) X chi 20 degrees M) + B. The values of constants A and B in the equation were estimated for horseradish peroxidase, methemoglobin, and metmyoglobin. On varying the temperature in the range of 0 to 40 degrees C, the oscillator strength of the metmyoglobin-azide complex changed in parallel with the change in the spin state. Taking advantage of the fact that fluoride complexes of many hemoproteins show 10(6) X chi 20 degrees M values close to 14,500 and also that the values of intersection B are around 86.4% of the respective values of the fluoride complexes of ferric horseradish peroxidase, methemoglobin, and metmyoglobin, an empirical equation was evolved for the calculation of an approximate 10(6) X chi 20 degrees M value from the f value of a given complex (fobs) and that of the fluoride complex (fF) of a hemoprotein. The approximate magnetic susceptibilities of various ligand complexes of bovine lactoperoxidase could be thus calculated with the equation. The oscillator strengths of ferrous hemoproteins were also investigated and ligand-dependent regular changes were found.