Kinetics and equilibria in systems containing haem, carbon monoxide and pyridine

Abstract

Spectrophotometric observations of heme and CO-heme(CO-H) in aqueous solution and in ethylene glycol suggest that CO-H is monomeric in both solvents, whereas heme itself is dimeric in aqueous solution but is depolymerized in ethylene glycol. The dissociation curve of CO-H in aqueous solution was found to be a rectangular hyperbola. Different heme solutions gave 50% saturation at 0.75 and 3.5-4.5 mm-Hg at 20[degree]. The material also obeyed Beer''s law over the range of CO concentration 0.075-65[mu][image]. The photochemical absorption spectrum of CO-H indicated that only a part of the material was being dissociated on flash photolysis. A new peak appeared at 420 mu, corresponding in position to the peak of monomeric heme in glycol. It is suggested that this new peak is caused by the absorption spectrum of the heme nucleus subsequent to the photo-decomposition. The dissociation of pyridine hemochromogen shows 3 stages. Spectrophotometric evidence supports the belief that 3 compounds of heme and pyridine are formed consecutively, the last of which is the well-known "pyridine hemochromogen". It is suggested in explanation of these observations that 2 of the compounds are dimeric, only "pyridine hemochromogen" itself being monomeric. The rate of combination of CO with 3 closely related hemes was measured by the flash-photolysis technique. The rates of combination with proto-, meso-, and deutero-heme respectively were found to be approximately 16, 85 and 85/uM-1 sec.-1 in 0.02N NaOH at room temperature. The rate of combination of Co with the pyridine hemochromogens of the same 3 hemes was similarly measured. The rates were found to be respectively 0.42, 1.04 and 1.30uM-1 sec.-1 under the same conditions. A theoretical explanation of the variation in these results, and also of those reported in the next paper, is attempted.