Diffusion coefficients of CO2 molecule and bicarbonate ion in hemoglobin solution measured by fluorescence technique

The diffusion process of CO2 within a thin layer of hemoglobin (Hb) solution was followed by pH-sensitive fluorescence of 4-methylumbelliferone. In the presence of sufficient carbonic anhydrase in the layer, the pH change was accelerated and determined only by the diffusion. The diffusion rate was reduced as the Hb concentration was increased. The pH rise observed in the CO2 diffusion out of the layer was slower than the pH fall caused by inward diffusion. However, the outward diffusion rate was faster than the inward diffusion rate in contrast to the pH change. The diffusion coefficients of molecular CO2 and bicarbonate ion were separately determined from the simulated and experimental PCO2-time curves by utilizing the non-linearity of the CO2 dissociation curve. The simulated curve was obtained from a numerical solution of the differential equation for diffusion by using the measured CO2 dissociation curve. The diffusion coefficient of CO2 was assumed to decrease exponentially with increasing Hb concentration. The diffusion coefficient of HCO3- was found to be reduced hyperbolically as the Hb concentration was increased. The diffusion coefficients of CO2 and HCO3- of 100% hemolysate were 0.34 X 10(-5) and 0.14 X 10(-5) cm2/sec, respectively.