Ionic strength dependence of the electric dissociation field effect. Investigation of 2,6-dinitrophenol and application to the acid-alkaline transition of metmyoglobin and methemoglobin.

We present data on the ionic strength dependence of the dissociation field effect (2nd Wien effect) of the protolytic reaction of 2,6-dinitrophenol in aqueous solution, monitored through optical absorption changes in high electric fields. The results are in very good agreement with the Onsager-Liu theory. We then investigate the field strength dependence of the acid-alkaline transition, i.e., the hydrolysis reaction of the water coordinated to the heme iron, in metmyoglobin and methemoglobin. The true field effect of the reaction is determined from measurements in buffers which exhibit no field effect. We conclude that negative charges on the protein influence the field effect in methemoglobin but not in metmyoglobin. The Onsager-Liu theory is applied to estimate the number of charges involved.