The carbamate reaction of carbon dioxide with glycyl-glycine

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Abstract
The carbamate equilibrium between carbon dioxide and glycyl-glycine is believed to form a model for the similar, but more complex, equilibria of carbon dioxide with the terminal [alpha]-NH2 groups of the haemoglobin molecule. In the case of glycyl-glycine the carbamate equilibrium constant, Kc, has been estimated from the fall, after one second, in the concentration of dissolved CO2 when a solution of the latter is rapidly mixed with a solution of glycyl-glycine and the continuously flowing, mixed fluid is caused to impinge on a CO2 electrode situated at a distance from the mixing chamber equivalent to a lapsed time of 1 s. In this period a rapid non-carbonate equilibrium between CO2 and the amine is established, without significant hydration of CO2 to bicarbonate and carbonate. The values of Kc, thence obtained, tally reasonable with those given by the classical barium precipitation method and by the total equilibrium method, but exceed considerably the values of Kc given by the manometric methods hitherto used for quantitative studies on the rapid non-carbonate equilibrium. The cause of this discrepancy has been investigated and has been shown to be due to diffusion errors associated with the manometric method. A theoretical investigation, with some experimental confirmation, is also reported of the progressive change in pH in CO2-amine systems as they pass from the rapid non-carbonate equilibrium to the total equilibrium. According to circumstances this change of pH may be positive, negative or practically zero. In the course of this study evidence has been obtained that the carbamate compound of glycyl-glycine is a stronger acid than carbonic acid in the first stage (pK = 6.26 at 20[degree]C), so that the pK of glycyl-glycine carbamic acid probably does not exceed 6 and might be appreciably less.

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