Constants of Appropriate Small Molecules

In most common proteins, one out of every three or four amino acid residues contains a titratable acidic or basic group. The number of titrat-able groups per molecule thus ranges from about 20 to about 250 for the many common proteins with molecular weights below 100,000, and it is even larger for proteins with very high molecular weight. 

The relation between structure and acidity of organic compounds has been the subject of much study. Those aspects which are of interest in connection with protein titration curves have been reviewed in definitive manner by Edsall and Wyman (1958) and by Edsall (1943), and the reader is referred to these reviews for a discussion of the theoretical and empirical principles which are involved. For the present purpose it is sufficient to extract the data which will lead to the expected pK values of the titratable groups of proteins, and this has been done in Table I. 

Dissociation Constants of Model Compounds in Aqueous Solution at or Near M°C  

Compounds resembling 3-COOH group of aspartic acid side chain  

Compounds resembling 7-COOH group of glutamic acid side chain  

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