What is the pKa of histidine?


Quick Answer

Francis A. Carey's "On-Line Learning Center for Organic Chemistry" provides three distinct pKa values. The pKa for the a-carboxyl group is 1.82. The pKa for the a-ammonium ion is 9.17. The pKa for the side chain is 6.00.

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Full Answer

All three values are only correct in the free amino acid and can be either irrelevant or different in a polypeptide. In the interior of a polypeptide, both the a-carboxyl group and the a-ammonium group would be occupied by peptide bonds and would no longer have a pKa. Side chains are more complicated, as noted in the Wikipedia article on calculating the pKa in proteins. A side chain in a folded protein might be in the interior of the molecule and not exposed to water. Even if the side chain is on the surface of the protein, exposed to water, the local environment of a polypeptide influences the pKa of histidine's side chain. This is partly because of changes to pH from other side chains that are acidic or basic, and in part because of dipole interactions and similar effects. Wikipedia notes several methods for calculating the pKa in proteins, including finite difference Poisson-Bolzmann, empirical methods, molecular dynamics, and titration curves or free-energy calculations.

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