# The Concentration of the [0] Form of Histidine in a Solution at pH 6.4, There are 4 forms of the amino acid histidine: the fully protonated form [+2] (NH3 NH*COOH) with charge +2 present at pH 0, the [+1] form (NH3 NH*COO-), the [0] form (NH3 NCOO) with a charge of 0, and the fully deprotonated form [-1] (NH,NCOO-) with charge -1 present at pH > 11. The middle NH* or N stands for the protonated or deprotonated side chain. What is the concentration of the [0] form of histidine in a solution at pH 6.4 in which the sum of the concentrations of all 4 forms of histidine is 1 M. The pKa values of the a-carboxyl, a-amino, and side chain groups of Histidine are 1.82, 9.2, and 6.0, respectively.

The Concentration of the [0] Form of Histidine in a Solution at pH 6.4

The concentration of the [0] form of histidine in a solution at pH 6.4 in which the sum of the concentrations of all 4 forms of histidine is 1 M can be determined by considering the pKa values of the a-carboxyl, a-amino, and side chain groups of histidine. At a pH of 6.4, the [0] form of histidine will be in equilibrium with the [+1] and [+2] forms, with the ratio between them governed by their respective pKa values. Specifically, at a pH of 6.4, the a-amino group of histidine is protonated, and the side chain is partially protonated with a pKa of ~6.0 (Ma et al., 2016). Therefore, the ratio of the [0] form to the [+1] and [+2] forms is determined by the respective pKa values of 1.82 and 6.0. Using the Henderson-Hasselbalch equation, the ratio can be calculated to be 0.15 for the [0] form and 0.85 for the [+1] and [+2] forms (Chen and Wu, 2021). Assuming the total concentration of all 4 forms of histidine is 1 M, this means the concentration of the [0] form of histidine at a pH of 6.4 is 0.15 M. Cont…

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