Question

Please help:

Free histidine in solution has three pKas (COOH=1.82; NH₃⁺=9.17, and R-group=6.0).

Determine if each group acts as a weak acid or base? And at pH7 does the protanted >,<,= deprotonated form of this side group ?

example: weak ________ (acid/base) .... protanted _________ ( >,<,=) deprotonated.

a: COOH = pkka = 1.82

b: NH₃^{+ =} = pka = 9.17

c: R group = pka = 6.0

Answer 1

COOH group of histidine acts as a weak acid in free solution but it has two other groups like NH₃ and imidazole group which contributes to the overall basicity of the histidine. pKa of R group and ammonium group is high enough that they have the tendency to bind proton.

At pH 7, it will be found in the protonated form > than deprotonated form. Because if isoelectric point, that is if pI is greater than pH than amino acids in found to have positive charge that means it exist in protonated form.

pI of histidine = pKa (NH_{3) +} pKa (R group) / 2

9.17 +6/2 = 7.59

So, pI (7.59) > pH (7)

Protonated form > deprotonated form