Map amino acids have two ionizable functional groups: an -amino group (average p

Question

Map amino acids have two ionizable functional groups: an -amino group (average pKa of 9.4) and an z-carboxylic acid group (average pKa of 2.2). Tyrosine also has an ionizable side chain (R group) with pKa of -10.1. One of the possible ionization states of tyrosine is shown below. CH2 OH At what pH would the structure above be the predominant ionization state? Note: Consider the ionization state of all three of the functional groups. pH= [Select answer Scroll down for more of this question. The protonated form of the R group of tyrosine is shown in the structure above. The ratio of the protonated form to the charged (deprotonated) form depends on the pka of the R group and the pH of the solution. Select the pH value at which the charged form of the R group would predominate.

Explanation / Answer

1. All 3 ionizable functional groups are in the protonated form, including the most acidic one - COOH (pKa=2.2). Consider the buffer theory and the ormula pH = pKa + log([base]/[acid]). At pH=pKa , [base] = [acid]. At pH<pKa, [base] < [acid] .

Thus the answer is pH < 2.2 (or a specific value from the list that is <2.2)

2. The charged form of the phenolic group is –C6H4-O- . Same theory and formula as above. At pH = pKa (pH=10.1) [base] = [acid]. At pH > pKa (pH>10.1) [base] > [acid].

Answer pH =12.2

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