Question

Question B1 The zwitterionic structure for the amino acid phenylalanine is shown below (pKai = 2.0, pka2 = 8.8). + NH3 phenylalanine (a) On the graph below, draw an accurate speciation diagram for this amino acid over the pH range from 1-12. [4 marks] fraction mole 1.0 2.0 3.0 4.0 5.0 8.0 9.0 10.0 11.0 12.0 6.0 7.0 pH

Answer 1

Ans b) At pH1, the medium is highly acidic and rich in protons so the amine group of the amino acid phenylalanine will occur in protonated form at this pH leaving a charge of +1 on the molecule.

At pH 6, The medium has both protons and hydroxyl ions in the solution. In this case, the amino acid will occur in the form of zwitterion predominantly.That is, amino group in protonated form and hydroxyl group in deprotonated form and total charge is 0 on the molecule as +1 and -1 cancel each other.

At pH 12, The medium is rich in hydroxyl ions, so both the amino group and hydroxyl group will be deprotonated leaving a -1 charge on the molecule.

All these structures are shown in the figure below:

At pH LHD Form Chargest At pH 6 LH Form (Zwitterion) charges o At pH 12 L Form charge = -1

Ans a). Speciation diagram of phenylalanine - Phenylalanine exists in the form of LH2+ from pH range of 1.6-6. LH or the zwitterion form exists from 1.7 to 11 pH and the L- form exists from pH 6.0 to 12. I do not have the graph paper so i have made graph on simple paper with markings. Kindly draw yours on a graph paper. The picture is given below:

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