Tutorial Practice Questions - Week 3

Question 1

Calculate the H+ concentration of a solution with a pH of
a) 4
b) 11

Answer 1

a) pH = -log [H+], [H+] = 10-4 M= 0.0001 M = 1 x 10-4 M = 0.1 mM

b) pH = -log [H+], [H+] = 10-11 M

Question 2

HCl is a strong acid

a) Write out the dissociation reaction for hydrochloric acid (HCl) in water

b) Calculate the pH of a solution of 5.0 x 10-4 M HCl.

Answer 2

a) HCl - -> H+ + Cl-

b) pH = -log (5.0 x 10-4) = 3.3

Question 3

Identify the conjugate base in each of the pairs below:
a) NH3+/NH2
b) COOH/COO-
c) H2PO4-/H3PO4

Answer 3

a) NH2
b) COO-
c) H2PO4-

Question 4

The amino acid glycine is often used as main ingredient of a buffer in biochemistry.
This amino acid only 2 ionizable groups: the amino group (NH3+) with a pKa of 9.6,
and the Carboxyl group (COOH) with a pKa of 2.34.

a) In what pH range can glycine be used as a buffer due to its amino group?

b) What is the isoelectric point of glycine?
c) What is the charge of glycine and pH 12.

Answer 4

a) pKa = 9.6, buffering region is the pH ranging between 8.6-10.6.

b) pI = (2.34 + 9.6)/2 = 5.97

c) Charge is -1, as all the H+ have been dissociated, NH2 and COO-

Question 5

Look at the structure, the pkas and the side chain of the glutamate amino acid.

a) Calculate the pI of the amino acid glutamate.

b) Calculate the charge of the amino acid glutamate at pH 7.4.

Answer 5

a) pI is the pH at which a molecule has an overall charge of zero.
pKa (COOH in alpha carbon) = 2.19, pKa (COOH in side chain) = 4.25
pI= (2.19 + 4.25)/ 2 = 3.22

b) The overall charge is -1: the COOH in the side chain is ionised to COO- (-1), the
COOH in the alpha carbon is COO- (-1), and the amino group NH3+ is protonated
(+1).

Question 6

1. Look at the amino acid table, and identify the amino acid residues in the sequences
   below that have an ionisable side chain (i.e. are classified as positive and negative).

a) Cys-Glu-Tyr-Lys-Lys-Arg-Cys-Asp-Glu-Phe-His-Pro-Val

b) Met-Ala-Gly-Trp-Glu-Asn-Gln-Cys-Arg-Gln-Tyr-Lys-Leu

2. Based on the amino acid residues identified as charged, and on the pKa of their side
   chain, calculate the charge of the peptides a) and b) at pH7.4

Answer 6

1.a) Lys, Lys, Arg, His are positive; Glu, Asp and Glu are negative

1.b) Glu is negative; Arg, Lys are positive.

2.a) Overall charge is 0 (the side chain of His is not protonated at this pH, it loses the
H+ at pH 6)

2.b) Overall charge is +1