Answer:
a) Arginine will have a positively charged R- group at pH 7.00
b) Aspartate will have a negatively charged R- group at pH 7.00
c) Asparagine has the neutral polar R- group
d) Valine has an isopropyl group a side chain.
e)Tryptophan has an aromatic side chain
Explanation:
List the amino acids given and note down the pKa values of the -COOH group, the -NH2 group and the side chain.
Asparagine
Tryptophan
Aspartate
Valine
Arginine
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Asparagine:
pka1: 2.02
pka2: 8.80
pl: 5.41
Tryptophan:
pka1: 2.83
pka2: 9.39
pl: 5.89
Aspartate:
pka1: 1.88
pka2: 9.60
pka3: 3.65
pl: 2.77
Valine
pka1: 2.32
pk2: 9.62
pl: 5.96
Arginine
pka1: 2.17
pka2: 9.04
pka2: 6.00
pl: 7.59
When the pH of the amino acid solution is less than the pI of the amino acid, the amino acid will be positively charged. The amino acid will be negatively charged when the pH < pI. When the pH of the solution equals the pI of the amino acid, the amino acid will be neutral and exist as a Zwitterion.
1) The pI of Arginine is greater than the pH of the solution; hennce Arginine will have a positively charged R- group at pH 7.00 (ans).
2) The pI of Aspartate is much lower than the pH of the solution at pH 7.00; hence, Aspartate will have a negatively charged R- group at pH 7.00 (ans).
3) Asparagine has a –CONH2 group as a side chain. Amide (-CONH2) group is polar, but neutral; hence Asparagine has the neutral polar R- group (ans).
4) Valine has an isopropyl group a side chain. An alkyl group is non-polar and neutral; hence, Valine is the correct answer (ans).
5) Tryptophan has an aromatic side chain and is the correct choice (ans).
