Question

One method for separating polypeptides makes use of their different solubilities. The solubility of large polypeptides in water depends upon the relative polarity of their R groups, particularly on the number of ionized groups: the more ionized groups there are, the more soluble the polypeptide.
1. Which of each pair of the polypeptides that follow is more soluble at the indicated pH?
(a) (Gly)20 or (Lys-Ala)3 at pH 7.0:
O (Gly)20
O (Lys-Ala)3
(b) (Glu)20 or (Phe-Met)3 at pH 7.0
O (Glu)20
O (Phe-Met)3

Answer 1

Answer:

(a) (Glu)zo or(Phe-Met)3 at pH 7.0

O (Glu)zo ✔

O (Phe-Met)s ❌

(b) (Gly) zo or (Lys-Ala)3 at pH 7.0:

O (Gly12) ❌

O (Lys-Ala)✔

(c) (Ala-Asp-Gly)s or (Asn-Ser-His)s at pH 3.0:

O (Asn-Ser-His)s ✔

O (Ala-Asp-Gly)s ❌

(d) (Ala-Ser-Gly)s or (Asn-Ser-His)s at pH 6.0:

O(Ala-Ser-Gly)s ❌

O (Asn-Ser-Hish)s ✔

Explanation:

Polypeptides that has polar or charged side chains are more soluble than polypeptides with nonpolar side chains.

(a) At ph 7.0

(Glu)20 is negatively charged at pH 7 and more soluble

(Phe-Met)3 is observed to be less polar and less soluble

(b)At ph 7.0

(Lys-Ala)3 is positively charged (polar) and more soluble

(Gly)20 is uncharged as only the amino- and carboxyl-terminal groups are charged as its less polar and less soluble too.

(c) At pH 6.0

(Asn-Ser-His)5 has polar Asn side chains and partially protonated His side chains and it's more soluble unlike the (Ala-Asp-Gly)s at that pH.

(d) At pH 3.0

(Asn-Ser-His)s as partially protonated carboxylate groups of Asp residues and it is also neutral but the imidazole groups of His residues are fully protonated and positively charged. Hence it's more soluble than the (Ala-Ser-Gly)s at that particular pH.