Question

One method for separating polypeptides makes use of their different solubilities. The solubility of large polypeptides in water depends on 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. Which of each pair of polypeptides that follow is more soluble at the indicated pH?

(a) (Gly)20 or (Glu)20 at pH 7.0
(b) (Lys–Ala)3 or (Phe–Met)3 at pH 7.0
(c) (Ala–Ser–Gly)5 or (Asn–Ser–His)5 at pH 6.0
(d) (Ala–Asp–Gly)5 or (Asn–Ser–His)5 at pH 3.0

Answer 1

The answers are below:

1.

(Gly)20 or (Glu)20 at ph 7.0

The answer is (Glu)20 because at a pH of 7 it has high negative charges so (Gly)20 is less soluble since it would remain unchanged

2.

Lys–Ala)3 or (Phe–Met)3 at pH 7.0

The answer is (Lys-Ala)3 because it has high positive charges at this pH while (phe-met)3 is lower this it is less soluble.

3.

Ala–Ser–Gly)5 or (Asn–Ser–His)5 at pH 6.0:

(Asn–Ser–His)5 is more soluble because though both of these polymers have the side chain Ser, (Asn–Ser–His)5 also have polar Asn side chain also it's His side chain is partially proponated.

4.

(Ala–Asp–Gly)5 or (Asn–Ser–His)5 at pH 3.0:

(Asn–Ser–His)5 is more soluble at ph of 3. this is because Asp residues are not fully proponated and they are also neutral, while (Asn–Ser–His)5 has its His residues fully proponated and also positively charged.