One method for separating polypeptides makes use of their different solubilities

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? Please provide reasoning. Thank you!!

(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.

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

Explanation / Answer

(a) (Glu)20 is more soluble at pH 7.0

Explanation- (Gly) 20 is uncharged except for the amino- and carboxyl-terminal groups. (Glu)20 is highly negatively charged at pH 7. So (Glu)20 is more soluble. Polypeptides with polar or charged side chains are more soluble than polypeptides with nonpolar side chains.

(b) (Lys–Ala)3 is more soluble at PH 7.0

Explanation-(Lys-Ala)3 is positively charged at pH 7 ad polar. (Gly (Phe-Met)3 is much less polar and hence less soluble. Polypeptides with polar or charged side chains are more soluble than polypeptides with nonpolar side chains.

(c) (Asn–Ser–His)5 is more soluble at pH 6.

Explanation-Both polymers have polar Ser side chains, but (Asn–Ser–His)5 also has the polar Asn side chains and partially protonated His side chains at pH 6.0.(Asn-Ser-His)5 has polar Asn side chains and partially protonated His side chains. So it is more soluble.

(d) (Asn–Ser–His)5 is more soluble at pH 3.0

Explanation- At pH 3.0, the carboxylate groups of Asp residues are partially protonated and neutral, whereas the imidazole groups of His residues are fully protonated and positively charged. So it is more soluble.

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