14. Solubility of Polypeptides One method for separating polypeptides makes use

Question

14. Solubility of Polypeptides One method for separating polypeptides makes use of their different solubilities. The sol- ubility polarity of their R groups, particularly on the number of ion- ized groups: the more ionized groups there are, the more solu- ble the polypeptide. Which of each pair of polypeptides that follow is more soluble at the indicated pH? of large polypeptides in water depends on the relative (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

Explanation / Answer

Answer Polypeptide with polar or charged side chains are more soluble than polypeptide with non polar side chain.

a (Glu )20 is negatively charged at pH 7, and (Gly)20 is uncharged except amino and carboxyl terminal group.

B (Lys-Ala)3 is positive charge (Phe-Met)3 is much less polar hence less soluble.

C (Asn-Ser-His)5 has polar at pH 6, ASN side chains and partially protonated His side chains.

D Carboxylate group of Asp residue are partially protonated and neutral at pH 3 whereas the imidazole groups of His residues are fully protonated and positively charged

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