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. Which of each pair of the polypeptides that follow is more soluble at the indicated pH?
(a) (Gly) zo or (Lys-Ala)3 at pH 7.0: O (Gly12 O (Lys-Ala)
(b) (Glu)zo or(Phe-Met)3 at pH 7.0 O (Glu)zo O (Phe-Met)s
(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: (Ala-Ser-Gly)s O (Asn-Ser-Hish

Answer 1

Answer:

The solubility of any substance in water depends on its polarity. The polar molecule dissolves more easily in the water than the non polar molecule. Amino acids have different polarity at different pH.

(a) (Lys-Ala)3 : The (Lys-Ala)3 contain more positive charge and highly polar the at pH 7 as compared with the  (Gly)20 that remains uncharged and has less polarity.

(b) (Glu)20 :This amino acids negatively charged at pH 7 and highly polar than the (Phe-Met)3 which is non polar or less polar and hence least soluble  in water.

(c) (Asn-Ser-His)5 is highly polar at pH 3 as compared with the (Ala-Asp-Gly)5 that contains the carboxylate groups of Asp residues that are partially neutral. The  (Asn-Ser-His)5 contains = the imidazole groups of His residues are fully polar and soluble in water.

(d) (Asn-Ser-His)5 is polar at pH 6.0 because both have polar side chains Ser but (Asn-Ser-His)5 is only partially partially protonated due to the His side chains. Thus,  (Asn-Ser-His)5 is more soluble in water.