One method for separating polypeptides utilizes their different solubilities. The solubility of large polypeptides in water depends upon the relative polarity of their R groups, particularly the number of ionized groups. The more ionized groups there are, the more soluble the polypeptide. Which polypeptide, ( Phe − Met ) 3 or ( Glu ) 20 , is more soluble at pH 7.0 ?

Polypeptides are composed of amino acids, and we know amino acids are differently charged, have different R groups, and also have different isoelectric points. Depending on different isoelectric points and charged groups, the polypeptides can be separated and because a protein has its lowest solubility on its isoelectric point.

So in this question,

(a) (Lys-Ala)3 ; this is highly positively charged (polar) at pH 7 than (Gly)20 which is uncharged except for the amino and carboxyl terminal.

(b) (Glu)20 ; it is highly negatively charged at pH 7 whereas (Phe-Met)3 is much less polar and hence less soluble.

(c) (Asn-Ser-His)5 ; at pH 3, because in (Ala-Asp-Gly)5 the carboxylate groups of Asp residues are partially protonated and neutral, whereas in (Asn-Ser-His)5 , the imidazole groups of His residues are fully protonated and positively charged.

(d) (Asn-Ser-His)5; at pH 6.0; both polymers have polar Ser sidechains, but (Asn-Ser-His)5 also has the polar Asn side chains and partially protonated His side chains.

Explanation: