Map 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) s

Answer: For (a):

(Lys-Ala) 3 is soluble and (Gly) zo is not soluble.

(b) (Glu) zo is soluble and (Phe-Met) is not soluble.

(c) (Asn-Ser-His) is soluble and (Ala - Asp-Gly) is not soluble.

(d) (Asn-Ser-His) is soluble and (Ala-Ser-Gly).

Explanation:Firstly, the solubility of polypeptides relies on polar groups in the R group of the side chain and also ionized groups because these groups can interact with dipole bond of water.

For (a) : (Gly) zo, no R group of the side chain of (Gly) zo is polar nor ionizable and solubility is based on the ability of R group of polypeptides to be polar or ionized while in polypeptides of (Lys-Ala), Lys is polar and therefore soluble though Ala in the compound is not polar nor ionizable but the presence of Lys makes the polypeptide to be soluble.

(b) (Glu) zo is a polar peptide, therefore soluble peptides while both the combination of (Phe-Met) 3 gives polypeptides that are not polar therefore not soluble.

(c) In poypeptide of (Ala-Asp-Gly) s, the R group on the side chain of both Ala and Gly are neither polar nor ionizable therefore cannot be soluble even though Asp (which at PH 3 can partially be protonated and partially deprotonated to make it soluble) is present in the polypeptide. Therefore the polypeptide of Ala-Asp-Gly is not soluble compared to Asn - Ser-His, where Asn has polar covalent bond which can interact with water, His which at pH 6 will be half protonated and half deprotonated making the substance to react with water and also Ser is a soluble polypeptide too.

(d) Also between (Ala - Ser-Gly) and (Asn - Ser-His), polypeptide with (Asn-Ser-His) will be considered soluble because Ala and Gly have R groups that neither polar nor ionizable though Ser is soluble but common to both substances, its influence will not be effective and (His) is a soluble which at pH 6 get partially protonated and deprotonated.