Which pH values would potentially provide the best separation of the three peptides?

[andy]

A solution contains a mixture of three peptides, Tyr-Arg-Thr, Glu-Ala-Phe and Asp-Pro-Lys. The ?-COOH groups have a pKa of 3.8; the ?-NH3+ groups have a pKa of 8.5.

(a) What is the net charge on each of these three peptides at pH 2.0, 6.0 and 13.0?
(b) Which of these pH values would potentially provide the best separation of the three peptides in the mixture and why?

[Kunal Pandya]

Tyr-Arg-Thr (Tyr and Arg having ionizing side chain while threonine not)

At pH~2 :- NH3+ [+1], TyrR [0] ,Arg(R+) [+1] , COOH [0]
Net Charge = 1+0+1+0= +2

At pH~6 :- NH3+ [+1], TyrR [0] ,Arg(R+) [+1] , COO-1 [-1]
Net Charge = 1+0+1-1 = + 1

At pH~13 :- NH2 [0], TyrR [-1] ,Arg(R) [0] , COO-1 [-1]
Net Charge = 0-1+0-1 = -2


2) Glu-Ala-Phe (Glu having ionizing side chain while Ala,Phe not)

At pH~2 :- NH3+ [+1], GluR [0], COOH [0]
Net Charge = 1+0+0 = 1

At pH~6 :- NH3+ [+1], GluR [-1], COO -1[-1]
Net Charge = 1-1-1 = -1

At pH~13 :- NH2 [0], GluR [-1], COO -1[-1]
Net Charge = 0-1-1 = -2

3) Asp-Pro-Lys (Aspartic and Lysine having Ionizing side chain while pro not)

At pH~2 :- NH3+ [+1], AspR [0] ,Lys(R+) [+1] , COOH [0]
Net Charge = 1+0+1+0 = + 2

At pH~6 :- NH3+ [+1], AspR [-1] ,Lys(R+) [+1] , COO-1 [-1]
Net Charge = 1-1+1-1 = 0

At pH~13 :- NH2 [0], AspR [-1] ,Lys(R) [0] , COO-1 [-1]
Net Charge = 0-1+0-1 = -2


pH value ~6 provide the best separation for all the three peptide in a mixture because at this pH value all have different charge value i.e. +1, 0 ,-1