Question

13. Net Electric Charge of Peptides A peptide has the sequence Glu–His–Trp–Ser–Gly–Leu–Arg–Pro–Gly. a. Calculate the net charge of the molecule at pH 3, 8, and 11. (Incorporation into a peptide can alter pKa values somewhat, but for this exercise, use pKa values for side chains and terminal amino and carboxyl groups as given in Table 3-1.) b. Estimate the pI for this peptide.

          13. Net Electric Charge of Peptides A peptide has the sequence Glu–His–Trp–Ser–Gly–Leu–Arg–Pro–Gly.
a. Calculate the net charge of the molecule at pH 3, 8, and 11.
(Incorporation into a peptide can alter pKa values somewhat, but for this exercise, use pKa values for side chains and terminal amino and carboxyl groups as given in Table 3-1.)
b. Estimate the pI for this peptide.

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13. Net Electric Charge of Peptides A peptide has the sequence Glu–His–Trp–Ser–Gly–Leu–Arg–Pro–Gly.
a. Calculate the net charge of the molecule at pH 3, 8, and 11.
(Incorporation into a peptide can alter pKa values somewhat, but for this exercise, use pKa values for side chains and terminal amino and carboxyl groups as given in Table 3-1.)
b. Estimate the pI for this peptide.

Added by Angela B.

Chemistry: Structure and Properties

Nivaldo Tro 2nd Edition

Instant Answer

Solved by Expert Madhur L

10/05/2023

Step 1

For this peptide, the ionizable groups are the N-terminal amino group, the C-terminal carboxyl group, and the side chains of Glu (glutamic acid), His (histidine), and Arg (arginine). The pKa values for these groups are typically as follows (assuming the values are Show more…

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Transcript

00:01 Hello, in this question we are given with this peptide chain, we have to calculate the net charge and pi of the peptide.

00:09 The solution is, we have this criteria, at low ph all ionizable group will be in protonated form, at very high ph all ionizable group will be in deprotonated form.

00:22 There is a rule, the ionizable group at ph less than its pka, that ionizable group will be in protonated form.

00:31 Now, the general ionizable groups in a given peptide are c -terminal carboxy group pka3, n -terminal amino group pka9 .6, r -group of arginine pka12 .9, r -group of glutrate pka4, r -group of tyrosine pka10, r -group of lysine pka10, r -group of arginine pka12 .5, r -group of hestadine pka6.

00:57 So, these are the different ionizable groups in the given peptide with their pka values.

01:04 So, in this given peptide chain, this hestadine r -group has a plus 1 charge and this arginine group, it carries plus 1 charge and this glutrate, it contains nh3 +, which also possess plus 1 charge.

01:19 So, total charge is plus 3 and at ph 1, all r -groups will be in protonated form, possessing plus 1 charge.

01:27 Now, we have to calculate net charge at ph equal to 3.

01:32 At ph equal to 3, only alpha carboxyl group loses proton, that is minus 1 will be the charge.

01:40 Now, total charge will be plus 3 plus minus 1, that is plus 3 minus 1, we will get plus 2.

01:54 So, this is the net charge at ph equal to 3.

01:58 At ph equal to 8, alpha carboxyl group loses proton.

02:03 So, minus 1 charge, this hestadine plus charge is gone, 0, total charge plus 2 plus minus 1.

02:11 So, total charge at ph equal to 8 will be plus 2 plus 0 plus minus 1...

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