At what pH does peptide the His-Arg have a net charge of+1.7?

Recall that:Amino acid sequences are written with N-terminal amino acid on the left and C-terminal…

Answered: At what pH does peptide the His-Arg…

9th Edition

ISBN:9781319114671

Author:Lubert Stryer, Jeremy M. Berg, John L. Tymoczko, Gregory J. Gatto Jr.

Publisher:Lubert Stryer, Jeremy M. Berg, John L. Tymoczko, Gregory J. Gatto Jr.

Chapter1: Biochemistry: An Evolving Science

Section: Chapter Questions

Problem 1P

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Expert Solution

Step 1: Ionization of a peptide

Recall that:

Amino acid sequences are written with N-terminal amino acid on the left and C-terminal amino acid on the right.
The peptide bond is formed between the carboxyl group of N-terminal amino acids and the amino group of C-terminal amino acids.
The charge on the amino acid residue will depend on the charge of the ionisable group because charges on alpha-amino/carboxyl groups are neutralised due to the formation of a peptide bond.
Terminal amino/carboxyl groups are ionisable.

pKa is the pH at which the weak acid is 50% dissociated.

When the pH < pKa of the ionising group, the group remains protonated.

When pH > pKa of an ionising group, the group is deprotonated.

If it is an acidic amino acid then, deprotonation causes the development of a -1 charge.
if it is a basic amino acid then deprotonation causes 0 charges.

If pH = pKa of the ionising group, 50% of the ionising group is protonated.