Question

The structure of a peptide is given below:

If the absolute values of the net charge of the peptide at $pH =2, pH =6$, and $pH =11$ are $\left| z _{1}\right|,\left| z _{2}\right|$, and $\left| z _{3}\right|$, respectively, then what is $\left|z_{1}\right|+\left|z_{2}\right|+\left|z_{3}\right|$ ?

Answer 1

(I) At pH = 2
(highly acidic), the structure of the peptide will be as shown below.

Net charge +2

|Z1| = 2 at pH = 2

(ii) At pH = 6 (neutral solution)

In neutral medium, the given tripeptide exists as Zwitter ion.

net charge = 0

|Z2| = 0 at pH = 6

(iii)At pH = 11 (basic medium)

In basic medium the given tripeptide exists in anionic form.

Net charge = –3
So, |Z3| = |-3|

|Z1| + |Z2| + |Z3| = 2 + 0 + 3 = 5