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

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

So, $|Z_1| = 2$
At $pH = 6$ it will exist as neutral or zwitter-ion structure as shown below.

So, according to this condition, $|Z_2| = 0$
At $pH = 11$ (highly basic), the given peptide will exist as:

So, $\left|Z_{3}\right|=|-3|=3$
$\left|Z_{1}\right|+\left|Z_{2}\right|+\left|Z_{3}\right| =2+0+3=5$