Which $\alpha$-amino acid can cross-link peptide chains?

Thiol group is present in:

Which of the following is not an amino acid?

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 $|z_1|$,$|z_2|$,and $|z_3|$,respectively,then what is $|z_1| + |z_2| + |z_3|$?

Assertion $(A)$: The denaturation of proteins can destroy all $1^{\circ}, 2^{\circ}$ and $3^{\circ}$ protein structures.
Reason $(R)$: Curdling of milk is due to denaturation of proteins.
The correct option among the following is

Which of the following molecules forms a Zwitter ion?