Which structure$(s)$ of proteins remain$(s)$ intact during the denaturation process?

Sanger's reagent is used for the identification of which of the following?

Assertion : At isoelectric point,the amino acid does not migrate under the influence of an electric field.
Reason : At isoelectric point,amino acid exists as a zwitterion.

The general formula of an $\alpha$-amino acid is shown below. Identify the side chain $-R$ for the amino acid serine.

Which of the following statements is/are true for amino acids?
$(A)$ The solubility of an amino acid in water is maximum at its isoelectric point $(pH)$.
$(B)$ It gives a ninhydrin test.
$(C)$ It releases nitrogen gas upon reaction with nitrous acid $(HNO_2)$.

Amide group is present in