Amino acids contain which of the following functional groups?

Which of the following is an acidic amino acid?

Which of the following is a Zwitter ion?

$A$ tetrapeptide $x$ on complete hydrolysis produced glycine $(Gly)$,alanine $(Ala)$,valine $(Val)$,and leucine $(Leu)$ in equimolar proportions. The number of possible tetrapeptide sequences involving each of these amino acids exactly once is:

When $HNO_3$ is dropped into the palm and washed with water,it turns yellow. The above yellow colour is due to:

Which of the following is $NOT$ a function of proteins?