If there are $20$ naturally occurring amino acids,what is the maximum number of tripeptides that can be formed?

The compound with the formula $NH_2CH_2COOH$ acts as:

The optically inactive amino acid is

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 $|z_1| + |z_2| + |z_3|$?

Among the $\alpha$-amino acids - threonine,tyrosine,methionine,arginine and tryptophan,those which contain an aromatic group in their side chain are

Amide group is present in