What is the number of amino acids present in a single turn of an $\alpha$-helix of a protein?

The following tripeptide structure is composed of three amino acids. Identify the correct sequence of amino acids in the tripeptide:
(Structure: $H_2N-CH(CH_2Ph)-CO-NH-CH(CH_3)-CO-NH-CH(CH_2OH)-COOH$)

The main structural feature of proteins is

The amino acid containing an indole part is

Which of the following is an amphoteric acid?

The number of tripeptides formed by $3$ different amino acids using each amino acid once is.................