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

The following tripeptide can be represented as:

The artificial sweetener aspartame $(A)$,$H_3N^{+}-CH(CH_2COO^{-})-CONH-CH(CH_2Ph)-COOCH_3$,is converted to . . . . . . on storage for extended periods of time in aqueous solution:

In an oligopeptide named Alanyl-glycyl-phenyl-alanyl-isoleucine,the number of $sp^2$ hybridised carbons is $...........$.

The correct structure of histidine in a strongly acidic solution $(pH = 2)$ is

What does the secondary structure of proteins represent?