The main structural linkage of protein is

An amino acid may be represented by the general formula $H_2N-CH(R)-COOH$. If $R = -CH_2C_6H_5$,it is phenylalanine $(Phe)$,and if $R = -CH_3$,it is alanine $(Ala)$. Identify the correct reagents from the list below to synthesize the dipeptide $Phe-Ala$:
$(1) ZNH-CH(CH_3)-COOH$
$(2) H_2N-CH(CH_3)-COOCH_2C_6H_5$
$(3) ZNH-CH(CH_2C_6H_5)-COOH$
$(4) H_2N-CH(CH_2C_6H_5)-COOCH_2C_6H_5$
(Note: $Z$ represents the benzyloxycarbonyl protecting group).

Choose the correct option for structures of $A$ and $B,$ respectively.

When urea is heated,it forms biuret,the alkaline solution of which forms $.....$ with $CuSO_4$ solution.

What are the common types of secondary structure of proteins?

The secondary structure of proteins is stabilized by which bond?