Which one is not a protein?

Which of the following amino acids contains a polar and negatively charged $R$ group?

What does $L$ indicate in $L$-lysine and $L$-malic acid?

What is a polypeptide chain of amino acids called?

Proteins have a primary structure. If you are given a method to know which amino acid is at either of the two termini (ends) of a protein,can you connect this information to the purity or homogeneity of a protein?

Which of the following molecules is amphoteric in nature?