Which $\alpha-$amino acid forms cross-links in peptide chains?

Which is the monomer of polypeptide?

The increasing order of $pKa$ of the following amino acids in aqueous solution is $Gly, Asp, Lys, Arg$.

The number of possible tripeptides formed involving alanine $(ala)$,glycine $(gly)$,and valine $(val)$,where no amino acid has been used more than once is:

Protein can be most easily removed from which of the following?

Amino acids are the building blocks of