(a) which amino acid residue\'s backbone forms a hydrogen bond with the backbone of the second (2nd) residue? choose the correct residue from the drop-down menu.

In order for hydrogen bonding to occur between the residual molecules of the second molecule, the hydrogen atoms on both of the residual groups must be bonded to electronegative atoms. These atoms include fluorine, oxygen, nitrogen and chlorine. These atoms are able to induce a charge on the hydrogen atom, which allows hydrogen bonding to occur.

Solubility would be your answer.