Why does adding copper(II) sulfate to each of the solutions in Part B reveal if peptides are present in solution? Peptides are more polar than amino acids. The solute needs to be polar enough to coordinate with the copper cation. Amino acids are free to coordinate with the copper cation, allowing for an efficient positive result. Due to the larger size of peptides, the London forces turn the copper solution pink. The distance between peptide bonds is optimal for coordination with the copper cation, turning the solution pink