At low pH, there are more free H+ molecules floating around in the substance (which causes the acidity of the substance). The free H+ molecules interact with the negatively charged part of the amino acid. All 20 amino acids have the same basic part as the head of their molecule. COO- is the topmost part... the negative oxygen bonds with the free floating H+ to become COOH. This COOH is stable and will not react with the reducing sugars to create a Maillard reaction.