Many biomolecules contain functional groups that may accept or donate protons. As a result, these groups can act as weak acids or bases. These biomolecules are referred to amphoteric and react both as acids or bases depending on pH. For example, amino acids contain a carboxyl group that is negatively charged at high pH and an amino group that is positively charged at low pH. Although these functional groups are involved in peptide bonds in proteins, some amino acids contain R-groups that may accept or donate protons changing their charge (i.e., imidazole in histidine). Nitrogenous bases also have functional groups that change their charge and may act as weak acid or bases depending on pH.