Low pKa values of acid hydrogen donor groups in mimosine enable efficient iron chelation at alkaline pH

• 1 H NMR is an efficient method for pKa determination of mimosine and 3H4P. • The pKa’s of mimosine were calculated to be 1.763, 2.607, 7.345, and 8.767. • The pKa’s of 3H4P were calculated to be 3.550, 7.037, and 9.177. • The α-amino and α-carboxylate groups of mimosine bind zinc at alkaline pH. • Mimosine has relatively low pKa’s compared to other amino acids. Mimosine and its degradation product 3-hydroxy-4-pyridone (3H4P) are found in leucaena plant species. Mimosine contains four potential acid hydrogen donor groups and 3H4P has three. The acid dissociation constant or pKa values of mimosine have not been updated and the pKa values of 3H4P have not been determined. Determining pKa values of these acid hydrogen donor groups of mimosine and 3H4P would provide insights into their metal-binding properties, which is important for iron acquisition from soil by leucaena plants. 1 H NMR, 13 C NMR, UV-Vis spectrophotometry, and potentiometry were used to analyze the pKa’s of mimosine. 1 H NMR and potentiometry were used to analyze the pKa of 3H4P. UV-vis spectrophotometry, 1 H NMR, and FTIR were used to analyze mimosine-metal complexes. 1 H NMR was found to be the most efficient method to determine the pKa’s of mimosine and 3H4P. Using 1 H NMR, the pKa’s of mimosine were calculated to be 7.345 for the α-amino group, 1.763 for the α-carboxy group, 8.767 for the phenolic hydroxyl group and 2.607 for the 4-hydroxy-pyridinium (lactam/lactim) group of mimosine. For 3H4P, the pKa’s were calculated to be 3.570 for the C-4 group, 7.037 for the ring N-1 group, and 9.205 for the C-3 group of 3H4P ring. 1 H NMR analysis also revealed increased binding capabilities of the α-amino and α-carboxy groups of mimosine at alkaline pH. The relatively low pKa’s of mimosine have biological implications and could be a significant reason for the stress-tolerance of leucaena and for the toxic and potential therapeutic properties of mimosine.