Interaction of L-3,4-dihydroxyphenylalanin (L-DOPA) as a coordinating ligand with a series of metal ions; reaction of L-DOPA

L-DOPA is an important neurotransmitter that is found in the brain and as a hormone in the circulatory system. We report in this article the similarities and differences in behaviour of this important neurotransmitter as a chelating agent among some divalent and trivalent metal ions using potentiometric titration in aqueous solutions at 25.0?±?1.0°C. The careful and detailed potentiometric titrations of L-DOPA with Al³⁺, Cr³⁺, Fe³⁺, Cu²⁺, and Zn²⁺ are discussed and compared. UV-Vis-spectroscopy is utilized for both the free L-DOPA and for the Fe³⁺/L-DOPA system. The characteristic peak due to the π?→?π* transition of the free L-DOPA at ～280?nm (ε_280?nm?=?1927?±?65?M^?1?cm^?1 between pH values of 2.0 to 3.0) disappeared when the iron solution was added to the L-DOPA sample in the same pH range. For the Fe³⁺/L-DOPA system we have observed a new peak at 470?nm with ε₄₇₀?=?800?±?50?M^?1?cm^?1. These comparison studies of the similarities and differences among these di- and tri-valent metal ions shed light on these systems in aqueous solutions. The appropriate metal simulation and speciation diagrams were constructed using the model that fit the titration data points.