1. Chemistry Aqueous solution chemistry of Zn(II) and Cd(II) with L-DOPA
233rd ACS National Meeting, Chicago, IL March 25-29, 2007
Cassie Rogers and Yahia Z. Hamada, LeMoyne-Owen College, Memphis, TN
Chemistry
Abstract
L-DOPA is a naturally occurring compound. Thus far it is the only drug that reduces the symptoms of Parkinson’s disease. It is also an important neurotransmitter. We believe that the interaction of L-DOPA with a variety of essential metal ions plays a very important role related to Parkinson’s disease. We are reporting in this study the interaction of L-DOPA with Zn (III) and Cd(II) using the potentiometric titration technique in aqueous solutions in milli-molar concentrations at room temperature. The condition set for the experiments mimics the physiological conditions. These studies are novel and will be used to measure the stability constants of this important neurotransmitter with both the essential Zn(II) and the toxic Cd(II).
Experimental
The potentiometric titration solutions were contained at 25 oC in a 250 mL beaker equipped with a magnetic stirring bar. The beaker was covered with a custom made Teflon cover. In a typical titration the ligand (L-DOPA) was added first then the metal ion solution was added followed by the addition of the appropriate amount of DI water to take the total volume to 100 mL. The total concentration of the metal ions was in the range of mM. Before each titration, the titration solution mixtures were allowed to stir for minutes to allow the titration system to attain complete equilibrium. The NaOH titrant was added in the 100 L increments using an Eppendorf micropipette with continuous stirring. The time intervals between the additions of the NaOH solution were set to 3-5 minutes, which was sufficient to get each of the pH values stabilized and reach complete equlibrium. To ensure that the titration systems reached complete equilibrium, the pH meter gave the ready signal after each data-point prompting the operator for the next aliquot’s addition of the titrant.
References
[1] Y. Z. Hamada and C. Rogers Potentiometry of L-3,4-dihydroxyphynlealanine (L-DOPA) with Al3+, Fe3+, Cr3+, Cu2+, and Zn2+ metal ions in aqueous solutions at the physiological pH J. Coord. Chem. In press. and references therein.
[2] L. Alderighi, P. Gans, A. Ienco, D. Perters, A. Sabatini, and A. Vacca, Hyperquad simulation and speciation (Hyss): a utility program for the investigation of equilibria involving soluble and partially soluble species, Coord. Chem. Rev., 184, (1999).
[3] A. E. Martell, R.-M. Smith, and R. J. Motekaitis, Critical Stability Constants Database, Version 6.0, NIST, Texas A & M University, College Station, TX, USA (2001).
[4] B. A. Hasan, K. D. Khalaf, and De La Guardia, Flow analysis spectrophotometric determination of L-DOPA in pharmaceutical formulations by reaction with P-Amin-o-phenenal, Talanta 42, (1995).
Conclusion
[1] Speciation plots for the free L-DOPA ligand were constructed using data from references two and three.
[2] Free Zinc and free Cadmium titrations revealed the release of two equivalents of protons per metal ion.
[3] Titrations of the Zn:L-DOPA in 1:1, 1:2, and 1:3 molar ratios show that the 1:1 titration system releases three equivalents off the three L-DOPA protons forming the simple one-to-one complex.
[4] the UV-Vis of the free L-DOPA is in agreement with the literature findings See reference number four.
[5] The cadmium titrations are under investigation. It is showing similar behavior to the Zn titration system.
Acknowledgements
NSF HBCU-UP Grant
#HRD
Department of Natural and Mathematical Sciences