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Colorimetric Determination Of Catechol Drugs L-Dopa, D-Dopa, And Carbidopa In Aqueous And Solid Samples Utilizing Complexation With A Transition Metal Cation

Ronald Bartzatt

The compound L-dopa is an important therapeutic tool for the clinical treatment of Parkinsons disease. L-dopa and D-dopa drug structures include a primary amine (-NH2), a carboxylic acid group (-C(O)OH), and an aromatic ring having two adjacent hydroxyl (-OH) substituents, a structure referred to as a catechol. The D- and L- forms of dopa are soluble in water and form an iron(III)-catechol complex seen in solution as a brown to dark-brown mixture. The complex is stable in aqueous solution with a strong absorbance peak at 400 nm which was utilized to identify the presence and quantitate the concentration of dopa. The extinction coefficient of a mixture having 0.167% FeCl3 •6H2O with 6.76E-04 molar catechol drug results in ε400 nm =2910.38 Liter/(mole/cm). The methods described here produce a colored endpoint that indicates the presence of these catechol drugs. The colored endpoint is readily seen by eye to an amount of drug < 53.0 μg/ml (53 parts per million) and can be utilized to determine the concentration from 53.0 μg/ml to 533.0 μg/ml. To assay multiple samples a standard curve may be constructed utilizing UV-Visible spectrometers that monitor a strong absorbance peak at 400 nm. As little as 1.50 μg/ml (parts per million) can be detected by utilizing a standard curve. Many inorganic salts such as LiBr and NaCl do not interfere with the colorimetric results. Many medicinal compounds such as aspirin, ampicillin, tetracycline, erythromycin, and quinine sulfate do not interfere with the determination. A Jobs Plot was accomplished to determine the ratio of iron(III) to catechol drugs within the complex to be 3:2, respectively.