Optical properties of aqueous solutions of manganese (II) complexes with l-cysteine, thioglycolic acid and citrate - ion

Abstract

Complexes of manganese (II)-ions with two thiols (amino acid L-cysteine and thioglycolic acid TGA) as well as with citrate ions are of a grate interest due to their important role in biosystems. Besides, they take attention as precursors for synthesis of nano sized semiconductors with various useful physical-chemical properties. In this work the results of the complexes aqua solutions’ optical properties monitoring are presented. It is known that SH-group of the L-cysteine and TGA can be oxidized by air oxygen spontaneously especially in neutral and alkaline medium so the studies was done at slightly acid conditions. The experiments in the Mn- L-cysteine system were performed at near physiological pH values ranged 5.5÷6.0 based on a previous theoretical consideration of possible ionic and molecular manganese and L-cysteine forms. The Mn-TGA- complexes were synthesized at pH = 6.0 and the Mn-citrate at pH=7.6. A spectrophotometrical Ostromisslensky-Job’s method of continuous variations as commonly used procedure for determining the composition of stable complexes in solution was used. In this method, a series of solutions are prepared by mixing varying proportions of the metal and ligand in such manner that the sum of the total molar concentrations remained constant. The absorbance of each solution at a selective wavelength is then plotted against the mole fraction of either the ligand or metal. The ratio that corresponds to the mole fractions of the components in the complex will attain a maximum absorbance value. Two total concentrations of the metal-ions plus the ligand constant equal to 0.05 M and 0.02 M were maintained. It was determined that though the Mn-L-cys mixtures were stable during more than a week, later tendency to cystin formation was observed. Absorbance spectra of the Mn-TGA solutions varied with time demonstrating rather slow complexation. The Mn-citrate complexes forming occurred most rapidly that observed by the variable absorbance spectra. The optical studies data treating by Ostromisslensky-Job’s method gives a possibility to reveal a clear peak in isomolar curves in Mn-citrate case only and concluded about the complexes forming in a stepwise fashion. The obtained results led to conclusion that despite the presence of three functional groups capable to the manganese ions chelating, both thiols exhibit weak complexation compared to citrate ions under the comparable conditions.