Kinetics and mechanism of anation of hydroxopenta-aquo chromium(III) ion byDL-alanine in aqueous solution

Ligand Sequential Replacement on Chromium(III)–Aqua Complexes by l-Alanine and Other Biological Amino Acids: A Kinetic Perspective

“…The spectroscopic data shown in Figures S7, S8, and indicate that the wavelength for the first visible peak of the final reacting mixture was in the range 392–409 nm and for the second in the range 528–548 nm, increasing both with the initial concentration of metal ion and decreasing both with the initial concentrations of either ligand or potassium hydroxide. The values reported for the number of l -alanine ligands coordinated to each Cr­(III) center in the final violet complex are 2 according to some authors and 3 according to others . Actually, the results shown in Figures S7, S8, , and cannot be explained unless we assume the presence of at least four different complexes in the final reacting mixture.…”

“…The program so developed yielded a good enough concordance between the calculated and experimental absorbances (E = 3.40 × 10 −4 to 1.50 × 10 −3 ). Furthermore, given that the Cr(III)−alanine reaction (under conditions of a large excess of organic ligand) has been previously reported in the chemical literature to follow a pseudofirst-order behavior, 29,30 it might be worth comparing the quality of the absorbance−time fits attained with two mathematical alternatives: the biparametric pseudo-first-order kinetic model (involving A λ,0 and k) and the tetraparametric model (involving A λ,0 , ε I,λ , k 1 , and k 2 ) used in the present work. As can be seen in Figure 3, the ratio between the experimental and calculated absorbances during the course of the reaction was much closer to unity when the two rate constant kinetic model was employed, the difference between the two models being especially notable at the beginning of the reaction.…”

“…The substitution reactions on aqua, − hydroxo, and ammonia complexes of Cr­(III) by amino acids have been the subject of several kinetic studies. Although the results of two independent investigations of the Cr­(III)–alanine reaction have already been reported, , the process under a large excess of organic ligand being classified in both cases as a pseudo-first-order reaction, some clear-cut deviations from this simple kinetic behavior have been observed. The main objective of the present work will be to search for and eventually find a kinetic model capable of accounting for those deviations.…”

“…In particular, the reactions of complexation of Cr(III) by amino acids might be related to the problem of the origin of the first peptides on prebiotic Earth, 17 since transition metal ions have been shown to catalyze the formation of peptide bonds between the amino acid monomers acting as ligands. 18 The substitution reactions on aqua, 19−26 hydroxo, 27 and ammonia 28 complexes of Cr(III) by amino acids have been the subject of several kinetic studies. Although the results of two independent investigations of the Cr(III)−alanine reaction have already been reported, 29,30 the process under a large excess of organic ligand being classified in both cases as a pseudo-firstorder reaction, some clear-cut deviations from this simple kinetic behavior have been observed.…”

Kinetics and Mechanism of the Anation and Complexation between Hydroxopentaaquochromium(III) Ions and the Hexacyanoferrate(II) Ion

“…The plot of log k obs vs. log [H ǹ ] (pH = 5.0-5.4) shows a fractional order in [H ǹ ]. It was observed that the reaction rate increases with increasing ionic-strength, indicating the easy removal 8,17,29 of coordinated water molecules and the formation of outer-sphere complexes. Under our experimental conditions (pH = 4.5-5.4), Cr(H 2 O) 6 3ǹ and Cr(H 2 O) 5 OH 2ǹ were the main reactive species.…”