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Abstract
Kinetics and mechanism of electron transfer to pyridinium chlorochromate (VI) from sulfur containing amino acid, L-cysteine in aqueous and micellar media The electron transfer reaction of L-cysteine (RSH) with pyridinium chlorochromate (PCC) has been studied spectrophotometrically over the range 2.0 ≤ 103 [RSH] ≤ 6.0; 0.01 ≤ [H+] ≤ 0.2; 298 ≤ T ≤ 318 K and I = 0.3 mol dm-3 (NaClO4). The electron transfer reaction has also been carried out in the presence of anionic, cationic and neutral micelle. The reaction in acid medium is strongly catalyzed by changing [SDS]T (sodium dodecyl sulfate) up to 3 × 10-2 mol dm-3, beyond this concentration of SDS, the rate is retarded. The cationic and neutral micelle has a small effect on the rate. ΔH≠ (kJ mol-1) and ΔS≠ (JK-1 mol-1) values for the k1 and k2 paths are 30.20 ± 0.25, -159.65 ± 0.83 and 29.60 ± 0.62, -127.09 ± 2.17, respectively. The negative activation entropy is indicative of the ordered transition state for the electron transfer reaction. Formation of 2-amino-3-(2-amino-2-carboxy-ethyl) disulfanyl-propanoic acid as product is strongly supported by IR spectra.
Highlights
In recent years there has been a great upsurge of interest on mechanistic studies of reactions of amino acids with transition metal complexes, due to their biological relevance[1,2,3,4,5,6,7,8]
The electron transfer reaction between L-cysteine and pyridinium chlorochromate (PCC) has been studied over the range 2.0 ≤ 103 [cysteine]T ≤ 6.0; 0.01≤[H+] ≤ 0.2, 298 K ≤ T ≤ 318 K and I = 0.3 mol dm-3 (NaClO4)
The UV-VIS spectral scan (Fig. 2) of the reaction mixture of L-cysteine and PCC over the range 200 ≤ λ(nm) ≤ 600 shows shifting of absorption maxima from 350 nm to 425 nm, indicating the formation of an intermediate species between cysteine and PCC
Summary
In recent years there has been a great upsurge of interest on mechanistic studies of reactions of amino acids with transition metal complexes, due to their biological relevance[1,2,3,4,5,6,7,8]. Amino acids are important biomolecules which are building blocks of proteins but they function as chemical messengers in bringing about communication between cells. This prompted us to undertake the present study which includes interaction of sulfur containing amino acid, cysteine with Cr(VI) in the form of pyridinium chlorochromate (PCC). Since most biological processes occur at interfaces, structure, dynamics and the reactivity of biomolecules differ at an interface than those observed in the bulk. Keeping this in view the present study was carried out both in the aqueous and micellar media
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