Cysteine and carbonyl compounds. Part I. The dissociation of cysteine and its reaction with 4-oxopentanoic acid
The reaction between L-cysteine and 4-oxopentanoic acid in water has been studied by means of pH determinations, polarimetry, and n.m.r. technique in the pH region 3–11. There exists an equilibrium dependent upon pH with a maximum at pH 9 in favour of reaction products. Kinetically the forward reaction, in the pH region 6·5–9, was of the first order in levulate ion and also in the species –O·CO·CH(+NH3)·CH2SH (Cy+) and –O·CO·CH(NH2)·CH2SH (Cy0) or –O·CO·CH(+NH3)·CH2S–(Cy±). The reverse reaction was formulated in the kinetic calculations as being of the first order in a compound R, with pK 6·4–6·5, and with the [M] values of 410° and 120° for the concerned species R0 and R+. The [M] values for cysteine were found to have an extreme value at pH 9. The findings are correlated with the dissociations of cysteine and reaction product(s), particularly in regard to the reactivities of Cy+ and Cy0 and rapid proton exchange in Cy0⇌ Cy±. The reaction product(s) could not be isolated because of the easily disturbed equilibrium, but in the region pH 6·5–9 they were shown to be, most likely, diastereomers of 2-(β-carboxyethyl)-2-methylthiazolidine-4-carboxylic acid from L-cysteine, and have been defined by AMX spectra of the methylene and methine groups in the L-cysteine part of the reaction product.