Multivariate curve resolution of spectral data for the pH-dependent reduction of ferrylmyoglobin by cysteine

The reduction of ferrylmyoglobin by cysteine was investigated in the pH range of 6–9 at 25 °C with varying amounts of cysteine in excess ensuring pseudo first-order conditions by UV–VIS spectroscopy from 450 to 700 nm. Spectral data was analysed using multivariate curve resolution to identify intermediate and reaction products. At pHs 6, 7 and 8, ferrylmyoglobin was reduced by cysteine forming mainly sulfmyoglobin together with some metmyoglobin. At pH 9, two consecutive reactions were observed. Ferrylmyoglobin was reduced to metmyoglobin by cysteine, which was further reduced to oxymyoglobin. The different behaviours observed at pHs 6, 7 and 8 compared to pH 9 may be explained by the thiol acid/base equilibrium of cysteine with a pKa value of 8.33. At pHs 6 and 7, and partly pH 8, the protonated form of the cysteine thiol is dominating, causing formation of sulfmyoglobin by sulphur atom transfer. MCR proved to be a useful tool for analysing the reduction of ferrylmyoglobin by cysteine by extracting information about the time dependent course of the absorbing components occurring during reduction of ferrylmyoglobin by thiol.

► The reduction of ferrylmyoglobin by cysteine was studied using UV/VIS spectroscopy.
► Multivariate curve resolution was applied for interpretation of data.
► The pH significantly affected the reaction products formed.
► Protonated/deprotonated cysteine led to diverse ferrylmyoglobin reduction pathways.