Interaction of L-cysteine with Dimethyl Sulfoxide in Mild Conditions

Introduction. Cysteine is the one of the sulfur containing non-essential amino acids along with methionine, which are found naturally in many proteins such as α and β-ceratins, and in free state. The ability of cysteine residues to form disulfide bridges plays an important role on secondary structure and folding of proteins. Another prominent quality is that sulfur can be found in different oxidation states in living systems, which is a key for various biological functions. More about the role and importance of sulfur containing amino acids as well as redox chemistry of cysteine residues in enzymes is summarized in [1, 2]. Few factors determine the importance of the investigation of interactions between L-cysteine and dimethyl sulfoxide (DMSO). Besides being a polar aprotic solvent DMSO is widely used in biology and medicine for drug delivery and as a cryoprotective agent [3, 4]. It is also known, that in some reactions DMSO can serve as an oxidizer by giving it’s oxygen atom [5]. However, reports in literature about the possible interactions of L-cysteine with DMSO are scarce and provide various, sometimes contradicting information on the resulting products. According to [6], the interactions in the ternary system L-cysteine–H2O–organic solvent (including DMSO) are limited to the solvation processes at room temperature, while in [7] a disulfide formation is stated in similar conditions. Similarly an analytical procedure for cysteine determination is described [8], where the reaction in hot acidified solutions yields mainly cysteic acid, whereas in [9] a general procedure for various organic disulfide preparations is given from starting thiols and DMSO.