Thiols, also known as mercaptans, are a class of organic compounds that contain a sulfhydryl group (–SH) composed of a sulphur atom and a hydrogen atom attached to a carbon atom. The plasma thiol pool is mainly formed by albumin thiols, protein thiols and slightly formed by low-molecular-weight thiols such as cysteine (Cys), cysteinylglycine, glutathione, homocysteine and γ- glutamylcysteine. 
Thiols (RSH) can undergo oxidation reaction via oxidants and form disulphide (RSSR) bonds. A disulphide bond is a covalent bond; the linkage is also called a SS-bond or disulphide bridge. Under conditions of oxidative stress, the oxidation of Cys residues can lead to the reversible formation of mixed disulphides between protein thiol groups and low-molecular-mass thiols. The formed disulphide bonds can again be reduced to thiol groups; thus, dynamic thiol–disulphide homeostasis is maintained.