Dimer formation during the thermo-oxidation of stigmasterol

The monomeric oxidation products formed during the thermo-oxidation of phytosterols do not explain the loss of unoxidized phytosterols, leaving a gap in the knowledge on the products that are being formed during thermo-oxidation. Previous research showed that the gap can be explained by the formation of dimers and polymers. The aim of this research was to investigate the dimers that are formed during the thermo-oxidation of phytosterols, using stigmasterol as a model compound. Following the oxidation of stigmasterol at 180 °C, several dimeric fractions were obtained by solid phase extraction and high-performance size-exclusion chromatography. Individual dimers were separated by reversed-phase high-performance liquid chromatography, and their molecular masses were determined by combining atmospheric pressure chemical ionization-mass spectrometry and coordination-ion spray-mass spectrometry. This study resulted, for the first time, in the assignment of several individual stigmasterol dimers. The most dominant dimer was found to have a molecular mass of 822.7 Da and is suggested to represent a C-C linked dimer of stigmasterol, in accordance with literature. Several other stigmasterol dimers were found to be formed during thermo-oxidation, which are suggested to differ in monomeric subunit composition, linkage type, and stereochemistry.