Fragmentation study of clerodane diterpenes from Casearia species by tandem mass spectrometry (QToF and Ion Trap).

Clerodane-type diterpenes from Casearia species show important pharmacological activites such as antitumor, antimicrobial and anti-inflamatory. There are several MS-based methods for identification of diterpenes; however, there is still a lack of mass spectrometry procedures capable of providing characteristic fragmentation pathways for a rapid and unambiguous elucidation of casearin-like compounds.Casearin-like compounds were investigated by ESI-MS/MS. The fragmentation studies were carried out by tandem mass spectrometry in space (QToF) using different collision energies and also by tandem mass spectrometry in time (QIT) by selective isolation of product ions.

Casearin-like compounds presented a predominance of sodium and potassium cationized precursor ions. Both QIT and QToF techniques provided sequential neutral losses of esters related to the R₁ to R₅ substituents linked to the nucleus of the clerodane diterpenes. The fragmentation pathway is initiated with a cleavage of the ester moieties R₂ followed by the elimination of the ester groups R₃ , both losing neutral carboxylic acids. Using tandem mass spectrometry in time, it was also possible to observe the cleavage of the ester groups R₁ or R₅ by MS⁴ experiments.

Through a rational analysis of the fragmentation mechanisms of Casearia diterpenes it was possible to suggest an annotation strategy based on the sequential cleavages of the ester groups related to the R₂ , R₃ and R₅ substituents. These results will assist studies of the dereplication and metabolomics involving casearin-like compounds present in complex extracts of Casearia species.