Letter: Fragmentation studies of ergosterol. The formation of the fragment ion at m/z 337

Abstract

Listen

Translate article

Dear Sir Ergosterol (Figure 1) is the most abundant sterol produced by yeasts and a precursor of vitamin D. In the electron impact mass spectrum of ergosterol there is an abundant fragment ion at m/z 337. This fragment ion is also present in the mass spectrum of the trimethylsilyl (TMS) derivative of ergosterol. A mechanism to account for the formation of this important ion has been reported. It is similar to the mechanism for the TMS derivative of cholesterol, whereby ring A fragments with loss of carbon atoms 1 to 3 and their substituents with transfer of hydrogen from C-9. The reported mechanism is presented in Scheme 1. This involves loss of 13l mass units from the molecular ion. The presence of a fragment ion at m/z 131 supported the proposed mechanism. During the course of studies on sterol donors in planthoppers several isotopically labelled steroids were synthesized. Ergosterol labelled with two deuterium atoms at C-4 was prepared. The electron impact mass spectrum of 4,4-d2-ergosterol is presented in Figure 2. Comparison with the electron impact mass spectrum of unlabelled ergosterol, which is shown in Figure 3, reveals that any fragment ions containing C-4 will be shifted by two mass units. What is noticeable is that the fragment ion at m/z 337 remains unchanged whereas all the other ions in the upper mass range have been shifted by two mass units. This indicates quite clearly that the C-4 in ring A has been lost and thus this disagrees with the mechanism reported. This ion at m/z 337 is also present in the electron impact mass spectrum of the Figure 1. Ergosterol.