STRUCTURE ELUCIDATION OF FUROSTANOL GLYCOSIDES USING LIQUID SECONDARY-ION MASS-SPECTROMETRY

The structures of five genuine furostanol glycosides isolated from Metanarthecium luteo-viride MAXIM. (Liliaceae) were determined on the basis of liquid secondary ion mass spectrometric (LSIMS) analysis including liquid secondary ion mass spectrometry/ mass spectrometry (LSIMS/MS). These glycosides were elucidated as bisdesmosides of furostanols (i.e. 2-O-acetyl-furometagenin, furometagenin, furonogiragenin, 2-O-acetyl-furometanarthogenin and 2-O-acetyl-3-oxo-furometagenin) as aglycons, which have cyclic hemiacetal moieties, bearing 2,3,4-tri-O-acetyl arabinopyranose at 11-C1) and glucopyranose at 26-C. In the LSIMS of these compounds, the protonated molecular ion [M + H]+ was not observed but the fragment ion [M - OH]+ corresponding to the loss of the hydroxyl group at 22-C was observed. By addition of NaCl to the sample matrix, the ion peaks for [M + Na]+ appeared in the spectra, which were used to determine the molecular formulae. Molecular orbital calculation of a model compound indicated that the ions [M + Na]+ were stabilized by formation of a four-membered ring structure bonding two oxygen atoms at the hemiacetal moiety and a sodium ion. Since the energy required for sodium ion addition to the neutral molecule was less than for proton addition, the sodium ion addition is more favorable. In addition, the repulsion energy of the protonated hemiacetal hydroxyl group is only 4.25 kcal/mol, and the potential energy of the fragmentation products ([M - OH]+ + H2O) is 18.13 kcal/mol less than that of [M + H]+. These data rationalized the easy dehydration from the protonated furostanol glycoside.