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

The structures of five genuine furostanol glycosides isolated from Met anarthecium luteo-viride MAXIM. (Liliaceae) were determined on the bas is of liquid secondary ion mass spectrometric (LSIMS) analysis includi ng 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-acet yl-furometanarthogenin and 2-O-acetyl-3-oxo-furometagenin) as aglycons , which have cyclic hemiacetal moieties, bearing 2,3,4-tri-O-acetyl ar abinopyranose at 11-C1) and glucopyranose at 26-C. In the LSIMS of the se compounds, the protonated molecular ion [M + H]+ was not observed b ut the fragment ion [M - OH]+ corresponding to the loss of the hydroxy l 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 f or sodium ion addition to the neutral molecule was less than for proto n addition, the sodium ion addition is more favorable. In addition, th e 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.