Application of Liquid Chromatography-Atmospheric Pressure Chemical Ionization Mass Spectrometry to the Differentiation of Stereoisomeric Diterpenoid Alkaloids

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High-performance liquid chromatography-atmospheric pressure chemical ionization mass spectrometry (HPLC-APCI-MS) was successfully applied to seven stereoisomeric diterpenoid alkaloids at position 1 or 12. Comparison of the breakdown curves, observed by changing the potential difference between the first electrode and the second electrode of the APCI ion source, revealed stereochemical dependence of different fragmentations. The APCI spectra of alkaloids were predominantly the [M+H]⁺ ion and the major fragment ion, corresponding to the [M+H-H₂O]⁺ ion or the [M+H-CH₃COOH]⁺ ion, and comparison of the APCI spectra showed that the abundance of fragment ions was significantly higher for C-1 β-form alkaloids than for C-1 α-form alkaloids, and for C-12 β-form alkaloids than for C-12 α-form alkaloids. The characteristic fragment ions were formed due to the loss of an acetic acid or a water molecule at position 12. The fragmentation mechanisms depending on the stereochemisry of the precursor ion could be discerned by recording the spectra in a deuterated solvent system of 0.05 M ammonium acetate in D₂O-acetonitrile-tetrahydrofuran. Loss of CH₃COOD or D₂O from the precursor ion gave the fragment ion. This result indicated that the proton of protonation was included in the leaving acetic acid and water molecule, respectively. The peak intensity ratio for R=[M+H]⁺/[M+H-H₂O]⁺+[M+H-CH₃COOH]⁺ manifested the stereochemical differentiation of alkaloids at position 1 or 12.