Identification of diastereomeric chlorophyll allomers by atmospheric pressure chemical ionisation liquid chromatography/tandem mass spectrometry

Atmospheric pressure chemical ionisation liquid chromatography/mass spectrometry (APCI‐LC/MS) has been used for identification of the epimers of hydroxy, methoxy and methoxylactone allomers of chlorophyll a (13 2 ‐HO‐chl a , 13 2 ‐MeO‐chl a and 15 1 ‐MeO‐lact‐chl a ), the hydroxy allomer of bacteriochlorophyll a (13 2 ‐HO‐bchl a ) and the hydroxy and methoxylactone allomers of bacterioviridin a (13 2 ‐HO‐bvir a and 15 1 ‐MeO‐lact‐bvir a ). The APCI mass spectra show that facile fragmentations involve the methoxyl or hydroxyl groups at the C‐13 2 or C‐15 1 chiral centres. Losses involving the C‐13 2 or C‐15 1 hydroxyl or methoxyl groups occur more easily from the S ‐epimer than from the R ‐epimer due to the greater relief of the steric strain associated with interaction with the bulky C‐17 substituent. The differences in mass spectrometric fragmentation can be used as a diagnostic tool for the assignment of the stereochemical configuration at the C‐13 2 or C‐15 1 chiral centres. Copyright © 2003 John Wiley & Sons, Ltd.