Analysis of cyclic and acyclic analogs of retinol, retinoic acid, and retinal by laser desorption ionization-, matrix-assisted laser desorption ionization mass spectrometry, and UV/Vis spectroscopy

Laser desorption ionization (LDI)- and matrix-assisted laser desorption ion ization (MALDI)-mass spectrometry (LDI-MS, MALDI-MS) at 337-nm laser wavele ngth were used to analyze retinol (vitamin A), retinoic acid, and retinal a nd their analogs 3-hydroxy-retinol, 3-hydroxyretinoic acid, 3-hydroxyretina l, 4-oxoretinol, 4-oxoretinoic acid, 4-oxoretinal, 3,4-didehydroretinol (vi tamin A(2)), 3,4-didehydroretinoic acid, 3,4-didehydroretinal, acycloretino l, acycloretinoic acid, and acycloretinal. The compounds exhibit sufficient ionizability which allows to obtain mass spectra by LDI which are similar in quality to those obtained by MALDI. Mass spectra were recorded with a li near time-of-flight (TOF) instrument or a reflectron-type (RETOF) instrumen t in positive-ion mode. Under the conditions of LDI-MS the compounds form a bundant radical molecular ions (M+.), whereas in the MALDI mass spectra abu ndant protonated molecular ions ([M + H](+)) are observed. Characteristic f ragment ions provide additional structural information. High-performance li quid chromatography (HPLC) coupled with UV/Vis photodiode detection was use d to assist in retinoid characterization. Synthesis of 3-hydroxyretinal, 4- oxoretinal, and acycloretinal was performed by oxidative cleavage of the al l-trans-carotenoids of zeaxanthin, canthaxanthin, and lycopene. (C) 1999 Ac ademic Press.