Atmospheric pressure chemical ionization mass spectrometry of retro-carotenoids

Rationale The single and double bonds of the polyene chain of the studied retro-carotenoids are located at the neighboring positions compared to those of regular carotenoids. Our mass spectrometry approach targeted at facilitating the characterization of retro-carotenoids as their structural diversity in nature is not yet fully elucidated. Moreover, extended pi-electron systems endow several retro-carotenoids with exceptional colors from golden-orange to vibrant red that stimulate the food industry's interest. Methods Atmospheric pressure chemical ionization-quadrupole time-of-flight-high-resolution mass spectrometry (APCI-QTOF-HRMS) experiments of the three structurally related retro-carotenoids rhodoxanthin, eschscholtzxanthone, and eschscholtzxanthin were performed to elucidate the formation of specific ion species compared to those of the common carotenoids lutein and zeaxanthin. Mass fragmentations of the aforementioned retro-carotenoids were unraveled using APCI-tandem mass spectrometry (MS/MS) in the negative and positive ion modes. Results Abundant in-source fragment ions [M + H - H2O](+) of eschscholtzxanthin and eschscholtzxanthone were formed in the positive ion mode owing to the loss of water at the hydroxylated epsilon-rings. Eliminations of the epsilon-rings at the characteristic exocyclic double bonds at C-6,7 and C-6 ',7 ' were observed after the resonance-stabilized loss of water. Distinct product ions were yielded for all retro-carotenoids assessed because of the cleavage at their typical central single bond at C-15,15 '. Conclusions Detailed APCI-QTOF-HRMS analyses enabled a highly accurate detection of the most abundant ion species and respective signal intensity ratios of retro-carotenoids, facilitating their further screening and reliable identification in natural sources. Mass fragmentations of the studied retro-carotenoids were found to be substantially impacted by the extraordinary configuration of their polyene backbone.

Automated summary

The mass spectrometry approach used in this study aimed at characterizing retro-carotenoids, which have unique structural features and colors, and are of interest to the food industry. APCI-QTOF-HRMS analysis enabled the detection and identification of retro-carotenoids in natural sources by analyzing their specific ion species and fragmentation patterns. The results showed that the polyene backbone of retro-carotenoids greatly influenced their mass fragmentation.