Molecular Characterization of Ongoing Enzymatic Reactions in Raw Garlic Cloves
Using Extractive Electrospray Ionization Mass Spectrometry
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Abstract
Characterization
of enzymatic reactions occurring in untreated
biological samples is of increasing interest. Herein, the chemical
conversion of alliin to allicin, catalyzed by allinase, in raw garlic
cloves has been followed in vivo by internal extractive electrospray
ionization mass spectrometry (iEESI-MS). Both precursors and products
of the enzymatic reaction were instantaneously extracted by infused
solution running throughout the tissue and directly electrospray ionized
on the edge of the bulk sample for online MS analysis. Compared to
the room-temperature (+25 °C) scenario, the alliin conversion
in garlic cloves decreased by (7.2 ± 1.4) times upon heating
to +80 °C and by (5.9 ± 0.8) times upon cooling to −16
°C. Exposure of garlic to gentle ultrasound irradiation for 3
h accelerated the reaction by (1.2 ± 0.1) times. A 10 s microwave
irradiation promoted alliin conversion by (1.6 ± 0.4) times,
but longer exposure to microwave irradiation (90 s) slowed the reaction
by (28.5 ± 7.5) times compared to the reference analysis. This
method has been further employed to monitor the germination process
of garlic. These data revealed that over a 2 day garlic sprouting,
the allicin/alliin ratio increased by (2.2 ± 0.5) times, and
the averaged degree of polymerization for the detected oligosaccharides/polysaccharides
decreased from 11.6 to 9.4. Overall, these findings suggest the potential
use of iEESI-MS for in vivo studies of enzymatic reactions in native
biological matrices