production of selected secondary metabolites in transformed ...

3. RESULTS AND DISCUSSION
The main task of this study was the application of the negative-ion mode MALDI-TOF MS
to the determination of a structure of storage oligosaccharides isolated from plants.
The proper experimental conditions (the most convenient matrix, optimal concentrations of
samples and matrix, optimal laser power etc.) were determined for inulin and MOSs standard
samples (the details are not shown). Moreover, these initial experiments showed a potential of
negative-ion mode MALDI-TOF MS for the differentiation of reducing (MOSs) and
nonreducing (inulin) oligosaccharides, because of easy fragmentation of reducing end ring
(the production of in-source fragment ions [M – H – 120] – ; see Figure 1). This in-source
fragmentation has already been described for negative-ion mode MALDI-TOF mass spectra
of dextrans (the polymer containing the main chain with 1–6 glycosidic bonds and different
degree of branching) [7]. This information is very useful for the identification of storage
carbohydrates isolated from plants as is shown below.
All real samples (LGS and oligosaccharides isolated from Jerusalem artichoke and red
onion) were analyzed with THAP that was selected as the most convenient matrix. While
[M – H] – ions formed the dominant distribution for oligosaccharides from both vegetables, the
main distribution of LGS was formed by the in-source fragment ions [M – H – 120] – (see
Figure 2). LGS showed this fragmentation because of its structure that contains the main
chain formed by 1–4 glycosidic bonds and branches with 1–6 glycosidic bonds and a reducing
end group. The mass spectra differed in the quantity of the adducts which was dependent on
the amount of ions in the samples.
Thus there are some important conclusions on the mass spectrometric behavior of the
neutral oligosaccharides. Although the negative-ion mode MALDI-TOF MS is ignored in
connection with neutral carbohydrates it is possible to determine the main characteristics of
their distribution without any carbohydrate derivatization (see Table 1). In addition, the
negative-ion mode MALDI-TOF mass spectra is able to differentiate reducing
maltooligosaccharides and nonreducing fructooligosaccharides extracted from real samples,
because of easy fragmentation of reducing end ring, which is not evident in the positive-ion
mode MALDI-TOF MS where both types of oligosaccharides form the alkali-ion adducts.
REFERENCES
[1] Harvey, D. J. Matrix-assisted laser desorption/ionization mass spectrometry of
carbohydrates and glycoconjugates. Int. J. Mass Specrom. 2003, 226, 1-35.
[2] Zaia, J. Mass spectrometry of oligosaccharides. Mass Spectrom. ReV. 2004, 23, 161-227.
[3] Harvey, D. J. Mass Specrom. Reviews. 2006, 25, 595-662
[4] Štikarovská, M.; Chmelík, J. Analytica Chimica Acta 2004, 520, 47–55
[5] Robyt, J.D. Essentials of carbohydrate chemistry, Springer, New York, 1998
[6] Laštovičkova,M; Chmelik, J. J. Agric. Food Chem. 2006, 54,5092-5097
[7] Čmelík, R.; Štikarovská, M.; Chmelík, J. J. Mass Spectrom. 2004, 39, 1467-1473.
Sborník soutěže Studentské tvůrčí činnosti Student 2006 a doktorské soutěže O cenu děkana 2005 a 2006
Sekce DSP 2006, strana 202