3.5. Identification of cyanobacterial peptides
A library performance was also examined in the example of recognition
of cyclopeptides in extracts from the two M. aeruginosa
972 and 973 cyanobacterial strains. This qualitative analysis was
very briefly mentioned in our presentation [41]. Three identifications
related to principal peaks in the MALDI spectra of extracts
are considered below.
The 1st case refers to the strain 973 and the analyte corresponding
to the ion with integral-valued m/z 996. The library search withthe ToF/ToF spectrum, see Fig. 4b, resulted in the high MFs and
microcystin-LR as the same identification candidate in top lines
of the hit list leaves no room to doubt that this abundant microcystin
is reliably recognized.
There were more difficult problems to identify main components
of the extract of the strain 972 (Fig. 5a). Here, the nature of the ion
m/z 1025 is relatively clear (Fig. 5a). Indeed, HRMS demonstrated
that it is demethylmicrocystin-RR as its molecular formulaC49H73N11O13 had been confirmed with the accuracy of 3.1 ppm (the
[M+H]+ ion in MALDI) and 0.1 ppm (the [M+2H]2+ ion, ESI-MS2).
Library searches with ESI spectrum (Fig. 5b) led to the two above-
MFcut-off candidates which are isomeric [Dha7]microcystin-RR
(Fig. 5d) and [D-Asp3]microcystin-RR (Fig. 5f). However, because
of not very high MF values and a few reference spectra of different
demethylmicrocystins-RR including [D-Asp3,E-Dhb7]microcystin-
RR [42] available in the library, from one through three spectra,
we cannot definitely identify the particular isomer and prefer ‘‘demethylmicrocystin-
RR’’ as the result statement.
In the 3rd case, there can be no any identification of the ion m/z
1723 (Fig. 5a) based on library searches because of the absence of
corresponding library spectra. Hit list contained only spectra (e.g.
in Fig. 5e) not matching to the corresponding MALDI fragment
spectra (Fig. 5c) and therefore resulted in only TNs. So, the
approach to identification based on MS libraries is not applicable
for especially rare/novel compounds without references.
One might assume on mass level grounds that there was one
from microviridins, a group of cyclic depsipeptide (see [43]).
Recently, ‘‘microviridin 1721’’ with m/z 1722.8 of the [M+H]+ ion
was detected [43]. We measured the accurate mass of this ion
(1722.7516) and also the [M+2H]2+ precursor (m/z 861.8810) and
its fragments in ESI-MS2 and could not conclude on the structure
of this molecule. The reason is a variety of plausible candidate formulas
(tens) and sequences (hundreds) for these masses within the
common mass tolerance (
3.5. Identification of cyanobacterial peptides
A library performance was also examined in the example of recognition
of cyclopeptides in extracts from the two M. aeruginosa
972 and 973 cyanobacterial strains. This qualitative analysis was
very briefly mentioned in our presentation [41]. Three identifications
related to principal peaks in the MALDI spectra of extracts
are considered below.
The 1st case refers to the strain 973 and the analyte corresponding
to the ion with integral-valued m/z 996. The library search withthe ToF/ToF spectrum, see Fig. 4b, resulted in the high MFs and
microcystin-LR as the same identification candidate in top lines
of the hit list leaves no room to doubt that this abundant microcystin
is reliably recognized.
There were more difficult problems to identify main components
of the extract of the strain 972 (Fig. 5a). Here, the nature of the ion
m/z 1025 is relatively clear (Fig. 5a). Indeed, HRMS demonstrated
that it is demethylmicrocystin-RR as its molecular formulaC49H73N11O13 had been confirmed with the accuracy of 3.1 ppm (the
[M+H]+ ion in MALDI) and 0.1 ppm (the [M+2H]2+ ion, ESI-MS2).
Library searches with ESI spectrum (Fig. 5b) led to the two above-
MFcut-off candidates which are isomeric [Dha7]microcystin-RR
(Fig. 5d) and [D-Asp3]microcystin-RR (Fig. 5f). However, because
of not very high MF values and a few reference spectra of different
demethylmicrocystins-RR including [D-Asp3,E-Dhb7]microcystin-
RR [42] available in the library, from one through three spectra,
we cannot definitely identify the particular isomer and prefer ‘‘demethylmicrocystin-
RR’’ as the result statement.
In the 3rd case, there can be no any identification of the ion m/z
1723 (Fig. 5a) based on library searches because of the absence of
corresponding library spectra. Hit list contained only spectra (e.g.
in Fig. 5e) not matching to the corresponding MALDI fragment
spectra (Fig. 5c) and therefore resulted in only TNs. So, the
approach to identification based on MS libraries is not applicable
for especially rare/novel compounds without references.
One might assume on mass level grounds that there was one
from microviridins, a group of cyclic depsipeptide (see [43]).
Recently, ‘‘microviridin 1721’’ with m/z 1722.8 of the [M+H]+ ion
was detected [43]. We measured the accurate mass of this ion
(1722.7516) and also the [M+2H]2+ precursor (m/z 861.8810) and
its fragments in ESI-MS2 and could not conclude on the structure
of this molecule. The reason is a variety of plausible candidate formulas
(tens) and sequences (hundreds) for these masses within the
common mass tolerance (<5 ppm).
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