of peaks being strictly dependent upon the distance between the
cyclopropane and the aldehyde [10, 16, 17]. Similarly, fragmentation of aldehydes from oxygenated MA is obtained and allows
determining the position of oxygenated functions in keto- and
methoxy-MA. Analysis of trimethylsilyl (TMS) derivative
hydroxyl-MA may also help to determine the localization of the
hydroxyl group [18].
The functional groups in the meromycolic chain can be localized by charge remote fragmentation in FAB-MS [19]. Careful
studies on purified mero-MA of different classes (cyclopropanes,
keto, or methoxy) have allowed determining the exact location of
the functional groups within the meromycolate chain [19]. However, due to the complicated fragmentation patterns obtained using
the method, the determination of the molecular mass is not possible. In this context, matrix-assisted laser desorption/time-of-flight
ä
Fig. 2 (continued) 2-octadecanol and 2-eicosanol, respectively. They are
observed only after saponification, leading to the cleavage of the wax-ester
mycolic acids; consequently, their presence is related to the occurrence of
dicarboxylic mycolic acids (type VI mycolates in Fig. 1a) on TLC. M24 refers to
mycosanoic acid: methyl 2L, 4L docosanoic acid (dextrogyre). M29 and M32
correspond to mycocerosates or phthioceranates series 2, 4, 6-methyl branched
with 29 and 32 carbon atoms, respectively. Both series have the same chemical
structure but opposite configurations of the asymmetric carbons bearing the
methyl branches, respectively negative and positive values of the molecular
rotations. Mycocerosates are constituents of phthiocerol dimycocerosate (PDIM)
and phenol glycolipid (PGL) while phthioceranates are the main acyl groups of
sulfoglycolipid (SGL1). P27 and P25, the so-called phthienoic acids, a family of
dextrorotatory acids considered as virulence markers encountered only in virulent strain of M. tuberculosis and M. bovis. They are engaged in polyphtienoyl
trehalose (PPT) of M. tuberculosis. P27:1: phthienoic acid, also called mycolipenic acid, 2L,4L,6L-trimethyl tetracosen-2,3 oic acid. In addition to the peak at
m/z 88, these acids present characteristic mass peaks
*
at m/z 127 and
169 resulting from 4,5 and 6,7 cleavages. Molecular ion (M) is not always
detectable in EI MS. Instead, M-32 (loss of methanol) for FAME and M-18
(loss of water) for hydroxylated compounds are more generally observed.
However, interesting fragments useful for the identification are observed
(Table, Fig. 1). The base peak (peak of higher intensity in the mass spectrum)
is the marker of the structure of the acid: m/z 74 (Mc Lafferty rearrangement) for
linear FAME. A base peak at m/z 88 is indicative of FAME with a methyl branch
located at C2. The EI-mass spectrum of ethylenic FAME (oleic acid, for example)
presents a base peak at 55 and an intense molecular ion. In addition, characteristic peaks
*
from specific cleavage as mentioned for phthienoic acids are
determinant for identification. For hydroxylated compounds, such as
2-octadecanol and 2-eicosanol, an easy confirmation of the structure is realized
after silylation
#
: in addition to the base peak at m/z 73 an intense peak at m/z
117 corresponding to the fragment CH 3 -CH(OTMS) and M-15 at m/z 313 and
355 for octadecanol and eicosanol, respectively
Lipid and Lipoarabinomannan
113
cyclopropane and the aldehyde [10, 16, 17]. Similarly, fragmentation of aldehydes from oxygenated MA is obtained and allows
determining the position of oxygenated functions in keto- and
methoxy-MA. Analysis of trimethylsilyl (TMS) derivative
hydroxyl-MA may also help to determine the localization of the
hydroxyl group [18].
The functional groups in the meromycolic chain can be localized by charge remote fragmentation in FAB-MS [19]. Careful
studies on purified mero-MA of different classes (cyclopropanes,
keto, or methoxy) have allowed determining the exact location of
the functional groups within the meromycolate chain [19]. However, due to the complicated fragmentation patterns obtained using
the method, the determination of the molecular mass is not possible. In this context, matrix-assisted laser desorption/time-of-flight
ä
Fig. 2 (continued) 2-octadecanol and 2-eicosanol, respectively. They are
observed only after saponification, leading to the cleavage of the wax-ester
mycolic acids; consequently, their presence is related to the occurrence of
dicarboxylic mycolic acids (type VI mycolates in Fig. 1a) on TLC. M24 refers to
mycosanoic acid: methyl 2L, 4L docosanoic acid (dextrogyre). M29 and M32
correspond to mycocerosates or phthioceranates series 2, 4, 6-methyl branched
with 29 and 32 carbon atoms, respectively. Both series have the same chemical
structure but opposite configurations of the asymmetric carbons bearing the
methyl branches, respectively negative and positive values of the molecular
rotations. Mycocerosates are constituents of phthiocerol dimycocerosate (PDIM)
and phenol glycolipid (PGL) while phthioceranates are the main acyl groups of
sulfoglycolipid (SGL1). P27 and P25, the so-called phthienoic acids, a family of
dextrorotatory acids considered as virulence markers encountered only in virulent strain of M. tuberculosis and M. bovis. They are engaged in polyphtienoyl
trehalose (PPT) of M. tuberculosis. P27:1: phthienoic acid, also called mycolipenic acid, 2L,4L,6L-trimethyl tetracosen-2,3 oic acid. In addition to the peak at
m/z 88, these acids present characteristic mass peaks
*
at m/z 127 and
169 resulting from 4,5 and 6,7 cleavages. Molecular ion (M) is not always
detectable in EI MS. Instead, M-32 (loss of methanol) for FAME and M-18
(loss of water) for hydroxylated compounds are more generally observed.
However, interesting fragments useful for the identification are observed
(Table, Fig. 1). The base peak (peak of higher intensity in the mass spectrum)
is the marker of the structure of the acid: m/z 74 (Mc Lafferty rearrangement) for
linear FAME. A base peak at m/z 88 is indicative of FAME with a methyl branch
located at C2. The EI-mass spectrum of ethylenic FAME (oleic acid, for example)
presents a base peak at 55 and an intense molecular ion. In addition, characteristic peaks
*
from specific cleavage as mentioned for phthienoic acids are
determinant for identification. For hydroxylated compounds, such as
2-octadecanol and 2-eicosanol, an easy confirmation of the structure is realized
after silylation
#
: in addition to the base peak at m/z 73 an intense peak at m/z
117 corresponding to the fragment CH 3 -CH(OTMS) and M-15 at m/z 313 and
355 for octadecanol and eicosanol, respectively
Lipid and Lipoarabinomannan
113