265
9 Qualitative and Quantitative FT-Raman Analysis of Plants
laser lies within an electronic absorption of the sample. So when uv resonance
Raman microspectroscopy (λ exc = 244 nm) was applied for analysis of quinine in
Cinchona bark the methodology allowed Raman spectra of the active agents to be
obtained selectively, with enhanced signals by a factor of up to 10
6
compared to the
non-resonant Raman signal of that plant. It was found that the uv resonance Raman
spectrum obtained from Cinchona bark corresponds well with that of pure isolated
quinine. moreover, in situ uv-RR microspectroscopy is capable of differentiating
between quinine and its diastereomer quinidine, structurally very similar active
agents. this discrimination is possible based on a marker band at 831 cm
−1
in the
Raman spectrum of quinine, which is shifted to 843 cm
−1
in the case of quinidine.
this vibration involves a banding motion within the side chain around the chiral
centre of quinine. vibrations belonging to the quinoline ring, important for its antimalarial activity in forming p–p interactions to hemozoin, and the vinyl group are
resonantly enhanced in the uv-Raman spectra. the interpretation of the experimental spectra was based on dFt calculations combined with Ft-Raman spectroscopy
of the pure isolated standards. the solvent effect was correlated with the shift of the
band at 1,362 cm
−1
to 1,371 cm
−1
for anhydrous quinine. this vibration is sensitive
to the presence of an aqueous environment and is assigned to a C = C stretching
mode [67].
Some other antimalarial alkaloids (dioncophylline A, dioncophylline C, and dioncopeltine A) belonging to the group of naphthylisoquinoline alkaloids were isolated from the tropical liana Triphyophyllum pelatatum [68]. In order to locate those
parts of the plant containing the highest concentration of these bioactive substances,
Raman microspectroscopy was found to be a very efficient tool capable of differentiating between various structurally similar naphthylisoquinoline alkaloids [69]. In
this context, the signals registered at 1,356 and 1,613 cm
−1
assigned to C = C stretching and C–h bending vibrations were especially useful for the reliable distinction of
the different alkaloid structures [70]. Raman spectra obtained from the individual
pure alkaloid standards, as well as in vivo measurements on the plant tissue, were
presented and discussed in detail. most of the identified signals could be successfully assigned to various vibrational modes of the alkaloid molecular structures.
9.3.3 Essential Oils
Raman spectra obtained from essential oils also show characteristic key bands
which can be used as marker bands to discriminate different plant species, cultivars
and chemotypes. Characteristic group frequencies of numerous pure essential oil
components have been listed to support the identification of unknown samples [34,
35, 71–74].
In some cases chemotaxonomic discrimination of plant species is possible measuring the individual fresh plant tissue [3, 75]. As can be seen in Fig. 9.4 micro-Raman measurements performed in situ on the essential oil cells of Eucalyptus cinerea
leaves provide an impression of the individual essential oil distribution [76].
9 Qualitative and Quantitative FT-Raman Analysis of Plants
laser lies within an electronic absorption of the sample. So when uv resonance
Raman microspectroscopy (λ exc = 244 nm) was applied for analysis of quinine in
Cinchona bark the methodology allowed Raman spectra of the active agents to be
obtained selectively, with enhanced signals by a factor of up to 10
6
compared to the
non-resonant Raman signal of that plant. It was found that the uv resonance Raman
spectrum obtained from Cinchona bark corresponds well with that of pure isolated
quinine. moreover, in situ uv-RR microspectroscopy is capable of differentiating
between quinine and its diastereomer quinidine, structurally very similar active
agents. this discrimination is possible based on a marker band at 831 cm
−1
in the
Raman spectrum of quinine, which is shifted to 843 cm
−1
in the case of quinidine.
this vibration involves a banding motion within the side chain around the chiral
centre of quinine. vibrations belonging to the quinoline ring, important for its antimalarial activity in forming p–p interactions to hemozoin, and the vinyl group are
resonantly enhanced in the uv-Raman spectra. the interpretation of the experimental spectra was based on dFt calculations combined with Ft-Raman spectroscopy
of the pure isolated standards. the solvent effect was correlated with the shift of the
band at 1,362 cm
−1
to 1,371 cm
−1
for anhydrous quinine. this vibration is sensitive
to the presence of an aqueous environment and is assigned to a C = C stretching
mode [67].
Some other antimalarial alkaloids (dioncophylline A, dioncophylline C, and dioncopeltine A) belonging to the group of naphthylisoquinoline alkaloids were isolated from the tropical liana Triphyophyllum pelatatum [68]. In order to locate those
parts of the plant containing the highest concentration of these bioactive substances,
Raman microspectroscopy was found to be a very efficient tool capable of differentiating between various structurally similar naphthylisoquinoline alkaloids [69]. In
this context, the signals registered at 1,356 and 1,613 cm
−1
assigned to C = C stretching and C–h bending vibrations were especially useful for the reliable distinction of
the different alkaloid structures [70]. Raman spectra obtained from the individual
pure alkaloid standards, as well as in vivo measurements on the plant tissue, were
presented and discussed in detail. most of the identified signals could be successfully assigned to various vibrational modes of the alkaloid molecular structures.
9.3.3 Essential Oils
Raman spectra obtained from essential oils also show characteristic key bands
which can be used as marker bands to discriminate different plant species, cultivars
and chemotypes. Characteristic group frequencies of numerous pure essential oil
components have been listed to support the identification of unknown samples [34,
35, 71–74].
In some cases chemotaxonomic discrimination of plant species is possible measuring the individual fresh plant tissue [3, 75]. As can be seen in Fig. 9.4 micro-Raman measurements performed in situ on the essential oil cells of Eucalyptus cinerea
leaves provide an impression of the individual essential oil distribution [76].