6.1.2 ROA Research on Chiral Compounds and Its
Applications, Now and Over the Years
ROA spectroscopy provides information about structure, dynamics, and behavior of
chiral compounds. Therefore, it allows determining an absolute configuration as
well as studying conformational equilibrium of chiral molecules [16]. ROA spectra
of two enantiomers are mirror reflections like their structures. Moreover, ROA
enables to measure an enantiomeric excess in samples containing unequal amounts
of mirror image enantiomers. Hecht et al. [17] described a method for determination
of the enantiomeric composition with the accuracy of 0.1%, using a-pinene
enantiomers mixture as a test compound. On account of the exquisite sensitivity to
chirality, ROA is also applied to study conformational changes of important ‘life’
molecules like carbohydrates, proteins and nucleic acids, all in the aqueous solution
to reflect their natural environment (Fig. 6.1) [16]. ROA spectroscopy also appears
to be useful even in the structural studies of viruses. A few virus ROA spectra were
recorded, and the first one was reported for filamentous bacteriophages by Blanch
et al. in 1999 [18].
A wide range of carbohydrates was studied by ROA [19–21] providing a solid
base of structural information, often inaccessible in other spectroscopic methods.
ROA spectra reveal in detail the stereochemical arrangement of carbohydrates in
the aqueous solution, including the anomeric configuration, the ring conformation,
the relative orientation of the substituents around the ring, the absolute configuration at anomeric centers and the glycosidic link configuration.
Studying protein structure, the ROA technique has much more to offer in
comparison with standard X-ray diffraction (XRD) and nuclear magnetic resonance
(NMR) spectroscopy. Various degrees of protein folding can make crystallization
difficult, or the structures are too large to be accessible to mentioned methods.
Protein ROA spectra reveal characteristic spectral patterns, sensitive to secondary
and tertiary structures of the polypeptide backbones, backbone hydration, and side
chain conformations, as well as structural elements in denaturation states [22].
Recently, Kessler et al. [23] shed a new light on ROA as a convenient optical tool to
monitor folding of proteins into amyloid fibrils, involved in serious medical disorders as neurodegenerative diseases or diabetes type 2. ROA spectra of amyloidal
insulin, supported for the first time by the theoretical model, were presented. So far,
ROA has not been commonly used to study such structural variations due to
experimental artifacts.
Another valuable application of ROA is the structural evaluation of nucleic
acids, especially when it comes to study interaction of DNA with drugs.
Gąsior-Głogowska et al. [24] demonstrated that ROA spectroscopy is a sensitive
technique to follow the structural alteration of the DNA molecule upon binding
with anticancer drug cisplatin. In general, ROA spectra of nucleic acid provide
information on the sugar ring conformation, the base-stacking arrangement of base
rings, and mutual orientation of the sugar and base rings.
6 Small and Large Molecules Investigated by Raman Spectroscopy
163
Applications, Now and Over the Years
ROA spectroscopy provides information about structure, dynamics, and behavior of
chiral compounds. Therefore, it allows determining an absolute configuration as
well as studying conformational equilibrium of chiral molecules [16]. ROA spectra
of two enantiomers are mirror reflections like their structures. Moreover, ROA
enables to measure an enantiomeric excess in samples containing unequal amounts
of mirror image enantiomers. Hecht et al. [17] described a method for determination
of the enantiomeric composition with the accuracy of 0.1%, using a-pinene
enantiomers mixture as a test compound. On account of the exquisite sensitivity to
chirality, ROA is also applied to study conformational changes of important ‘life’
molecules like carbohydrates, proteins and nucleic acids, all in the aqueous solution
to reflect their natural environment (Fig. 6.1) [16]. ROA spectroscopy also appears
to be useful even in the structural studies of viruses. A few virus ROA spectra were
recorded, and the first one was reported for filamentous bacteriophages by Blanch
et al. in 1999 [18].
A wide range of carbohydrates was studied by ROA [19–21] providing a solid
base of structural information, often inaccessible in other spectroscopic methods.
ROA spectra reveal in detail the stereochemical arrangement of carbohydrates in
the aqueous solution, including the anomeric configuration, the ring conformation,
the relative orientation of the substituents around the ring, the absolute configuration at anomeric centers and the glycosidic link configuration.
Studying protein structure, the ROA technique has much more to offer in
comparison with standard X-ray diffraction (XRD) and nuclear magnetic resonance
(NMR) spectroscopy. Various degrees of protein folding can make crystallization
difficult, or the structures are too large to be accessible to mentioned methods.
Protein ROA spectra reveal characteristic spectral patterns, sensitive to secondary
and tertiary structures of the polypeptide backbones, backbone hydration, and side
chain conformations, as well as structural elements in denaturation states [22].
Recently, Kessler et al. [23] shed a new light on ROA as a convenient optical tool to
monitor folding of proteins into amyloid fibrils, involved in serious medical disorders as neurodegenerative diseases or diabetes type 2. ROA spectra of amyloidal
insulin, supported for the first time by the theoretical model, were presented. So far,
ROA has not been commonly used to study such structural variations due to
experimental artifacts.
Another valuable application of ROA is the structural evaluation of nucleic
acids, especially when it comes to study interaction of DNA with drugs.
Gąsior-Głogowska et al. [24] demonstrated that ROA spectroscopy is a sensitive
technique to follow the structural alteration of the DNA molecule upon binding
with anticancer drug cisplatin. In general, ROA spectra of nucleic acid provide
information on the sugar ring conformation, the base-stacking arrangement of base
rings, and mutual orientation of the sugar and base rings.
6 Small and Large Molecules Investigated by Raman Spectroscopy
163