67
4 Raman optical Activity of Biological Samples
only in that it has an exocyclic hydroxymethyl group. the rest of the two structures
possess the same configuration at the individual chiral centres which is reflected
in the similarity of the spectra in the range from 950–1200 cm
−1
. As is seen in
the fingerprint region of both spectra the characteristic sign pattern is observed,
starting from high wavenumber for d-glucose: positive at 1155 cm
−1
, negative at
1107 cm
−1
, positive 1053 cm
−1
and negative at 994 cm
−1
and for d-xylose: positive
at 1128 cm
−1
, negative at 1094 cm
−1
, positive at 1018 cm
−1
and negative at 980 cm
−1
[37]. In this region in the spectra of d-glucose in d 2 o and also d-glucose-1-d the
same sign pattern was found [27], although wavenumber shifts were observed in the
spectrum of d-xylose. Figure 4.1. shows also that the spectra of these two monosaccharides differ significantly in other regions, which is due to the absence of the
− CH 2 oh group in the structure of d-xylose.
Although the RoA spectra of carbohydrates are highly informative on their
structure and many successful studies have been presented in the literature, the interpretation of these spectra is still difficult and limited, because there are no clear
empirical rules relating the sign and magnitude of observed RoA signals to the molecular structure. Nowadays, we see that the most effective step towards complete
understanding of RoA spectra is the use of quantum chemical calculations which
are giving better results from year to year, starting from geometry prediction and
ending with the computation of RoA observables [38–41]. In 2011, reliable calculations of the Raman and ROA spectra of methyl-β-D-glucose using a combination of
density functional theory (dFt) and full molecular dynamics (md) simulation of
the aqueous environment were reported [42].
4.3.1 Calculation of ROA Spectra
generally, the computational complexity of the molecular properties that are
needed for simulation of an RoA spectrum and low experimental activity in RoA
spectroscopy have been responsible for the rather slow development of methods
for calculation of vibrational RoA spectra [43]. Calculations of vibrational Raman
optical activity observables, usually based on the Placzek approximation, can be
conducted in several ways. this multiplicity is a result of the existence of a variety
of theoretical models of RoA [44–46]. Calculation by using such RoA models as
the atom-dipole interaction [47] and the bond-polarizability [13, 48] didn’t result in
broad satisfactory results. however, they can still provide valuable physical insight
into the generation of RoA [13, 45]. In 1990 the first complete ab initio calculation
of a Raman optical activity spectrum was presented in the literature by Polavarapu
and co-workers [49, 50] and in 2001 the first calculations of RoA CIds at the
dFt level of theory was made by Ruud et al. [51] which was a huge turning point
in terms of presenting improvements in the quality of the obtained calculations.
Nowadays, dFt theory has become the dominant approach for the calculation of
vibrational Raman optical activity spectra.
4 Raman optical Activity of Biological Samples
only in that it has an exocyclic hydroxymethyl group. the rest of the two structures
possess the same configuration at the individual chiral centres which is reflected
in the similarity of the spectra in the range from 950–1200 cm
−1
. As is seen in
the fingerprint region of both spectra the characteristic sign pattern is observed,
starting from high wavenumber for d-glucose: positive at 1155 cm
−1
, negative at
1107 cm
−1
, positive 1053 cm
−1
and negative at 994 cm
−1
and for d-xylose: positive
at 1128 cm
−1
, negative at 1094 cm
−1
, positive at 1018 cm
−1
and negative at 980 cm
−1
[37]. In this region in the spectra of d-glucose in d 2 o and also d-glucose-1-d the
same sign pattern was found [27], although wavenumber shifts were observed in the
spectrum of d-xylose. Figure 4.1. shows also that the spectra of these two monosaccharides differ significantly in other regions, which is due to the absence of the
− CH 2 oh group in the structure of d-xylose.
Although the RoA spectra of carbohydrates are highly informative on their
structure and many successful studies have been presented in the literature, the interpretation of these spectra is still difficult and limited, because there are no clear
empirical rules relating the sign and magnitude of observed RoA signals to the molecular structure. Nowadays, we see that the most effective step towards complete
understanding of RoA spectra is the use of quantum chemical calculations which
are giving better results from year to year, starting from geometry prediction and
ending with the computation of RoA observables [38–41]. In 2011, reliable calculations of the Raman and ROA spectra of methyl-β-D-glucose using a combination of
density functional theory (dFt) and full molecular dynamics (md) simulation of
the aqueous environment were reported [42].
4.3.1 Calculation of ROA Spectra
generally, the computational complexity of the molecular properties that are
needed for simulation of an RoA spectrum and low experimental activity in RoA
spectroscopy have been responsible for the rather slow development of methods
for calculation of vibrational RoA spectra [43]. Calculations of vibrational Raman
optical activity observables, usually based on the Placzek approximation, can be
conducted in several ways. this multiplicity is a result of the existence of a variety
of theoretical models of RoA [44–46]. Calculation by using such RoA models as
the atom-dipole interaction [47] and the bond-polarizability [13, 48] didn’t result in
broad satisfactory results. however, they can still provide valuable physical insight
into the generation of RoA [13, 45]. In 1990 the first complete ab initio calculation
of a Raman optical activity spectrum was presented in the literature by Polavarapu
and co-workers [49, 50] and in 2001 the first calculations of RoA CIds at the
dFt level of theory was made by Ruud et al. [51] which was a huge turning point
in terms of presenting improvements in the quality of the obtained calculations.
Nowadays, dFt theory has become the dominant approach for the calculation of
vibrational Raman optical activity spectra.