42
1
General Principles
Interpretation of nOe results must be performed with caution because a number of difficulties,
both technical, such as spin diffusion, and theoretical, such as the use of oversimplified or
incorrect models can lead to incorrect interpretations.
2.4 Circular Dichroism and Optical Rotatory Dispersion
Only chiral molecules exhibit circular dichroism (CD) or optical rotatory dispersion (ORD)
spectra. ORD is circular birefringence spectroscopy, that is, the difference in refractive index
for circularly polarized light as a function of wavelength, or simply, the measure of optical
rotation as a function of wavelength. Each individual UV absorption appears as an S-shaped
or mirror-image S-shaped curve that is null at 8 max . CD is the difference in absorption of right
and left circularly polarized light and appears as a Gaussian peak centered on 8 max for the UV
absorption [442,443]. CD has been employed more for carbohydrates than ORD [444,445].
Since absorption maxima for most carbohydrates occur below 185 nm, vacuum CD must be
used [444]. Considerable conformational information can be obtained because the appearance
of the curves is very sensitive to the three-dimensional orientations of groups with respect to
the chromophores. The most important factors in carbohydrate CD [445] are the anomeric configuration and the orientation about the angle, normally exo-syn. Simple monosaccharides
and some disaccharides have been studied [446] and conformational conclusions have been
drawn for disaccharides and polymers [447,448,449]. Compounds containing other longer
wavelength chromophores have also been studied [450,451].
Alternatively, chromophores can be added to carbohydrates through the formation of parasubstituted benzoate esters or other functional groups containing chromophores. The high
intensities of the CD spectra of these derivatives make this technique quite sensitive [452]. The
orientations of the chromophores with respect to the sugar can be estimated and the resulting
CD curves are very sensitive to substituent orientations. Thus, this technique is also useful for
conformational studies [199,200,203,453].
2.5 Molecular Modeling
Molecular modeling has become an essential technique to outline the possibilities inherent
for a particular system. The experimental observations, which are usually weighted averages,
can then be interpreted in terms of these possibilities. Information about structure, conformation, and dynamics can be obtained. It is impossible to interpret complex sets of experimental observations, such as the multiple sets of average internuclear distances provided by nOe
experiments, without knowledge about the geometries of two, three or more potential energy
minima and without having some information about molecular dynamics. Methods for performing molecular modeling studies of carbohydrates are well established [329,347,454,455].
Initial studies used the hard sphere method to estimate non-bonded interactions [456] calculated with the potential of Kitaigorodsky [457]:
V = 3.5
−0.04(r o /r)
6
+ 8.6 · 10
3 exp
−13
r o
r
.
(10)
Addition of terms to account for the exoanomeric effect in α and β glycosides gave the Hard
Sphere Exo-Anomeric (HSEA) force field [458,459], which was very successful in predict-
1
General Principles
Interpretation of nOe results must be performed with caution because a number of difficulties,
both technical, such as spin diffusion, and theoretical, such as the use of oversimplified or
incorrect models can lead to incorrect interpretations.
2.4 Circular Dichroism and Optical Rotatory Dispersion
Only chiral molecules exhibit circular dichroism (CD) or optical rotatory dispersion (ORD)
spectra. ORD is circular birefringence spectroscopy, that is, the difference in refractive index
for circularly polarized light as a function of wavelength, or simply, the measure of optical
rotation as a function of wavelength. Each individual UV absorption appears as an S-shaped
or mirror-image S-shaped curve that is null at 8 max . CD is the difference in absorption of right
and left circularly polarized light and appears as a Gaussian peak centered on 8 max for the UV
absorption [442,443]. CD has been employed more for carbohydrates than ORD [444,445].
Since absorption maxima for most carbohydrates occur below 185 nm, vacuum CD must be
used [444]. Considerable conformational information can be obtained because the appearance
of the curves is very sensitive to the three-dimensional orientations of groups with respect to
the chromophores. The most important factors in carbohydrate CD [445] are the anomeric configuration and the orientation about the angle, normally exo-syn. Simple monosaccharides
and some disaccharides have been studied [446] and conformational conclusions have been
drawn for disaccharides and polymers [447,448,449]. Compounds containing other longer
wavelength chromophores have also been studied [450,451].
Alternatively, chromophores can be added to carbohydrates through the formation of parasubstituted benzoate esters or other functional groups containing chromophores. The high
intensities of the CD spectra of these derivatives make this technique quite sensitive [452]. The
orientations of the chromophores with respect to the sugar can be estimated and the resulting
CD curves are very sensitive to substituent orientations. Thus, this technique is also useful for
conformational studies [199,200,203,453].
2.5 Molecular Modeling
Molecular modeling has become an essential technique to outline the possibilities inherent
for a particular system. The experimental observations, which are usually weighted averages,
can then be interpreted in terms of these possibilities. Information about structure, conformation, and dynamics can be obtained. It is impossible to interpret complex sets of experimental observations, such as the multiple sets of average internuclear distances provided by nOe
experiments, without knowledge about the geometries of two, three or more potential energy
minima and without having some information about molecular dynamics. Methods for performing molecular modeling studies of carbohydrates are well established [329,347,454,455].
Initial studies used the hard sphere method to estimate non-bonded interactions [456] calculated with the potential of Kitaigorodsky [457]:
V = 3.5
−0.04(r o /r)
6
+ 8.6 · 10
3 exp
−13
r o
r
.
(10)
Addition of terms to account for the exoanomeric effect in α and β glycosides gave the Hard
Sphere Exo-Anomeric (HSEA) force field [458,459], which was very successful in predict-