274
D. Toczek et al.
The above considerations apply to neutral conditions since in basic environment
sugars even possessing only hydroxyl groups can be in the anionic form due to dissociation of hydroxyl groups, and then the situation becomes different. The general rule
presented in 70th years of XX century by Angyal [17] is based on his experimental
research and defines the preferred spatial arrangement of hydroxyl group in the carbohydrate structure for complex formation. The results of his work indicated that for
cyclitols and carbohydrate in the six-membered ring form, the preferred arrangement
for the hydroxyl group are 1,3,5-triaxial ( ax–ax–ax) or 1,2,3-axial–equatorial–axial
( ax–eq–ax) sequences and also quasi axial–quasi equatorial–quasi axial sequence of
three adjacent hydroxyl group for the furanose form. These rules by some are called
as Angyal rules[12]. In further Angyal further showed [18] structural arrangement of
hydroxyl group for acyclic compounds. Gyurcsik and Nagy [11] collected all possible spatial positions, arranged in the order according to the decreasing ability to form
the complex (Fig. 9.1). All above mentioned ligands form 1:1 complexes with metal
cations in the hydrophilic solvent [11, 17–19]. Further Angyal studies [20] specified
another factor, which leads to the formation of carbohydrate complexes. Namely, it
is that the size of the ionic radius of complexed ion. The cation with the ionic radius
lower than about 80 pm is better bounded by the ax–ax–ax arrangement, whereas the
cation with larger ionic radius prefers the ax–eq–ax sequence for the coordination.
Since the ionic radius of calcium is estimated at 114 pm, the preferable carbohydrate
sequence for calcium ions is ax–eq–ax. Molecular mechanics computations, carried
out by Hancock and Hegetschweiler [21], confirmed such preferences. However, as
noted by Alekseev et al. [12] only three complexes in solid state are known, which
are in accordance with Angyal rules. On the other hand, there exist carbohydrate
complexes with metal ions (e.g. α-d-glucopyranose) without special spatial arrangement required by Angyal rules. Another independent computations carried out by
Palma and Pascal [22] have proved that in the gaseous phase general Angyal rules
are not fullfilled. It is not surprising that tetra- and pentacoordinated complexes for
β-anomers are characterized by the lowest energy. The structural analysis of carbohydrate–metal complexes in water solution is very difficult. The weak complexation
results in the equilibrium shifted strongly toward the uncomplexed form. In addition, sugar in water exists in many equilibria, and each conformer can form different
complex with the metal cation with the different complex constant. Constants of
complexes for simple carbohydrates are in the range between 0.1 and 6.0. Alekseev
pointed out that complexes between metal ions of sugar acids are characterized by
the one order higher complex constant. In the next part of this section we will present some examples of calcium complexes with carbohydrates and their derivatives.
9.2.1 Calcium–Carbohydrate Complexes in the Crystal Form
Since, as mentioned above, calcium plays very important role in biological systems,
our studies are devoted to calcium complexes with polyphenol glycosides. So this
review will be devoted mostly to describe the research done on calcium–carbohy-
D. Toczek et al.
The above considerations apply to neutral conditions since in basic environment
sugars even possessing only hydroxyl groups can be in the anionic form due to dissociation of hydroxyl groups, and then the situation becomes different. The general rule
presented in 70th years of XX century by Angyal [17] is based on his experimental
research and defines the preferred spatial arrangement of hydroxyl group in the carbohydrate structure for complex formation. The results of his work indicated that for
cyclitols and carbohydrate in the six-membered ring form, the preferred arrangement
for the hydroxyl group are 1,3,5-triaxial ( ax–ax–ax) or 1,2,3-axial–equatorial–axial
( ax–eq–ax) sequences and also quasi axial–quasi equatorial–quasi axial sequence of
three adjacent hydroxyl group for the furanose form. These rules by some are called
as Angyal rules[12]. In further Angyal further showed [18] structural arrangement of
hydroxyl group for acyclic compounds. Gyurcsik and Nagy [11] collected all possible spatial positions, arranged in the order according to the decreasing ability to form
the complex (Fig. 9.1). All above mentioned ligands form 1:1 complexes with metal
cations in the hydrophilic solvent [11, 17–19]. Further Angyal studies [20] specified
another factor, which leads to the formation of carbohydrate complexes. Namely, it
is that the size of the ionic radius of complexed ion. The cation with the ionic radius
lower than about 80 pm is better bounded by the ax–ax–ax arrangement, whereas the
cation with larger ionic radius prefers the ax–eq–ax sequence for the coordination.
Since the ionic radius of calcium is estimated at 114 pm, the preferable carbohydrate
sequence for calcium ions is ax–eq–ax. Molecular mechanics computations, carried
out by Hancock and Hegetschweiler [21], confirmed such preferences. However, as
noted by Alekseev et al. [12] only three complexes in solid state are known, which
are in accordance with Angyal rules. On the other hand, there exist carbohydrate
complexes with metal ions (e.g. α-d-glucopyranose) without special spatial arrangement required by Angyal rules. Another independent computations carried out by
Palma and Pascal [22] have proved that in the gaseous phase general Angyal rules
are not fullfilled. It is not surprising that tetra- and pentacoordinated complexes for
β-anomers are characterized by the lowest energy. The structural analysis of carbohydrate–metal complexes in water solution is very difficult. The weak complexation
results in the equilibrium shifted strongly toward the uncomplexed form. In addition, sugar in water exists in many equilibria, and each conformer can form different
complex with the metal cation with the different complex constant. Constants of
complexes for simple carbohydrates are in the range between 0.1 and 6.0. Alekseev
pointed out that complexes between metal ions of sugar acids are characterized by
the one order higher complex constant. In the next part of this section we will present some examples of calcium complexes with carbohydrates and their derivatives.
9.2.1 Calcium–Carbohydrate Complexes in the Crystal Form
Since, as mentioned above, calcium plays very important role in biological systems,
our studies are devoted to calcium complexes with polyphenol glycosides. So this
review will be devoted mostly to describe the research done on calcium–carbohy-