Most monosaccharides and their derivatives encountered in foods are polyhydric alcohols carrying a “reducing” keto or aldehydo unit, and they exist primarily in cyclic tetrahydropyran and tetrahydrofuran forms, with the latter occurring less frequently than the former. However, the common ketosugars are more likely than aldosugars to exist as furanoses. Seven-membered rings occur but are not common in foods. Free reducing sugars in solution may exist in different cyclic forms, which are in equilib- rium via the acyclic aldehydo or keto form.
There are three potentially stable shapes for the six- membered saturated sugar rings, namely chair, boat, and skew (Figure 2.1). The chair conformation predominates in most cases because the widest separation of the elec- tronegative oxygen atoms is usually achieved through equatorial orientations of most of the hydroxyl and CH2OH groups. The anomeric hydroxyl unit differs in
that it may adopt two orientations (α or β), which are strongly influenced by the ring oxygen. Similarly, there are two principal conformations for saturated furanoid rings, described as envelope (E) and twist (T) (Figure 2.1), each with four or three coplanar atoms, respectively. Because of the low energy barriers between the E and
FIGURE 2.1 Examples of chair (C), boat (B), and skew (S) forms for pyranoid rings and envelope (E) and twist (T) forms for furanoid rings. 4 5 O 1 2 3 S B C E T 3 2 O 5 4 4 3 5 O 2 2 O 3 4 5 1 5 O 1 2 3 4
T conformers, interconversions of these occur more read- ily than between the pyranoid forms. The shapes of acyclic aldehydo and keto carbohydrates and their reduced forms are usually described as either a linear (zig- zag) or a sickle shape. The advent of diffraction and NMR techniques has allowed the determination of the configu- ration and conformation of almost all the important mono- saccharides (1). In crystals, most molecules adopt a single conformation, whereas in solution there is generally more than one conformation undergoing fast intercon- version.
For a more detailed treatment of monosaccharide chemistry and nomenclature, the readers are referred to standard textbooks (2).
B. OLIGOSACCHARIDES
The conformation of oligosaccharides is less well docu- mented than that of monosaccharides, although the natu- rally occurring common oligosaccharides are well characterized. Most data from x-ray diffraction and NMR analysis are limited to oligosaccharides having less than four monomeric units. There is considerable experimental difficulty encountered when applying these techniques to large oligosaccharides (3). However, although based on limited amount of data, some general features about the conformation of oligosaccharides can be drawn. Once incorporated into an oligosaccharide or polysaccharide chain, the monosaccharide ring is relatively rigid and the ring geometry becomes effectively fixed. Thus, the overall shape of oligosaccharides become more determined by the two torsion (dihedral) angles φand ψacross the two single bonds of their connecting glycosidic linkage than by the unit geometries. Wells of minimum potential energy may be calculated, which limit the values adopted by φand ψ but not rigidly so. Generally speaking, disaccharides should have a preference for staggered conformations about the two linkage bonds, unless there are geometric constraints imposed by, for example, a hydrogen bond between the two rings. The crystal structures of many oligosaccharides have been elucidated (3). Monosaccharides and certain oligosac- charides possess definite crystalline structures, and thus have well-defined melting points and solubilities.
C. POLYSACCHARIDES
Similar to polypeptides, polysaccharides also have different levels of structures, although higher level structures are less well defined. The primary structure describes the covalent sequence of monosaccharide units and the respective glyco- sidic linkages. The secondary structure describes the char- acteristic shapes of individual chains such as ribbons and helices, which arise from repetition of units adopting a par- ticular average orientation in shape. Polysaccharide chains with well-defined secondary structure (or sufficient areas of such) may interact with each other, leading to further ordered organizations incorporating a group of molecules. This is known as the tertiary structure. Further association of these ordered entities results in large quaternary structures.
1. Ordered Structures in the Solid State
A repeated sequence of monomers or oligomers leads to an ordered and periodic conformation of polysaccharide mol- ecules. The different linkage types, arising from the anomeric nature of glycosidic linkage and the orientation of OH units through which it is attached, impose certain general features on oligosaccharide and polysaccharide conformations because of the limitations placed on the dihedral angles. Fundamentally there are four different types of chain shapes: ribbons, hollow helices, loosely jointed, and crumpled types (4). For example, for β-(1→4) linked D-glucopyranosyl units, the two bonds from the ring to its two bridging oxygens define a zig-zag form, which promotes a tendency to adopt a flat, extended, ribbon- like conformation, in the polymer (Figure 2.2a). In con- trast, when the links between the D-glucopyranosyl units are β-(1→3) or α-(1→4), they define a U-turn form (Figure 2.2b); this geometry extended over multiple units often produces a hollow helical conformation, which becomes stabilized in multiple helices. The linkage through the primary hydroxyl units, such as between (1→6) linked hexopyranose units, leads to a loosely jointed type of conformation and marked molecular flexi- bility in the resultant polysaccharides. This arises from the extra single bond and torsion angle (ω) between the two sugar rings that separates the rings, reducing inter-unit interactions and allowing a greater range of conformational
Carbohydrates: Physical Properties 2-3
FIGURE 2.2 Examples of geometrical relationships across sugar rings. (a) Zig-zag relationship across 1,4-linked β-D-glucopyra- nose; (b) U-turn relationship across 1,3-linked β-D-glucopyranose.
O H OH H H H H O O H O OH O H OH H H H H O O H OH O (a) O H OH H H H H O O O H OH O H OH H H H H O HO OH O (b)
possibilities. A further type of conformation known as “crumpled,” such as in β-(1→2) linked glucopyranoxyl units, is less common in food carbohydrates.
The regular conformation of polysaccharides can always be described as a helix, which may be defined by just two parameters, the number of units per turn of the helix, and the translation of each repeating residue along the helical axis. The resultant single helix may associate to form multiple helices, which are then further packed in various ways to form higher ordered structures in the solid state. The majority of polysaccharides in their native form exist in an amorphous structure, examples being the antiparallel, extended twofold ribbon-like organized chain structure in the family of mannans and galactomannans (5). A relatively small number of polysaccharides are organized into a repeating crystalline or partly crystalline structure, examples being cellulose, starches, chitin, and some β-D-glucans. The crystalline element is usually capable of existing in different polymorphic forms. The ordered structures of polysaccharides have been exten- sively studied by x-ray and electron diffraction (6), and the x-ray structures of more than 50 well-defined poly- saccharides are known (7).
2. Secondary and Tertiary Structures in Solutions and Gels
The extensively ordered conformation of a polysaccharide in the solid state may not be retained following hydration in solutions and gels. Polysaccharide chains tend to adopt a more or less coiled shape in solutions and fluctuate con- tinuously between different local and overall conforma- tions. A large group of non-gelling polysaccharides, or gelling polysaccharides in non-gelling conditions, exist in solutions with a conformation known as disordered ran- dom coils. Since polysaccharide molecules contain a large number of hydroxyl groups, they have a high tendency to associate into supramolecular aggregates through hydro- gen bonding in aqueous solutions. For example, combin- ing static and dynamic light scattering, a fringed micelle model was proposed for the aggregates formed in solu- tions by a number of neutral polysaccharides including tamarind xyloglucan (8) and cereal (1→3)(1→4)-β-glu- cans (9). The association of molecules in such a form markedly increases the stiffness of the single chains, lead- ing to enhanced solution viscosity. More ordered struc- tures may be developed, in solution through the so-called cooperative interactions, especially for polysaccharides in which identical repeat units result in a regularity of sequence. Conformational transitions in solution between random coils and helices have been well recognized and characterized for a number of polysaccharides such as curdlan, xanthan, and gellan (10). Under favorable condi- tions, these ordered structures may further associate, lead- ing to the formation of three-dimensional gel networks.