V amylose and inclusion complexes

Native starches of A, B, or C types are organized as a double helix. Katz (1930) has highlighted another type of crystalline amylose and starch. This new crystalline type is commonly called V “Verklesterung” (reprecipitation) or “Verkleistert” (gelatinized) for others and is obtained by reprecipitation of starch solutions in the presence of alcohols. In 1932, Katzet and Derksen noted several similarities between the diffraction patterns of X-ray powder V amylose and α-cyclodextrin. They then proposed for the V-type amylose, an arrangement of simple helix mimicking a stack of cyclodextrins. The helix thus arranged includes six glucose residues per turn. In 1943, Rundle and Edwards have shown the helical nature of amylose V by XRD (Rundle et al., 1944). This structure was also studied by NMR (Veregin et al., 1987a,b). These helices have a central cavity capable of receiving the molecule used for reprecipitation (alcohol) from where sometimes the term employed of inclusion complexes. Many molecules have complexing property with respect to amylose. The main ones are fatty acids, alcohols, dimethyl sulfoxide (DMSO), iodine, salts (KOH and KBr), and also some aroma compounds.

Specific Complexes

These complexes have mostly a constitutive left propeller and six glucose residues per turn but differ in the pitch of the helix and/or its stacking mode in the crystalline structure. These structures can only be obtained with a single type of sequestering molecule.

V_DMSO Complexes

Winter and Sarko (1974) proposed for complex amylose-DMSO, the following crystal structure:

The crystal lattice is pseudo-tetragonal with the P2₁2₁2₁ space group (a = b = 1.97 nm, c = 2.439 nm). It contains two antiparallel helices and six molecules of DMSO. The repeat unit along the chain axis consists of three turns of helix. Thus, each turn of the helix contains two molecules of DMSO. This structure is easily converted into V_h by exposing fibrillar crystals in alcoholic solutions.

V_glycerol Complexes

The complexation of glycerol and amylose may result in crystal packing and has been described by Hulleman et al. (1996) from lamellar crystals and electron diffraction. The lattice is orthorhombic, the most likely space group P2₁2₁2₁ with the following parameters: a = 1.93 nm, b = 1.86 nm, and c = 0.83 nm. The lattice contains two helices arranged in antiparallel directions. Neither the position of the glycerol molecules nor their number can be clearly established.

V_KOH Complexes

The structure of complex amylose-KOH was resolved by Sarko and Biloski (1980) with fibrillar crystals and XRD. Two amylose helices are arranged in an orthorhombic lattice with the parameters a = 0.884 nm, b = c = 1.231 nm, and 2.241 nm. It belongs to the space group P2₁2₁2₁. Each helix has six glucose residues (a helix turn) and each potassium ion is complexed with three glucosyl residues. This type of structure is also obtained with other salts such as LiOH and NaOH.

V_iode Complexes

Amylose complexes may also be formed in the presence of iodine. Rundle et al. (1944) showed that the amylose acquires a helical conformation, generating channel that is trapped in iodine atoms. The crystal lattice is orthorhombic with the parameters a = 1.360 nm, b = 2.342 nm, and c = 0.817 nm (Bluhm and Zugenmaier, 1981) and belongs to the space group P2₁(S). It contains two adjacent amylose helices, not allowing iodine to find its place between helices. According to Bluhm and Zugenmaier (1981), the iodine atoms are randomly aligned in the cavity of the helix at a distance of 0.31 nm. The formation of these complexes gives rise to a color, which depends on the DP of amylose, of the water content, and of the positioning of the iodine in the helix. This color variation is particularly marked by the DP of amylose: it can be brown (DP 21-24), red (DP 25-29), purple red (DP 30-38), blue-violet (DP 39-46), or blue (DP > 47) (John et al., 1983).

Aspecific V₆ Complexes

There are other types of complex amyloses, but these can be obtained with a wide variety of different molecules. The complexes using a helix with six, seven, or eight glucoses per turn are called, respectively, V₆, V₇, and V₈.

Three main types of structures have been highlighted from amylose reprecipitated with ethanol (V_h, for V hydrated Type), butanol (V_butanol), and isopropanol (V_isopropanol). These three complexes possess an identical constructive amylose helix; so their main differences lie in the dimensions of the crystal lattice but also in the number of helices contained in each mesh. Many complexing molecules can induce these three structures; this leads to define three “families” of complexes referred, respectively, as V_6I for V_h; V_6II for V_butanol, and V_6III for V_isopropanol in reference to the size of the crystal lattice. This name evokes a type of nonspecific structure of this molecule in the complex.

V_6I-Type Complexes

V_6I-Type complexes are obtained with certain alcohols (ethanol, methanol, n-propanol, etc.) (Buleon et al., 1984; Brisson et al., 1991), alcohols with long carbon chains (up to 18 carbons) (Kowblansky, 1985; Whittam et al., 1989) or lipids having a long aliphatic chain (Yamashita et al., 1973).

The diffraction patterns of X-rays sufficiently helped to define a structural model of V_6I type. Rappenecker and Zugenmaier (1981) showed an elementary lattice of orthorhombic pseudohexagonal type, belonging to the P2₁2₁2₁ space group with a = 1.365 nm, b = 2.370 nm, and c = 0.805 nm. The crystal lattice contains two antiparallel helices in the left and 16 water molecules (8 intrahelical and 8 between helices).

Furthermore, Brisson et al. (1991) proposed another model obtained from single crystals and electronic diffraction patterns. This model has a hexagonal lattice with symmetry P6522 (a = b = 1.365 nm and c = 0.805 nm, γ = 120 degrees). According to this model, the crystal lattice contains only one helix amylose. Stacking the helices in the edifice is then statistically random and not antiparallel as in the model of Rappenecker. Although models Rappenecker and Zugenmaier (1981) and Brisson et al. (1991) differ in the crystalline lattice, they are both based on the same helix of amylose. This is a simple left helix with six residues per turn (Rundle and French, 1943) whose advance by glucosyl residue (h) is between 0132 and 0136 nm. It has a hydrophobic cavity with a diameter of about 0.45 nm, which can accommodate the complexing agent (Godet et al., 1993a,b, 1995).

Molecules having an aliphatic chain are frequently included in the cavity of the helix. The aliphatic chain may be inserted into the helical cavity without undergoing any significant stresses. Indeed, Carlson et al. (1979) showed that the conformation of the aliphatic chain of a fatty acid included is identical to the conformation of the aliphatic chain of the same fatty acid in its crystalline state. An important feature of this inclusion is the almost perfect adequation between the container (helix) and content (the aliphatic chain of the ligand): Godet et al. (1993a,b) showed that the progress of a glucosyl residue perfectly corresponds to a CH₂ group of the aliphatic chain. The inclusion of fatty acid between helices in the V_6I lattice is not feasible because the space between the helices is very limited and does not allow the positioning of larger molecules as water molecules (Godet et al., 1993a,b). Water molecules, present between helices, play an important role. Indeed, under certain conditions of hydration, crystal packing can be modified and lead to a transition to a different crystal packing named V_a (a, for anhydrous).

This type of crystalline V_a can be formed from V_6I complex and also during a crystallization at a low water content or during extrusion. When drying fibrillar crystals (water activity Aw ≤ 0.6), a reversible transition of V_h (V_6I) type to another crystal type V_a can be observed (Hinkle and Zobel, 1968). This transition may lead to intermediate forms (Zobel et al., 1967) depending on the degree of hydration. The passage of V_h-type to V_a-type results in a change of XRD diagrams. Indeed, both crystalline types present a diffraction pattern each with characteristic peaks at 20 angles = 7.4, 13.9, and 19.8 degrees (λ = 0.15405 nm) for V_6I and 20 = 7.9, 13.6, and 20.9 degrees (λ = 0.15405 nm) for the type V_a.

V_6II-Type Complexes

The V_6II type complexes are known under the name V n-butanol from the work of Schoch (1942) on fractionating starch by precipitation with butanol. These crystals are very sensitive to desolvation, and their drying causes crystalline transition at solid state of V_6II type to V_6I type (Rundle and Edwards, 1943; Yamashita, 1965). They are in the form of crackled rectangular lamellae along its main axis. The crystal lattice is orthorhombic with the parameters a = 2.74 nm, b = 2.65 nm, and c = 0.8 nm and belongs to the space group P2₁2₁2₁ (Helbert and Chanzy, 1994). It contains approximately four molecules of n-butanol and four amylose helices, each being in contact with four portions of adjacent helices separated by 0.31 nm and from four other portions.

V_6III-Type Complexes

The V_6III complexes were obtained from isopropanol (Visopropanol) and have been highly studied (Yamashita and Hirai, 1966; Yamashita et al., 1973; Buleon et al., 1990). They are less sensitive to desolvation than the complex with n-butanol, but a polymorphic transition to V_6I type is still possible in the solid state (Yamashita and Hirai, 1966). Rectangular crystals crackle then perpendicularly to the main axis of the lamellae. The crystal lattice determined by Buleon et al. (1990) is orthorhombic and belongs to P2₁2₁2₁ space group or P2₁2₁2 with a = 2.83 nm, b = 2.93 nm, and c = 0.8 nm. These parameters have subsequently been refined from complexes with aroma compounds (Nuessli et al., 2003). It contains four amylose helices and the molecules are probably trapped between helices. As for complex V_6II, there is still no precise molecular model giving helix positions in the lattice.

Many molecules can complex amylose and lead to a crystalline V_6III type (Helbert and Chanzy, 1994; Le Bail et al., 2005; Biais et al., 2005).

V8-Type Complexes

Complexes having a helix of amylose with eight glucosyl units per turn have been obtained with molecules such as a-naphthol or quinoline (Yamashita and Monobe, 1971; Helbert and Chanzy, 1994). Relatively few studies focused on these complexes.