Helical Inclusion Complexes

To determine the amylose content of starch, the iodine reaction has been most commonly used because amylose and amylopectin have different abilities to bind iodine. The methods such as blue value (absorbance at 680 nm for starch-iodine complex using amylose and amylopectin standards), and potentiometric and amperometric titration have been used for more than 50 years. These procedures are based on the capacity of amylose to form helical inclusion complexes with iodine, which display a blue color characterized by a maximum absorption wavelength (kmax) above 620 nm. During the titration of starch with iodine solution, the amount (mg) of iodine bound to 100 mg of starch is determined. The value is defined as iodine-binding capacity or iodine affinity (lA). The amylose content is based on the iodine affinity of starch vs. purified linear fraction from the standard 100 mg pure linear amylose fraction has an iodine affinity of 19.5-21.0mg depending on amylose source. Amylopectin binds 0-1.2mg iodine per 100mg (Banks and Greenwood, 1975). The amylose content determined by potentiometric titration is considered an absolute amylose content if the sample is defatted before analysis. 

The limited solubility of starch and its modified products may affect the reversibility of many reactions. This may explain several, apparently unusual, reactions reported in starch chemistry. There are, for example, reports of starch esterification with sodium hydrogenphosphates, acylation of starch with acyl amides (which is equivalent to the transformation of an amide into an ester), and the formation of alkali-metal starchates upon treatment of starch with alkali (a reaction which fails for simple alcohols). A specific property of starch is its ability to form surface sorption and helical inclusion-complexes with many inorganic and organic guest molecules.4... 

The iodine-binding capacity of starch is dependent on the degree of polymerization (DP). Amylose forms with iodine a helical inclusion complex with an intense blue colour, which possesses an absorption maximum at wavelengths between 620 and 680 nm. Amylopectin has much less iodine-binding capacity because of its branched character, leading to a red-violet colour with absorption maximum of 540 nm 31. 

The outer branches of the amylopectin molecule assist in the formation of a helical inclusion complex with suitable lipids (Gudmudsson and Eliasson, 1990). Interactions depend on the starch origin. Mutant maize and mutant rice produce amylopectin with branched chains having about 15 residues rather than the normal 10 to 13 glucose residues. Instead of a normal A-pattem, mutant cereals gave B-pattems (Imberty et al., 1991). 

The melting of the lipid-amylose complex is completely reversible (see Figure 9.6) and can be observed on successive runs. It can be used as the basis for the assessment of the amylose content in starches [27], where the size of the melting peak in an excess quantity of lysophosphatidylcholine, a material known to form helical inclusion complexes, is dependent on the amylose level. This has been adapted to other starch-containing foods. 

X-ray diffraction studies on gramicidin commenced as early as 1949 218-219> and this early work pointed to a helical structure 220). Recent work by Koeppe et al. 221) on gramicidin A crystallised from methanol (/%) and ethanol (.P212121) has shown that the helical channel has a diameter of about 5 A and a length of about 32 A in both cases. The inclusion complexes of gramicidin A with CsSCN and KSCN (P212121) have channels that are wider (6-8 A) and shorter (26 A) than the uncomplexed dimer 221 222). Furthermore there are two cation binding sites per channel situated either 2.5 A from either end of the channel or 2.5 A on each side of its centre 222) Unfortunately these data do not permit a choice to be made from the helical models (i)—(iv) and it is not certain if the helical canals studied are the same as those involved in membrane ion transport. 

Figure 45 (a) ORTEP view of the molecular adduct 39 35 (H-bonds are represented by thin lines), (b) ORTEP view of the inclusion complex between benzene and adduct 39 35. (c) Side view of the H-bonding network of adduct 39 35. (d) Simplified representation of the view in (c) showing the right-handed helical motif of the ribbon like H-bonded core of the assembly, (e) Single strand for H-bonded units extracted from the triple-stranded heli-cate structure in 39 35 showing left-handed helicity. (f) Stereoview of the inclusion complex between benzene and adduct 39 35 [60],... 

It should be noted that the different structures of amylose and amylopectin confer distinctive properties to these polysaccharides (Table II). The linear nature of amylose is responsible for its ability to form complexes with fatty acids, low-molecular-weight alcohols, and iodine these complexes are called clathrates or helical inclusion compounds. This property is the basis for the separation of amylose from amylopectin when starch is solubilized with alkali or with dimethylsulfoxide, amylose can be precipitated by adding 1-butanol and amylopectin remains in solution. 

The discovery of the V-type, helical amylose (see p. 265) that forms when amylose interacts with 1-butanol was crucial for the development of the chemistry of starch inclusion complexes. It soon appeared that 1-butanol complexes solely with the amylose component. This selectivity became the first convenient method of fractionating starch. This method was first described by Schoch699 and later developed by Kerr et al.700-702 and oth-ers 680,703 An impr0ved procedure was subsequently patented.704 The amount of 1-butanol adsorbed in amylose is increased by the presence of moisture and is also dependent on two key factors the time of contact with that alcohol and the origin of the amylose, as shown in Table XXIX. 

When the racemic heterohelicenediol (PAf)-S4 forms an inclusion complex with EtOH through helical hydrogen bonding, the interplanar angle decreases to 37.96° <1995CC1873>. 

The anaesthetic steroid 3a-hydroxy-5a-pregnane-ll,20-dione has normal conformational features, both in the crystal and in solution. X-Ray data show that deoxycholic acid can form an inclusion complex in which alternate molecules of dimethyl sulphoxide and water are held in canals formed by helically arranged host molecules.Six different crystalline forms of 17a-ethynyloestradiol have been recognized. X-Ray structural data are reported for 3-methoxy-2-aza-oestra-l,3,5(10)-trien-17/3-yl acetate, 3/3-hydroxypregn-5-en-20-one (pregnenolone), 5a-cholest-2-ene, 3/3-bromo- and 3/3-chloro-cholest-5-enes, and cholesteryl acetate (at 123 K), benzoate, chloroformate, ° laurate, methyl carbonate, and 24-norcholesteryl acetate. ... 

The interaction of iodine and amylose involves formation of inclusion complexes in which iodine molecules are arranged, endwise and axially, inside a series of helices of a-(l 4)-linked n-glucose residues each helix... 

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