Starch conformation

Cyclodextrins are macrocyclic compounds comprised of D-glucose bonded through 1,4-a-linkages and produced enzymatically from starch. The greek letter which proceeds the name indicates the number of glucose units incorporated in the CD (eg, a = 6, /5 = 7, 7 = 8, etc). Cyclodextrins are toroidal shaped molecules with a relatively hydrophobic internal cavity (Fig. 6). The exterior is relatively hydrophilic because of the presence of the primary and secondary hydroxyls. The primary C-6 hydroxyls are free to rotate and can partially block the CD cavity from one end. The mouth of the opposite end of the CD cavity is encircled by the C-2 and C-3 secondary hydroxyls. The restricted conformational freedom and orientation of these secondary hydroxyls is thought to be responsible for the chiral recognition inherent in these molecules (77). 

Specific optical rotation values, [a], for starch pastes range from 180 to 220° (5), but for pure amylose and amylopectin fractions [a] is 200°. The stmcture of amylose has been estabUshed by use of x-ray diffraction and infrared spectroscopy (23). The latter analysis shows that the proposed stmcture (23) is consistent with the proposed ground-state conformation of the monomer D-glucopyranosyl units. Intramolecular bonding in amylose has also been investigated with nuclear magnetic resonance (nmr) spectroscopy (24). 

FIGURE 7.22 Suspensions of amylose in water adopt a helical conformation. Iodine (b) can insert into the middle of the amylose helix to give a bine color that is characteristic and diagnostic for starch. 

Staggered conformation, 94 molecular model of, 94 Stannous chloride, reaction with nitroarenes, 928 Starch, 1—>4- -links in, 1000 structure of, 1000 Stationary phase, chromatography and, 432... 

Approximate conformity of the composition of cellulose and starch with the formula CeHioOs was deduced very early, but often the literal exactness of this formula was obscured by failure to obtain the polymer in the anhydrous condition. In the first decade of the present century the CeHioOs formula was shown to represent the composition of... 

Chemically, both varieties of starch are polymers of glucose with the a-D-glucose units in the 4Ci conformation. The glucose units are linked -(1 ->4)- in both amylose and amylopectin but in amylopectin roughly one residue in twenty is linked -(1 ->6)-, which forms branch points. The proportion of branch points varies, depending on the source of the amylopectin. 

Repulsive forces between Fe oxide particles can be established by adsorption of suitable polymers such as proteins (Johnson and Matijevic, 1992), starches, non-ionic detergents and polyelectrolytes. Adsorption of such polymers stabilizes the particles at electrolyte concentrations otherwise high enough for coagulation to occur. Such stabilization is termed protective action or steric stabilization. It arises when particles approach each other closely enough for repulsive forces to develop. This repulsion has two sources. 1) The volume restriction effect where the ends of the polymer chains interpenetrate as the particles approach and lose some of their available conformations. This leads to a decrease in the free energy of the system which may be sufficient to produce a large repulsive force between particles. 2) The osmotic effect where the polymer chains from two particles overlap and produce a repulsion which prevents closer approach of the particles. 

Aberle, T., Burchard, W., Vorwerg, W., Radosta, S. (1994). Conformational contributions of amylose and amylopectin to the structural properties of starches from various sources. Starch/Stdrke, 46, 329-335. 

Gernat, C., Radosta, S., Anger, H., Damaschun, G. (1993). Crystalline parts of three different conformations detected in native and enzymatically degraded starches. Starch, 45, 309-314. 

The life and work of Dexter French, who contributed so much to our knowledge of starch, is sensitively treated by his student J. H. Pazur (University Park, Pennsylvania). The pioneering discovery by French and Rundle, in the carbohydrate field, of a helical biopolymer in complexes of amylose predates the widely celebrated work with proteins and nucleic acids where the concept of a helical conformation revolutionized... 

In both starch and glycogen the glucose emits of the main chains are linked with a-1,4 linkages. An extended conformation is not possible and the chains tend to undergo helical coiling. One of the first helical structures of a biopolymer to be discovered (in 1943)76 77 was the left-handed helix of amylose wound around molecules of pentaiodide (I5 ) in the well-known blue starch-iodine complex78 (Fig. 4-8). Tire helix contains six residues per turn, with a pitch of 0.8 nm and a diameter of nearly 14 nm. Amylose forms complexes of similar structure with many other small molecules.79... 

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