Xylan optical rotation

Xylan has the general properties of insolubility in water, solubility in alkaline solutions, ease of acid hydrolysis, high negative optical rotation, and non-reducing action toward Fehling s solution. It can be placed in three general polysaccharide classes (1) pentosan, (2) glycan, and (3) hemicellulose. It is classed as a pentosan because it is principally a polymer of a pentose. It is by far the most abundant pentosan. 

Optical Rotations of Xylan in Sodium Hydroxide Sol utions... 

On treatment with periodatd ion, straw and beechwood xylan oxidize rapidly.104 As with other polysaccharides, the oxidation comes to a more definite end point in solutions which are buffered to pH 4-5. While somewhat more than the theoretical amount of periodate ion is consumed, the reaction apparently proceeds uniformly with oxidative cleavage of the 2,3 carbon bond to produce the structure indicated in Figure 4. In the course of the reaction the xylan passes into solution. The optical rotation of the oxidized product is surprisingly high (co. 100°). 

Model building in the computer is used to compare the influence of remote contacts on the stereochemistry and properties of P-1,2-, p-1,3-, and P-1,4-glucans, including their optical rotation behavior, and to explore the possibility of chain folding in cellulose, chltin, and xylan derivatives. 

Methods used to obtain conformational information and establish secondary, tertiary, and quaternary structures involve electron microscopy, x-ray diffraction, refractive index, nuclear magnetic resonance, infrared radiation, optical rotation, and anisotropy, as well as a variety of rheological procedures and molecular weight measurements. Extrapolation of solid state conformations to likely solution conformations has also helped. The general principles of macromolecules in solution has been reviewed by Morawetz (17), and investigative methods are discussed by Bovey (18). Several workers have recently reexamined the conformations of the backbone chain of xylans (19, 20, 21). Evidence seems to favor a left-handed chain chirality with the chains entwined perhaps in a two fold screw axis. Solution conformations are more disordered than those in crystallites (22). However, even with the disorder-... 

Cori ester, III, 258, 264 IV, 31 Com cobs, optical rotation of xylan from, V, 282... 

Xylan from wood has received somewhat less attention from carbohydrate chemists than has the xylan from such rich sources as wheat straw, corn cobs, and esparto grass. However, specific optical rotations of the xylans obtained from the aforementioned sources are of the same general order of magnitude as those of some xylans obtained from wood. Yundt crystallized xylans from paper birch and from barley straw as sphero-crystals, and compared their properties (see Table VII). 

The interaction of agarose with two heavily substituted P1,4-1 inked D -xylans was studied. Using the same gelation and optical rotation criteria, the heavily substituted xylans... 

Since the days of Pasteur and Freudenberg, the value of optical rotation [a]o, as a means of characterizing polysaccharides has been appreciated. Timell isolated and purified glucuronoxylans and arabinoxylans to provide a series of samples whose optical rotation could be recorded in 10% aqueous sodium hydroxide. Figure 1 is a schematic of the molecular structure of the 4-0-methyl-glucuronoxylans which he isolated with a variable content of uronic acid. Figure 1 also shows the Equation which we used to account for the observed molar rotation of xylans (3). 

These linear plots are essentially physical proofs of structure of the respective polysaccharides and can be used for analytical purposes. Someone once referred to the Equation in Figure 1 as the Marchessault equation which I accept humbly but in fact the derivation follows directly from the teachings of C.S. Hudson and K. Freudenberg, used for proof of structure of cellulose. However, the optical rotation of the neutral xylan chain, derived from these data, is much greater than that of cellulose. 

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