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Cellulose1, hemicellulose2 (also called polyose), and lignin3 are the three types of molecules that form the lignocellulosic fibers of plants. Cellulose, a polysaccharide that consists of d-glucose units, was the Molecule of the Week for January 19, 2009. Lignin, with its aromatic polyether structure, was the MOTW for June 25, 2012.
The third type, hemicellulose, is a less ordered, more labile form of cellulose. It also is a polysaccharide, but its various forms can contain six-carbon sugars (hexoses), five-carbon sugars (pentoses), or both. After cellulose and lignin, hemicellulose is the third most abundant biopolymer on Earth. The most common hemicelluloses are glucuronoxylan, arabinoxylan, glucomannan, xyloglucan, and xylan, described here.
Xylan, as its more descriptive name poly(β-d-xylopyranose[1→4]) suggests, is composed primarily of five-carbon xylose units. It is commonly found in the cell walls of grasses and dicotyledon flowering plants. It is the major hemicellulose in cereals and hardwood, which also contain arabinoxylans and glucuronoxylans, respectively. The image shows the structure of the repeating unit of xylan in hardwood.
Xylan and other hemicelluloses derived from agricultural waste are renewable substances used to manufacture chemicals, packaging materials, and biomedical products. The hemicellulose xylan should not be confused with the trade name Xylan, a fluoropolymer coating used in nonstick cookware.
1. CAS Reg. No. 9004-34-6.
2. CAS Reg. No. 9034-32-6.
3. CAS Reg. No. 9005-53-2.
Xylan hazard information
|Hazard class*||GHS code and hazard statement|
|Not a hazardous substance or mixture|
*Globally Harmonized System (GHS) of Classification and Labeling of Chemicals.
Sodium chlorite1 (NaClO2) is a salt with chlorine in the 3+ oxidation state that was known to chemists at least as far back as 1915. It has historically been used to bleach textiles and paper pulp and to sterilize water in treatment plants. But in the mid-2000s, Michael S. McGrath at the University of California, San Francisco, discovered that chlorite could be a valuable prodrug for treating neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS, Lou Gehrig’s disease).
McGrath is now the chief scientific officer at the pharmaceutical firm Neuvivo (Palo Alto, CA), which owns the rights to his invention. This year, Neuvivo contracted with Thermo Fisher Scientific (Waltham, MA) to manufacture large supplies of NaClO2 at its Florence, SC, facility in anticipation of the US Food and Drug Administration’s approval of the drug for treating ALS.
Cesium triiodide2 (CsI3) was first described in 1903 by H. W. Foote at Yale University (New Haven, CT), who made it from cesium iodide and molecular iodine. Almost 30 years later, Foote*, W. M. Bradley, and Michael Fleischer more thoroughly investigated its properties. CsI3 crystals have a 2-D, layered, orthorhombic structure under standard conditions; but last month Tomasz Poręba, Stefano Racioppi, and coauthors at the European Synchrotron Radiation Facility (Grenoble, France) and the State University of New York at Buffalo reported that applying pressure of >1.24 GPa to orthorhombic CsI3 reorganizes the crystals into a trigonal system with 3-D architecture.
1. CAS Reg. No. 7758-19-2.
2. CAS Reg. No. 12297-72-2.
Molecules from the journals
MOTW briefly describes noteworthy molecules that appeared in recent ACS journal articles. See this week's
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Xylan fast facts
|CAS Reg. No.||9014-63-5|
|Molar mass||20,000–30,000 g/mol
150.13 g/mol monomer unit
|Appearance||White to off-white powdera|
|Melting point||200–>300 °Cb|
|Water solubility||Very slightb|
a. Natural substance may be beige to brown because of impurities.
b. Depends on degree of polymerization.
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