每周分子：木聚糖

Molecule of the Week: Xylan

纤维素、半纤维素和木质素是形成植物木质纤维素纤维的三种分子。纤维素是一种由D-葡萄糖单元组成的大分子多糖，为2009年1月19日的每周分子。具有芳香族聚醚结构的木质素为2012年6月25日的每周分子。

半纤维素的结构不像纤维素那么有序，且不如纤维素稳定。半纤维素也是一种多糖，它的各种形式可以包含六碳糖(己糖)、五碳糖(戊糖)或两者兼而有之。继纤维素和木质素之后，半纤维素是地球上含量第三丰富的生物聚合物。最常见的半纤维素包括葡糖醛酸木聚糖、阿拉伯木聚糖、葡甘聚糖、木葡聚糖和木聚糖，如下文所述。

木聚糖，如其更具描述性的名称poly (β-d-xylopyranose[1→4])所示，主要由五碳木糖单元组成，常见于草类植物和双子叶开花植物的细胞壁中。它是谷类和硬木中主要的半纤维素类型，它们还分别含有阿拉伯木聚糖和葡糖醛酸木聚糖。本文图片展示了硬木中木聚糖的重复单元的结构。

来自农业废弃物的木聚糖和其他半纤维素属于可再生物质，可用于制造化学品、包装材料和生物医学产品。不要将半纤维素木聚糖与商品名Xylan混淆，商品Xylan是一种用于不粘锅的含氟聚合物涂层。

木聚糖信息速览

a. 由于有杂质，天然物质可能是米色至棕色。

b. 取决于聚合的程度。

【关于每周分子】

Cellulose, hemicellulose (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.

Xylanfast facts

a. Natural substance may be beige to brown because of impurities.

b. Depends on degree of polymerization.

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