User:HaidongLiu/sandbox

We are a team from Rensselaer polytechnic Institute for a biology materials course. We are assigned a term project to improve a certain Wikipedia article related to biological materials. We choose to work on the page of hemicellulose page. The current page has enough sections, but each section does not have enough content. In the following weeks, We will edit the current page extensively, including changing the page layout and addition of more context. We hope our work could help the community to have better understanding the science and application of hemicellulose. We are open to any discussions.

Types and Structures of Hemicellulose

Primarily based on the structural difference, like backbone linkages and side groups, as well as other factors, like abundance and distributions in plants, hemicellulose could be characterized into four groups as following: 1) Xylans, 2) Mannans; 3) Mixed linkage β-glucans; 4) Xyloglucans

• Xylans
  • Xylans usually consist of backbone of β-(1→4)-linked xylose residues. And it could be further divided into homoxylands and heteroxylans. Homoxylans has a backbone of D-xylopyranose residues linked by β(1→3) or mixed ,β(1→3, 1→4)-glycosidic linkages. Homoxylans mainly carry structural functions. Heteroxylans such as glucuronoxylans, glucuronoarabinoxylans, and complex heteroxylans, have a backbone of D-xylopyranose and short carbohydrate branches. For examples, glucuronoxylan has substitution with α-(1→2)-linked glucuronosyl and 4-O-methyl glucuronosyl residues. And arabinoxylans and glucuronoarabinoxylans contain arabinose residues attached to the backbone.

    

    Xylan in hardwood.

• Mannans
  • The mannan-type hemicellulose can be classified into two types based on their main chain difference, galactomannans and glucomannans. Galactomannans have only β-(1→4) linked D-mannopyranose residues in linear chains. Glucomannans consist of both β-(1→4) linked D-mannopyranose and β-(1→4) linked D-glucopyranose residues in the main chains. As for the side chains, D-galactopyranose residues tend to be 6-linked to both types as the single side chains with various amount.

• Mixed linkage β-glucans
  • The conformation of the mixed linkage glucan chains usually contains blocks of β-(1→4) D-Glucopyranose separated by single β-(1→3) D-Glucopyranose. The population of β-(1→4) and β-(1→3) are about 70% and 30%. These glucans primarily consist of cellotriosly (C₁₈H₃₂O₁₆) and cellotraosyl (C₂₄H₄₂O₂₁)segments in random order. There are some study show the molar ratio of cellotriosly/cellotraosyl for oat (2.1-2.4), barley (2.8-3.3), and wheat (4.2-4.5).

    

    Beta-D-glucopyranose with carbon positions.

• Xyloglucans
  • Xyloglucans have a backbone similar to cellulose with α-D-Xylopyranose residues at position 6. To better describe different side chains, a single letter code notation is used for each side chain type. G -- unbranched Glc residue; X -- α-d-Xyl-(1→6)-Glc. L -- β-Gal , S -- α-l-Araf, F-- α-l-Fuc. These are the most common side chains.
  • The two most common types of xyloglucans in plant cell walls are identified as XXXG and XXGG.