Polysaccharides of grasses
Methylation and periodate studies have indicated that the hemicellulosic polysaccharides from the mesophyll cell-wall from Italian ryegrass leaf include rhamnogalacturonan, xylan and glucan. The rhamnogalacturonan readily undergoes p-eliminative degradations during extraction with alkali, methylation or any exposure to base and consequently the detection of low proportions of it does not indicate correspondingly low proportions in the parent material. The glucan is heterolinked and is indicated to be branched by the detection of 2,3-di-0 methyl-D-glucose in methylation analyses. The xylan is rich in uronic acid and arabinose and highly branched. There are doubly substituted xylose residues in which one substituent is L-arabinofuranose and the other glucuronic acid or its 4-methyl ether. Structural features similar to those in Italian ryegrass leaf mesophyll cell-wall were present in mesophyll cell-wall preparations from leaves of sea-lyme grass, bamboo, and perennial ryegrass and from perennial ryegrass stem. The mesophyll cell-walls of grasses are concluded to have similar chemical structural features. Fructosans were present in the cell-contents of the mesophyll cells. The epidermal cells from sea lyme grass have walls composed of cellulose, xylan, rhamnogalacturonan and heterolinked glucan. They are, relative to mesophyll cell-walls, poor in- the last two arid rich in xylan. The xylan is like many xylans commonly studied and unlike xylans from primary cell-walls, substituted on few main-chain residues by arabinose or glucuronic acid (or its 4-methyl ether) and contains no non-reducing terminal galactose residues. Chemically similar material can be isolated from sea lyme grass leaf by dissection. In the extraction and examination of polysaccharides, which are rich in uronic acid, B-eliminative degradations can occur. This can lead to extensive depolymerisation when the uronic acid residues are in the main-chain and are 1-4 linked. This is the case with the rhamnogalacturonan of the mesophyll cell-wall. If the uronic acid residues are present in side-chains on the main-chain, as in mesophyll xylan, then 3-eliminative degradations may cause cleavage and loss of those side-chains. The extent of degradation occurring during methy- lation is not, so far, controllable, "but it can be estimated by U.V. examination or, when there is knowledge of the spectrum of possible results, by methylation analysis. In the analysis by G.L.G. of neutral sugar composition of hernicelluloses certain components are eluted after the pentoses but before any of the common hexoses. They are concluded to be 4-deoxy-hexoses and to derive from uronic acid residues that have undergone elimination and subsequent reduction.