The extraction and characterization of soil polysaccharide by whole soil methylation

Abstract

SummaryA soil of the Countesswells series was repeatedly methylated by the Hakomori procedure and a chloroform‐soluble product isolated after each methylation. Ninety‐two per cent of the material engendered by seven methylations was released during the first four methylations. This had a methoxyl content of about 20% and contained 2% N. Later fractions had lower methoxyl and N contents.Residual carbohydrate in the soil had reducing sugar content on hydrolysis, equivalent to about 3% of the original value.The extracted material had the infrared spectrum of a methylated soil polysaccharide and, on hydrolysis by 2 m trifluoracetic acid, released methylated sugar derivatives of which 68 were characterised by GC‐MS.Derivatives corresponding to (1→4) linked sugars predominated for both hexose and pentose sugars but there was also a large proportion of (1→3) linkages, (1→3) linkages predominated for the deoxyhexose sugars. There were more sugars with only one or two methylated hydroxyl groups than could be accounted for as branching points because of the relatively small numbers of end groups.Prior reduction of the soil with sodium borohydride had no measurable effect on the nature or yield of the methylated product. This indicates that amino acid sugar linkages susceptible to β‐elimination reactions can have only a very small influence on the reaction.The isolated sugar derivatives accounted for 70% of the total soil sugars. The methylated material before hydrolysis had a low nominal molecular weight on diafiltration, with 68% < 10000. Some of the sugars unaccounted for were probably lost during the dialysis stage necessary to remove dimethyl sulphoxide.