Analysis of Uronic Acids

Uronic acids are sugars where the CH2OH group has been oxidised to form a carboxylic acid group. Uronic acids can be formed from hexoses, in which case they are referred to as hexuronic acids, and pentoses, where they are referred to as penturonic acids. The name of each uronic acid tends to be derived from the sugar it came from. We are able to determine the contents of five different uronic acids: glucuronic acid (derived from glucose), 4-O-Methyl-D-Glucuronic Acid (also from glucose), galacturonic acid (from galactose), mannuronic acid (from mannose), and guluronic acid (from gulose).

Uronic acids play important functions in nature and exist in varying amounts and proportions in different biomass types.

Uronic acids can be present as side chains attached to the main backbone of hemicelluloses. For example, the arabinoxylans of softwoods have partial substitution of the xylan main chain by 4-O-Methyl-D-Glucuronic Acid, with the ratio of xylose to uronic acids varying between 4:1 and 9:1. Other softwood hemicelluloses can also contain glucuronic acid as well as galactuornic acid.

In hardwoods, the xylan backbone of the glucuronoxylans can be substituted with 4-O-Methyl-D-Glucuronic Acid, with the xylose-to-uronic acid ratio varying from 3:1 to 20:1, with the most common ratio being from 7:1 to 12:1.

Uronic acids are also present in hemicelluloses in grasses and are typically more abundant in the early-stages of growth. Furthermore, the concentrations of uronic acids tends to be greatest in the nodes, lower in the internodes, and at intermediate levels in the leaves.

The uronic acids that are present in hemicelluloses also play an important role in the structure of the lignocellulosic matrix. For example, ester linkages between 4-O-Methyl-D-Glucuronic Acid and the lignin polymer have been documented.

Pectins are structural polysaccharides that are mostly contained within the primary cell walls of land plants. They are polyuronides that primarily consist of galacturonic acid units and are considered to be the most highly branched polysaccharides. Rhamnose (6- deoxymannose) is also a significant constituent. While pectins are acidic polysaccharides, they do not contain glucuronic acid and 4-O-Methyl-D-Glucuronic Acid, which are hemicellulose-only uronic acids.

Three of the most important types of pectic polysaccharides are rhamnogalacturonan I, rhamnogalacturonan II and homogalacturonan. The backbone of the pectin structure tends to consist of partially methylated galacturonic acid residues. However there are substantial 'hairy' areas of alternating rhamnose-galacuturonic-acid disaccharide sections as illustrated below.

While uronic acids are typically minor components of lignocellulosic biomass, they can form a signficiant proportion of the total mass of some seaweeds. Furthermore, whilst glucuronic acid, 4-O-Methyl-D-Glucuronic Acid, and galacturonic acid are by far the most abundant uronic acids in the hemicelluloses of terrestrial plants, in seaweed guluronic acid and mannuronic acid are the uronic acids of most importance. These are found within the alginic acid (or alignate) polysaccharides of seaweeds.

These polymers contain both guluronic acid and mannuronic acid in linear chains with the relative proportions of mannuronic acid to guluronic acid varying from ratios of 1.2 to 2.1 or greater. Within this linear chain these hexuronic acids tend to be arranged in C5 epimer blocks of one or the other, although less crystalline blocks involving both sugars can also be present.

The abundance and composition of uronic acids within seaweeds have been observed to vary greatly according to the season.

Using our ion chromatography system and chromatography methods developed in house, we are able to resolve between and quantify the contents of five different uronic acids: glucuronic acid, galacturonic acid, mannuronic acid, guluronic acid, and 4-O-Methyl-D-Glucuronic Acid.

We can determine the uronic acid composition of lignocellulosic biomass, seaweed, and liquids. This analysis is included as standard in most of our seaweed analysis packages and can be requested for the analysis of liquids using analysis package "P15 - Uronic Acids".

If you would like to determine the uronic acid composition of non-seaweed biomass samples it will be necessary to first obtain a hydrolysate suitable for analysis in package "P15 - Uronic Acids". Examples of analysis packages that will produce such a hydrolystae from biomass include "P7 - Lignocellulosic Sugars", "P9 - Lignocellulosic Sugars and Lignin", and "P10 - Sugars, Lignin, Extractives, Ash".

If you are interested in us determining the concentrations of different uronic acids than the five we currently analyse then please get in touch and we will try to develop a custom analytical method for you.