Mannuronate

Mannuronate refers to the uronic acid monomer associated with the polysaccharide alginate. The monomer is β-D-mannuronic acid, and in alginate polymers it is commonly denoted as the “M” residue. Mannuronate is the C-5 epimer of guluronate (the “G” residue) and together these two monomers compose the repeating units of alginate.

Alginate is produced by brown algae and certain bacteria, and its composition varies in the relative amounts and arrangement of mannuronate and guluronate. In alginate polymers, sequences occur as blocks of M residues (M-blocks), blocks of G residues (G-blocks), and mixed MG regions. The ratio and pattern of M and G units influence the physical properties of the polymer, including its viscosity and gel-forming behavior.

In solution and in gels, mannuronate contributes a negatively charged uronate unit that participates in ionic crosslinking with divalent cations such as calcium. Gel strength and elasticity are strongly affected by the M/G composition: G-block-rich regions tend to form stronger, more rigid gels through the egg-box crosslinking mechanism, whereas higher M content generally yields more flexible, softer gels and higher solution viscosity.

In practice, alginates with varying M/G ratios are used across industries. They are valued for their biocompatibility, gelling, thickening, and stabilizing properties, finding applications in food, pharmaceutical, and biomedical fields, including wound dressings and cell encapsulation. The term mannuronate can also refer to the uronate (carboxylated) form of the mannuronic acid moiety in salt forms.