Iduronate

Iduronate is the conjugate base of iduronic acid, formed when its carboxyl group loses a proton. Starting from neutral ${\mathrm{C}}_6{\mathrm{H}}_{10}{\mathrm{O}}_7$, proton loss gives the free-monomer molecular formula ${\mathrm{C}}_6{\mathrm{H}}_9{\mathrm{O}}_7^{-}$. The usual biological discussion concerns the L-iduronate series.^[ref]

Ionization and epimerization are different operations. Removing the acidic proton changes electric charge without changing the carbon stereochemistry. Conversely, heparosan-N-sulfate-glucuronate 5-epimerase converts D-glucuronate residues into L-iduronate residues by changing carbon-5 configuration within a polymer.^[ref]

Charge accounting makes the distinction quantitative. A solution containing $1\mathrm{mmol}$ of singly charged iduronate anions requires one millimole of monovalent positive counterions for electrical neutrality, if no other charged species contribute. That statement concerns charge balance, not a specification of which salt was prepared. In a polysaccharide, the iduronate name instead refers to a linked residue; its atom inventory cannot be substituted unchanged for a free monomer's formula.