Glucuronate

Glucuronate is the conjugate base of glucuronic acid, formed by losing the proton of its carboxyl group. In the usual D series, the free monomer has molecular formula ${\mathrm{C}}_6{\mathrm{H}}_9{\mathrm{O}}_7^{-}$, compared with ${\mathrm{C}}_6{\mathrm{H}}_{10}{\mathrm{O}}_7$ for the neutral acid. A six-membered-ring example is D-glucopyranuronate, whose official chemical record includes both alpha and beta anomers.^[ref]

Protonation and anomeric configuration answer different questions. Removing a carboxyl proton changes charge; selecting alpha rather than beta specifies the arrangement at the anomeric center. Neither operation is the carbon-5 epimerization that distinguishes the glucuronate and iduronate series.

The Henderson-Hasselbalch equation quantifies the acid–base distinction. In an ideal dilute solution, $[{\mathrm{A}}^{-}]/[\mathrm{HA}]=10^{\mathrm{pH}-\mathrm{p}{K}_a}$. At a pH two units above the relevant $\mathrm{p}{K}_a$, the ratio is $100:1$, giving an anionic fraction $100/101\approx 99\%$. This example uses an offset from the dissociation constant rather than assigning an unverified numerical value to it.

Biological names often refer to glucuronate residues inside a polysaccharide. Such a linked residue must be distinguished from a free monomer when counting atoms or preparing a reaction balance.