Beta-hydroxy acid

A beta-hydroxy acid is a carboxylic acid with a hydroxyl group on the beta carbon: the carbon two carbon–carbon bonds away from the carboxyl group carbon. For a simple chain numbered from the carboxyl carbon as carbon 1, the beta carbon is carbon 3. The hydroxyl within the carboxyl group itself does not establish the additional hydroxy substitution.

The small example 3-hydroxypropionic acid has structureStarting at the carboxyl carbon, the adjacent $\mathrm{C}{\mathrm{H}}_2$ is alpha and the terminal $\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$ is beta. The primary ChEBI record gives the systematic name 3-hydroxypropanoic acid, formula ${\mathrm{C}}_3{\mathrm{H}}_6{\mathrm{O}}_3$ and its classification as a 3-hydroxy monocarboxylic acid.^[ref]

An alpha-hydroxy acid instead places the additional hydroxyl group one carbon closer to the carboxyl group. Thus lactic acid, $\mathrm{C}{\mathrm{H}}_3\mathrm{CH}(\mathrm{OH})\mathrm{COOH}$, and 3-hydroxypropionic acid, $\mathrm{HOC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{COOH}$, have the same molecular formula but different atom connectivity. They are constitutional isomers; counting oxygen atoms or measuring molecular mass alone cannot distinguish them. The distinction is structural, not a statement about acidity or biological activity.^[ref]

The two functional groups also describe different possible transformations. Removing the carboxyl proton forms a carboxylate while preserving the beta-hydroxy carbon skeleton. Converting the hydroxyl group to a carbonyl changes the hydroxy-acid functionality. Which transformation occurs depends on reagents and conditions; the class name alone supplies no reaction rate or treatment claim. Tracking positions relative to the carboxyl group makes such structural changes easier to compare.

3-hydroxypropionic acid is a three-carbon beta-hydroxy acid with structureIts carboxyl group carbon is numbered 1; the additional hydroxyl group is on carbon 3. The primary ChEBI record identifies the systematic name 3-hydroxypropanoic acid, formula ${\mathrm{C}}_3{\mathrm{H}}_6{\mathrm{O}}_3$ and average molecular mass $90.078g/mol$. The acid and its conjugate base, 3-hydroxypropionate, are distinguished chemical species.^[ref]

Moving the hydroxyl group by one carbon gives the different connectivity of lactic acid, $\mathrm{C}{\mathrm{H}}_3\mathrm{CH}(\mathrm{OH})\mathrm{COOH}$. The two acids have the same molecular formula but are constitutional isomers. Molecular mass alone cannot decide which one a sample contains. The terminal alcohol and the carboxyl group also provide different sites for chemical transformation; the compound’s name specifies the initial structure rather than guaranteeing one reaction pathway.

An experimentally demonstrated motivation is production as a chemical building block by metabolic engineering. In the 2016 primary study by Chan Woo Song and colleagues, engineered Escherichia coli produced $31.1g/L$ of 3-hydroxypropionic acid in fed-batch culture. The study screened enzyme combinations and altered the strain, so the observation concerns a particular engineered production system rather than a universal property of the species.^[ref]

Using the record’s molecular mass, that mass concentration corresponds to approximatelyof acid equivalents. This conversion is an original calculation, not a separate measurement. Product concentration alone gives neither production rate without a time interval nor mass yield without substrate consumption. Chemical identity, accumulated amount and process performance remain different experimental questions.

3-hydroxypropionate is the conjugate base formed when the carboxyl group of 3-hydroxypropionic acid loses a proton. Its structure isThe primary ChEBI record gives formula ${\mathrm{C}}_3{\mathrm{H}}_5{\mathrm{O}}_3$, net charge $-1$ and the systematic name 3-hydroxypropanoate. The terminal hydroxyl group remains present; the acid-to-base change occurs at the carboxyl group, not by moving that hydroxyl or removing the carbon skeleton.^[ref]

The acid–base pair can be represented asIn a dilute-solution approximation with activity coefficients treated as equal, the Henderson-Hasselbalch equation givesAt a pH one unit above the relevant $\mathrm{p}{K}_a$, the ratio is $10:1$, so about $10/11$ of the acid-plus-conjugate-base pool is anionic in that model. This is an illustrative equilibrium calculation; no numerical $\mathrm{p}{K}_a$ for this compound is being supplied.

Speciation and total amount answer different analytical questions. If no other reactions occur, changing pH can redistribute $[\mathrm{HA}]$ and $[{\mathrm{A}}^{-}]$ while preserving their sum. A negatively charged 3-hydroxypropionate ion in a bulk solution is accompanied by compensating positive charge elsewhere; it is not the same molecular species as the neutral acid or a complete named salt.

The record’s average mass of $89.070g/mol$ applies to the anion’s composition. Reporting a concentration as acid equivalents instead uses the acid’s reference mass, while reporting a particular salt requires the salt’s complete composition. This distinction matters when comparing metabolic engineering measurements that use an acid name for a total analytical product pool.^[ref]