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1-aminocyclopropane-1-carboxylic acid is a non-proteinogenic alpha-amino acid with an amino group and a carboxyl group attached to the same carbon of a cyclopropane ring. Its molecular formula is ${\mathrm{C}}_4{\mathrm{H}}_7\mathrm{N}{\mathrm{O}}_2$, and it is commonly abbreviated ACC. The named acid and its conjugate base, 1-aminocyclopropane-1-carboxylate, distinguish protonation states of the same carbon framework.^[ref]

Its role in plant ethene biosynthesis was established through precursor experiments. In 1979, labelled methionine supplied to apple tissue produced ethylene in air but accumulated as ACC under nitrogen. Labelled ACC was itself converted into ethylene in air; further precursor and inhibitor comparisons supported its placement between S-adenosylmethionine and ethylene.^[ref]

The informative observation was not merely that ACC was present. Accumulation under one condition, conversion under another and differential effects on upstream and downstream precursors jointly constrained the pathway. A metabolite's concentration alone cannot establish which reaction produces or consumes it, because both processes affect its measured pool.

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]

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.

An alpha-hydroxy acid is a carboxylic acid with a hydroxyl group on the carbon immediately adjacent to the carboxyl group. That neighboring carbon is the alpha carbon; in simple systematic numbering it is carbon 2 because the carboxyl carbon is carbon 1. The hydroxyl oxygen within the carboxyl group itself is not the additional hydroxyl that defines this class.^[ref]

For a common saturated monocarboxylic example, the local structure can be writtenwith $R,R^{\prime }$ representing hydrogen or carbon-containing substituents. Glycolic acid, $\mathrm{HOC}{\mathrm{H}}_2\mathrm{COOH}$, has two hydrogens at that carbon. Lactic acid, $\mathrm{C}{\mathrm{H}}_3\mathrm{CH}(\mathrm{OH})\mathrm{COOH}$, has hydrogen and methyl groups there and is also called 2-hydroxypropanoic acid.^[ref][ref]

Position distinguishes the class. The structure $\mathrm{HOC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{COOH}$ has its additional hydroxyl group at carbon 3 and is a beta-hydroxy acid, even though it has the same molecular formula as lactic acid. A molecular formula therefore does not determine whether a compound is an alpha-hydroxy acid; the connectivity is essential.

The alpha carbon can also be a stereogenic centre. In lactic acid it is bonded to four different groups: hydrogen, hydroxyl, methyl and carboxyl. Its two enantiomers share the alpha-hydroxy classification but differ in three-dimensional arrangement. Glycolic acid does not have that stereogenic centre because two substituents are identical hydrogens.

Both functional groups participate in the chemistry. Loss of the carboxyl proton gives a carboxylate, such as lactate from lactic acid, while the additional hydroxyl remains part of the structure. Distinguishing group identity, position, protonation and stereochemistry makes the class useful in organic and biological chemistry without reducing it to a list of commercial uses.

Arachidonic acid is a twenty-carbon polyunsaturated fatty acid with four cis carbon–carbon double bonds, at positions 5, 8, 11 and 14 when counted from the carboxyl group. Its molecular formula is ${\mathrm{C}}_{20}{\mathrm{H}}_{32}{\mathrm{O}}_2$. The shorthand $20:4n-6$ records twenty carbons, four double bonds and the first double bond six carbons from the methyl end.^[ref]

The indexing conventions describe the same structure from opposite ends: the bond between carbons 14 and 15 from the acid end is the first encountered from the methyl end. The cis geometry also distinguishes this molecule from stereoisomers with the same formula. Composition, double-bond positions and geometry all belong to its chemical identity.

Aspartic acid is an alpha-amino acid with two carboxyl groups, represented in a neutral structural form as $\mathrm{HOOC}-\mathrm{CH}(\mathrm{N}{\mathrm{H}}_2)-\mathrm{C}{\mathrm{H}}_2-\mathrm{COOH}$. Its molecular formula is ${\mathrm{C}}_4{\mathrm{H}}_7\mathrm{N}{\mathrm{O}}_4$. One carboxyl group belongs to the amino-acid backbone and the other terminates the side chain.^[ref]

The protein-associated L configuration and the sequence symbols Asp and D are separate naming conventions. A one-letter D in a protein sequence denotes the aspartic-acid residue; it is not a declaration of D stereochemistry.^[ref]

Loss or redistribution of protons changes the ionic form. The name aspartate commonly denotes the anionic form. For example, a form with two carboxylates carrying $-1$ each and an ammonium group carrying $+1$ has net charge $-1-1+1=-1$. This charge count must be kept distinct from the L/D configuration and from atom connectivity.

The side-chain carboxyl chemistry also matters in aspartic proteases, where appropriately positioned acidic groups participate in peptide bond hydrolysis. Presence of an aspartic-acid residue alone does not establish catalytic activity: its geometry and chemical environment matter.^[ref]

An aspartic protease is a protease whose catalytic chemistry depends on aspartic acid side chains. In common examples, their carboxyl groups position and activate a water molecule for peptide bond hydrolysis. Water supplies the attacking nucleophile, unlike a mechanism in which an enzyme side chain itself makes the initial covalent attack.^[ref]

The name concerns catalytic groups, rather than requiring an aspartic-acid residue at every substrate cleavage site. Substrate recognition and catalytic mechanism are distinct properties.

HIV-1 protease provides an experimentally characterized example. In 1989, T. D. Meek and colleagues measured apparent masses of approximately 11,000 under denaturing conditions and 22,000 under nondenaturing conditions; protein cross-linking supplied additional evidence for an active protein dimer. The ratio $22,000/11,000=2$ explains the subunit interpretation.^[ref]

The same study found pH-dependent inactivation consistent with a required unprotonated carboxyl group in the active site. Combining this chemical perturbation with cleavage activity and subunit measurements supported a catalytic classification more specifically than the enzyme's molecular mass alone.^[ref]

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.

A carboxyl group is the functional group $-\mathrm{C}(=\mathrm{O})\mathrm{OH}$: one carbon has a double bond to oxygen and a single bond to a hydroxyl group. It gives a carboxylic acid its characteristic acid functionality. In acetic acid, the group is attached to a methyl group, giving $\mathrm{C}{\mathrm{H}}_3\mathrm{COOH}$.^[ref]

Loss of the acidic hydrogen produces a carboxylate, the conjugate base:The pH influences the balance between these forms. A structural drawing of the neutral acid therefore need not show the predominant charge state in every solution. The carboxyl group is a part of the molecule, while acetic acid names the entire compound.

Citrulline is an amino acid whose side chain has the structure $-(\mathrm{C}{\mathrm{H}}_2{)}_3-\mathrm{NH}-\mathrm{C}(=\mathrm{O})-\mathrm{N}{\mathrm{H}}_2$. Its free, uncharged molecular formula is ${\mathrm{C}}_6{\mathrm{H}}_{13}{\mathrm{N}}_3{\mathrm{O}}_3$; the biological L form has a specified arrangement at its chiral carbon atom. Like other amino acids, its amino group and carboxyl group can change protonation state, so a structural formula and the dominant ionic form in a solution need not look identical.^[ref]

An illuminating experimental role is as a reaction product of nitric oxide synthase. A 1990 purification from rat cerebellum associated conversion of arginine to nitric oxide with stoichiometric production of citrulline. The reaction required NADPH and calcium ions, and the purified enzyme required calmodulin. Measuring newly formed citrulline therefore provided a chemically distinct readout of the same enzyme reaction that generated a gaseous signalling molecule.^[ref]

For an idealized assay in which each turnover gives one molecule of citrulline and no competing reaction consumes it, formation of $10\mathrm{nmol}$ of citrulline would correspond to $10\mathrm{nmol}$ of nitric oxide generated. That inference requires the specified reaction and appropriate controls; an existing pool of citrulline in a cell is not by itself a measure of current nitric oxide synthase activity.

A dicarboxylic acid is a carboxylic acid containing two carboxyl groups. The groups may be attached to an aliphatic framework, as in malonic acid, or an aromatic framework, as in terephthalic acid.^[ref] The definition counts functional groups rather than specifying chain length, ring substitution or acid strength.

A dicarboxylic acid can supply two acidic protons through its carboxyl groups. In the simplest diprotic acid model, write the successive equilibriaThe two acid dissociation constants $K_1$ and $K_2$ characterize different equilibria. Two chemically similar groups do not justify treating the two proton-removal steps as identical. Other ionizable groups, if present, require additional equilibria.

For an original ideal dilute-solution example, use concentration-form constants $K_1=10^{-3}\mathrm{mol}{\mathrm{L}}^{-1}$, $K_2=10^{-5}\mathrm{mol}{\mathrm{L}}^{-1}$ and hydrogen-ion concentration $H=10^{-4}\mathrm{mol}{\mathrm{L}}^{-1}$. Mass balance and the two equilibrium relations give species fractionsHere $({\alpha }_0,{\alpha }_1,{\alpha }_2)=(1,10,1)/12$. The average charge is $-1$ per acid-derived species even though only $5/6$ of the molecules are monoanions: equal neutral and dianion populations balance around that mean. The constants are stipulated for illustration, not assigned to a particular compound. Precise thermodynamic calculations use activities rather than equating them with concentrations.

A titration that fully removes both carboxyl protons consumes two moles of hydroxide per mole of initially neutral dicarboxylic acid, in the absence of other reactions. For example, 0.50 mmol requires 1.00 mmol hydroxide by stoichiometry. Two stoichiometric equivalents do not guarantee two separately resolved features on a titration curve; the acid dissociation constants and measurement conditions determine their separation.

Two carboxyl groups also provide two possible connections in esterification. Terephthalic acid and a diol can therefore build a polyester, illustrating how the same functional-group count matters both to acid-base measurements and to polymer connectivity.^[ref]

A gamma-carboxyglutamate residue, or Gla residue, is a glutamate residue within a protein whose side chain carries an additional carboxyl group at its gamma carbon. It is the protein-bound form associated with gamma-carboxyglutamic acid. The amino acid residue remains part of the polypeptide backbone; the modification changes its side chain.

The distinction from the free amino acid matters experimentally. Identifying gamma-carboxyglutamic acid after breaking a protein into smaller components reveals chemical composition, while determining which residues are modified and how they are arranged requires structural or sequence evidence.^[ref]

In a Gla domain, multiple Gla residues help coordinate calcium ions. A 1992 X-ray crystallography study of a prothrombin fragment resolved seven bound calcium ions in its calcium-bound structure and found substantial disorder in the corresponding calcium-free region. This illustrates how a local chemical modification can contribute to an ion-dependent folded arrangement.^[ref]

Gamma-carboxyglutamic acid is a derivative of glutamic acid with an additional carboxyl group attached at the gamma carbon of the side chain. Its side chain can be written $-\mathrm{C}{\mathrm{H}}_2-\mathrm{CH}(\mathrm{COOH}{)}_2$ in the fully protonated representation. The extra carboxyl group changes the ion-binding properties of the corresponding gamma-carboxyglutamate residue within a protein.

In 1974, J. Stenflo and colleagues identified this unusual amino acid in prothrombin. Anomalous electrophoresis behaviour led to analysis by nuclear magnetic resonance and mass spectrometry, connecting a chemical modification with the special properties of this clotting protein.^[ref]

An illuminating comparison is to count the side-chain acid groups: glutamic acid has one, while gamma-carboxyglutamic acid has two. Their ionized forms provide additional negatively charged oxygen donors for calcium ions. The arrangement of several modified residues in a folded Gla domain gives the protein its calcium-binding architecture; an isolated modified amino acid is only one component of that structure.^[ref]

A Gla domain is a protein domain containing several gamma-carboxyglutamate residues. These are glutamate residues changed by a Vitamin K-dependent post-translational modification: an additional carboxyl group gives the modified residue extra capacity to bind calcium ions. In proteins such as prothrombin, the resulting calcium ion interactions help organize the domain's protein folding and its association with a phospholipid membrane. The residue chemistry, the folded domain and its membrane interaction are related but distinct levels of explanation.

The unusual residue was identified experimentally in 1974. J. Stenflo and colleagues found that a prothrombin fragment's electrophoretic mobility could not be explained by its ordinary amino acid composition. Nuclear magnetic resonance and mass spectrometry identified gamma-carboxyglutamic acid. The extra chemical group helped explain the fragment's calcium ion-binding behaviour.^[ref]

Later, M. Soriano-Garcia and colleagues used X-ray crystallography to resolve a calcium-bound prothrombin fragment at $2.2\text{A˚}$ resolution. Much of the Gla domain was disordered in the calcium-free crystal, whereas the calcium-bound structure was well defined and contained seven bound calcium ions. Their coordination by carboxylate groups and water connected residue chemistry to a three-dimensional structure. Seven is the count for that reported structure, rather than a universal definition of every Gla domain.^[ref]

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.

Glucuronic acid is the uronic acid corresponding to glucose; in the usual D form, carbon 6 of D-glucose carries a carboxyl group instead of its primary alcohol group. Its neutral free-acid molecular formula is ${\mathrm{C}}_6{\mathrm{H}}_{10}{\mathrm{O}}_7$. The reference carbon numbering remains that of glucose.^[ref]

Check the atom count: changing $\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$ into $\mathrm{COOH}$ adds one oxygen and removes two hydrogen atoms. Loss of the acidic proton gives glucuronate, with negative electric charge. The free monomer's formula should not be substituted unchanged for a residue inside a linked polysaccharide.

In a synthesis experiment, P. L. DeAngelis used UDP-glucuronic acid together with UDP-N-acetylglucosamine as activated substrates for hyaluronan synthase. Incorporation of the two sugar types into hyaluronic acid turns a repeating structural description into a reaction whose components can be varied.^[ref]

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.

Kainic acid is a naturally occurring, non-proteinogenic amino acid with molecular formula ${\mathrm{C}}_{10}{\mathrm{H}}_{15}\mathrm{N}{\mathrm{O}}_4$. Its structure is a substituted proline derivative containing two carboxyl groups. It acts as an excitatory amino-acid agonist, including activation of kainate receptors.^[ref][ref]

Its scientific usefulness comes from perturbing receptor activity with a chemically specified molecule. A voltage clamp experiment can measure the change in electric current associated with that perturbation while holding the membrane potential under a stated convention.

For a constructed example, suppose the current is $-5\mathrm{pA}$ before exposure and $-45\mathrm{pA}$ afterward. The change is $-40\mathrm{pA}$, not $-45\mathrm{pA}$: the baseline contributes to the recorded signal. A matched preparation lacking the receptor, together with suitable pharmacological comparisons, helps distinguish receptor-associated activity from background effects. These numbers illustrate analysis and are not reported experimental results.

Activation and cell injury are also different endpoints. Excessive excitatory signalling can cause excitotoxicity, but an observed current change alone does not measure the extent of cellular damage.^[ref] The molecular identity, receptor response and downstream outcome should each be established by an appropriate measurement.

Malonic acid is propanedioic acid, a dicarboxylic acid in which two carboxyl groups are separated by one methylene group:Its neutral molecular formula is ${\mathrm{C}}_3{\mathrm{H}}_4{\mathrm{O}}_4$ and its average molar mass is approximately $104.061\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$.^[ref]

Two of the four hydrogen atoms belong to carboxyl OH groups; the other two belong to the central carbon. Complete neutralization of both carboxyl groups therefore requires two equivalents of a suitable base, not four merely because the formula contains four hydrogens. An original stoichiometric model isFor $1.00\mathrm{mmol}$ acid, the two-equivalent amount of $0.100\mathrm{mol}{\mathrm{L}}^{-1}$ hydroxide solution is $20.0\mathrm{mL}$. The pH at an experimental endpoint also depends on acid-base equilibria and the measurement method; the equivalent count alone does not determine it. Malonate names the corresponding deprotonated system, whose protonation state should be specified.

Heating can promote decarboxylation, producing acetic acid and carbon dioxide. Malonic-acid derivatives underlie the carbon-chain construction described in the malonic ester synthesis.^[ref] For the unsubstituted acid, the stoichiometric balance isAn original ideal mass balance for $1.00\mathrm{mmol}$ converted by only this reaction gives about $60.052\mathrm{mg}$ acetic acid and $44.009\mathrm{mg}$ carbon dioxide from $104.061\mathrm{mg}$ starting material. Following evolved gas and residual material would test conversion, whereas a change in sample mass alone would not distinguish this reaction from evaporation or another decomposition pathway.

Nicotinic acid, ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{N}{\mathrm{O}}_2$, is pyridine-3-carboxylic acid: a pyridine ring bearing a carboxyl group at position 3. It is one form included under vitamin B3, and can contribute to production of the coenzymes nicotinamide adenine dinucleotide and nicotinamide adenine dinucleotide phosphate.^[ref][ref]

The related nicotinamide has a carboxamide group in place of the acid group. Their related names reflect related structures, while the different functional groups make them distinct chemical compounds. “Niacin” can be used broadly for the vitamin family, so an experiment reporting a particular chemical form should specify which molecule was used.