Butyric acid

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Butyric acid
General
Systematic name	butanoic acid
Other names	butyric acid n-butyric acid ethylacetic acid propane-1-carboxylic acid
Molecular formula	C4H8O2
SMILES	CCCC(O)=O
Molar mass	88.10 g·mol−1
Appearance	colorless oily liquid
CAS number	[107-92-6]
Properties
Density and phase	0.959 g·cm−3, liquid
Solubility in water	miscible
Melting point	-7.9 °C (265.1 K)
Boiling point	163.5 °C (436.5 K)
Acidity (pKa)	4.81
Hazards
MSDS	External MSDS
Main hazards	?
NFPA 704
Flash point	72 °C
R/S statement	R: 34 S: 26 36 45
RTECS number	ES5425000
Related compounds
Related compounds	butyraldehyde methyl butyrate
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Butyric acid, (from Greek βουτυρος = butter) IUPAC name n-Butanoic acid, or normal butyric acid, is a carboxylic acid with structural formula CH₃CH₂CH₂-COOH. It is notably found in rancid butter, parmesan cheese, and vomit, and has an unpleasant odor and acrid taste, with a sweetish aftertaste (similar to ether). Butyric acid can be detected by mammals with good scent detection abilities (e.g., dogs) at 10 ppb, while humans can detect it in concentrations above 10 ppm.

Butyric acid is a fatty acid occurring in the form of esters in animal fats and plant oils. The glyceride of butyric acid makes up 3% to 4% of butter. When butter goes rancid, butyric acid is liberated from the glyceride by hydrolysis leading to the unpleasant odor.

Normal butyric acid or fermentation butyric acid is also found as a hexyl ester in the oil of Heracleum giganteum (cow parsnip) and as an octyl ester in parsnip (Pastinaca sativa); it has also been noticed in the fluids of the flesh and in perspiration.

It is ordinarily prepared by the fermentation of sugar or starch, brought about by the addition of putrefying cheese, with calcium carbonate added to neutralize the acids formed in the process. The butyric fermentation of starch is aided by the direct addition of Bacillus subtilis.

Butyric acid is used in the preparation of various butyrate esters. Low-molecular-weight esters of butyric acid, such as methyl butyrate, have mostly pleasant aromas or tastes. As a consequence, they find use as food and perfume additives.

The acid is an oily colorless liquid that solidifies at -8 °C; it boils at 164 °C. It is easily soluble in water, ethanol and ether, and is thrown out of its aqueous solution by the addition of calcium chloride. Potassium dichromate and sulfuric acid (also known as sulphuric acid) oxidize it to carbon dioxide and acetic acid, while alkaline potassium permanganate oxidizes it to carbon dioxide. The calcium salt, Ca(C₄H₇O₂)₂·H₂O, is less soluble in hot water than in cold.

There is an isomer, isobutyric acid, which has the same chemical formula C₄H₈ O₂ but a different structure. It has similar chemical properties but different physical properties.

[edit] Butyrate fermentation

Butyrate is produced as end-product of a fermentation process solely performed by obligate anaerobic bacteria. Kombucha tea includes Butyric Acid as a result of fermentation. This fermentation pathway was discovered by Louis Pasteur in 1861. Examples of butyrate producing species :

• Clostridium butyricum
• Clostridium kluyveri
• Clostridium pasteurianum
• Fusobacterium nucleatum
• Butyrivibrio fibrisolvens
• Eubacterium limosum

The pathway starts with the glycolytic cleavage of glucose to two molecules of pyruvate, as happens in most organisms. Pyruvate is then oxidized into acetyl coenzyme A using a unique mechanism that involves an enzyme system called pyruvate-ferredoxin oxidoreductase. Two molecules of carbon dioxide (CO₂) and two molecules of elemental hydrogen (H₂) are formed in the process and exit the cell. Then:

• Acetyl coenzyme A converts into acetoacetyl coenzyme A; responsible enzyme: acetyl-CoA-acetyl transferase.
• Acetoacetyl coenzyme A converts into β-hydroxybutyryl CoA; responsible enzyme: β-hydroxybutyryl-CoA dehydrogenase.
• β-hydroxybutyryl CoA converts into crotonyl CoA; responsible enzyme: crotonase.
• Crotonyl CoA converts into butyryl CoA (CH₃CH₂CH₂C=O-CoA); responsible enzyme: butyryl CoA dehydrogenase.
• A phosphate group replaces CoA to form butyryl phosphate; responsible enzyme: phosphobutyrylase.
• The phosphate group joins ADP to form ATP and butyrate; responsible enzyme: butyrate kinase.

ATP is produced, as can be seen, in the last step of the fermentation. 3 ATPs are produced for each glucose molecule, a relatively high yield. The balanced equation for this fermentation is:

C₆H₁₂O₆ → C₄H₈O₂ + 2CO₂ + 2H₂

[edit] Acetone and butanol fermentation

Several species form acetone and butanol in an alternative pathway which starts as butyrate fermentation. Some of these species are:

• Clostridium acetobutylicum: the most prominent acetone and butanol producer, used also industrially
• Clostridium beijerinckii
• Clostridium tetanomorphum
• Clostridium aurantibutyricum

These bacteria begin with butyrate fermentation as described above, but, when the pH drops below 5, they switch into butanol and acetone production in order to prevent further lowering of the pH. Two molecules of butanol are formed for each molecule of acetone.

The change in the pathway occurs after acetoacetyl CoA formation. This intermediate then takes two possible pathways:

• Acetoacetyl CoA → acetoacetate → acetone, or
• Acetoacetyl CoA → butyryl CoA → butyraldehyde → butanol.

[edit] Butyric acid function/activity

Butyric acid has been associated with the ability to inhibit the function of histone deacetylase enzymes, thereby favouring an acetylated state of histones in the cell. Acetylated histones have a lower affinity for DNA than non-acetylated histones, due to the neutralisation of electrostatic charge interections. It is generally thought that transcription factors will be unable to access regions where histones are tightly associated with DNA (ie non-acetylated, eg heterochromatin). Therefore, it is thought that butyric acid enhances the transcriptional activity at promoters which are typically silenced/downregulated due to histone deacetylase activity.

This article incorporates information from the 1911 encyclopedia.