4-Aminobenzoic acid (also known as ''para''-aminobenzoic acid or PABA because the two functional groups are attached to the
benzene ring across from one another in the ''para'' position) is an
organic compound
In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. Th ...
with the
formula
In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwe ...
H
2NC
6H
4CO
2H. PABA is a white solid, although commercial samples can appear gray. It is slightly soluble in water. It consists of a
benzene ring substituted with
amino
In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituent ...
and
carboxyl group
In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic ...
s. The compound occurs extensively in the natural world.
Production and occurrence
In industry, PABA is prepared mainly by two routes:
* Reduction of
4-nitrobenzoic acid
4-Nitrobenzoic acid is an organic compound with the formula C6H4(NO2)CO2H. It is a pale yellow solid. It is a precursor to 4-nitrobenzoyl chloride, the precursor to the anesthetic procaine and folic acid. It is also a precursor to 4-aminobenzoic a ...
*
Hoffman degradation of the monoamide derived from
terephthalic acid
Terephthalic acid is an organic compound with formula C6H4(CO2H)2. This white solid is a commodity chemical, used principally as a precursor to the polyester PET, used to make clothing and plastic bottles. Several million tonnes are produced an ...
.
Food sources of PABA include liver, brewer's yeast (and unfiltered beer), kidney, molasses, mushrooms, and whole grains.
A review on this compound.
Biology
Biochemistry
PABA is an intermediate in the synthesis of
folate
Folate, also known as vitamin B9 and folacin, is one of the B vitamins. Manufactured folic acid, which is converted into folate by the body, is used as a dietary supplement and in food fortification as it is more stable during processing an ...
by bacteria, plants, and fungi.
Many bacteria, including those found in the human intestinal tract such as
''E. coli'', generate PABA from
chorismate by the combined action of the enzymes
4-amino-4-deoxychorismate synthase and
4-amino-4-deoxychorismate lyase. Plants produce PABA in their chloroplasts, and store it as a glucose ester (''p''ABA-Glc) in their tissues. Humans lack the enzymes to convert PABA to folate and so require folate from dietary sources, such as green leafy vegetables. In humans, PABA is considered nonessential and, although it has been referred to historically as "vitamin B
x", is no longer recognized as a
vitamin
A vitamin is an organic molecule (or a set of molecules closely related chemically, i.e. vitamers) that is an essential micronutrient that an organism needs in small quantities for the proper functioning of its metabolism. Essential nutrie ...
[ because the typical human gut ]microbiome
A microbiome () is the community of microorganisms that can usually be found living together in any given habitat. It was defined more precisely in 1988 by Whipps ''et al.'' as "a characteristic microbial community occupying a reasonably wel ...
generates PABA on its own.
Sulfonamide drugs are structurally similar to PABA, and their antibacterial activity is due to their ability to interfere with the conversion of PABA to folate by the enzyme dihydropteroate synthetase. Thus, bacterial growth is limited through folate deficiency.
Medical use
The potassium salt is used as a drug against fibrotic skin disorders, such as Peyronie's disease
Peyronie's disease is a connective tissue disorder involving the growth of fibrous plaques in the soft tissue of the penis. Specifically, scar tissue forms in the tunica albuginea, the thick sheath of tissue surrounding the corpora cavernosa, c ...
, under the brand name Potaba.[
] PABA is also occasionally used in pill form by sufferers of irritable bowel syndrome
Irritable bowel syndrome (IBS) is a "disorder of gut-brain interaction" characterized by a group of symptoms that commonly include abdominal pain and or abdominal bloating and changes in the consistency of bowel movements. These symptoms may ...
to treat its associated gastrointestinal symptoms, and in nutritional epidemiological studies to assess the completeness of 24-hour urine collection for the determination of urinary sodium, potassium, or nitrogen levels.
Nutritional supplement
Despite the lack of any recognized syndromes of PABA deficiency in humans, except for those who lack the colonic bacteria that generate PABA, many claims of benefit are made by commercial suppliers of PABA as a nutritional supplement. The benefit is claimed for fatigue, irritability, depression, weeping eczema (moist eczema), scleroderma (premature hardening of the skin), patchy pigment loss in the skin (vitiligo
Vitiligo is a disorder that causes the skin to lose its color. Specific causes are unknown but studies suggest a link to immune system changes.
Signs and symptoms
The only sign of vitiligo is the presence of pale patchy areas of depigmen ...
), and premature grey hair.
Commercial and industrial use
PABA finds use mainly in the biomedical sector. Other uses include its conversion to specialty azo dye
Azo dyes are organic compounds bearing the functional group R−N=N−R′, in which R and R′ are usually aryl and substituted aryl groups. They are a commercially important family of azo compounds, i.e. compounds containing the C-N ...
s and crosslinking agents. PABA is also used as a biodegradable pesticide, though its use is now limited due to evolution of new variants of bio-pesticides.
In the past, PABA was widely used in sunscreens as a UV filter. It is a UVB absorber, meaning it can absorb wavelengths between 290 and 320 nm. while still allowing UVA wavelengths between 320-400 nm to pass through, producing a tan.
Patented in 1943, PABA was one of the first active ingredients to be used in sunscreen
Sunscreen, also known as sunblock or sun cream, is a photoprotective topical product for the skin that mainly absorbs, or to a much lesser extent reflects, some of the sun's ultraviolet (UV) radiation and thus helps protect against sunbur ...
. The first ''in vivo'' studies on mice showed that PABA reduced UV damage. In addition, it was shown to protect against skin tumors in rodents.[H.; Thune, P.; Eeg Larsen, T. The inhibiting effect of PABA on photocarcinogenesis Arch. Dermatol. Res. 1990, 282, 38, 41.]
Animal and ''in vitro'' studies in the early 1980s suggested PABA might increase the risk of cellular UV damage.
Safety considerations
PABA is largely nontoxic; the median lethal dose
In toxicology, the median lethal dose, LD50 (abbreviation for " lethal dose, 50%"), LC50 (lethal concentration, 50%) or LCt50 is a toxic unit that measures the lethal dose of a toxin, radiation, or pathogen. The value of LD50 for a substance is ...
of PABA in dogs (oral) is 2 g/kg.[ Allergic reactions to PABA can occur. It is formed in the metabolism of certain ester local anesthetics, and many allergic reactions to local anesthetics are the result of reactions to PABA.Toxicity, Local Anesthetics: eMedicine Emergency Medicine]
/ref>
References
External links
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{{DEFAULTSORT:Aminobenzoic Acid, 4-
Anilines
Benzoic acids
IARC Group 3 carcinogens
Sunscreening agents
Non-proteinogenic amino acids