4-Aminobenzoic acid

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 with the formula H₂NC₆H₄CO₂H. 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 and carboxyl groups. The compound occurs extensively in the natural world.

Reduction of

4-nitrobenzoic acid

of the monoamide derived from terephthalic acid.^[3]

Hoffman degradation

In industry, PABA is prepared mainly by two routes:

Food sources of PABA include liver, brewer's yeast (and unfiltered beer), kidney, molasses, mushrooms, and whole grains.^[4] Other food sources of PABA include spinach, liver, and oat seeds.^[5]

Commercial and industrial use[edit]

PABA finds use in the biomedical sector. Its derivatives are found as a structural component in 1.5% of a database of 12111 commercial drugs.^[12] Other uses include its conversion to specialty azo dyes 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. Specifically, studies have shown that PABA photodegrades through an O2-mediated pathway in which PABA is oxidized by O2 via hydrogen abstraction and decarboxylation.^[13]

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.^[14] while still allowing UVA wavelengths between 320-400 nm to pass through, producing a tan.^[15] Patented in 1943, PABA was one of the first active ingredients to be used in sunscreen.^[16] 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.^[17] Animal and in vitro studies in the early 1980s suggested PABA might increase the risk of cellular UV damage.^[18] On the basis of these studies, as well as problems with allergies and clothing discoloration, PABA fell out of favor as a sunscreen. However, water-insoluble PABA derivatives such as padimate O are currently used in some cosmetic products including mascara, concealer, and matte lipsticks.^[19]

As of 2008, the advancement of new sunscreen is focused on developing a broad spectrum of active ingredients that provide consistent protection across all wavelengths, including UVA. Researchers are considering the PABA–TiO₂ Hybrid Nanostructures that result from the method of aqueous in situ synthesis with PABA and TiO₂.^[15]

Safety considerations[edit]

PABA is largely nontoxic; the median lethal dose of PABA in dogs (oral) is 2 g/kg.^[3] 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.^[20]