trans-Urocanic Acid: Properties, Uses & FAQs

trans-Urocanic Acid is a naturally occurring histidine metabolite found in the stratum corneum that acts as a major epidermal UV chromophore and plays a central role in UV-induced immunomodulation research.

CAS Number

104-98-3

Molecular Formula

C6H6N2O2

Molecular Weight

138.12 g/mol

UV Absorption Maximum

~277 nm (trans isomer in aqueous solution)

Appearance

White to off-white crystalline powder

Biological Origin

Deamination product of L-histidine by urocanase pathway in skin

Key Research Application

Photoimmunology, UV-induced immune suppression studies

What Is trans-Urocanic Acid?

trans-Urocanic acid (trans-UCA; CAS 104-98-3) is the thermodynamically stable geometric isomer of urocanic acid, a small organic molecule with the molecular formula C₆H₆N₂O₂ and a molecular weight of 138.12 g/mol. It is biosynthesized in the skin via the enzymatic deamination of L-histidine and accumulates in high concentrations in the stratum corneum, where it serves as the principal UV-absorbing chromophore of the epidermis.

Structurally, trans-UCA contains an imidazole ring conjugated through a trans double bond to an acrylic acid moiety, giving rise to its strong absorption in the UVB range (~277 nm). Upon UV irradiation, trans-UCA undergoes photoisomerization to its cis counterpart (cis-UCA), a transformation with significant immunological consequences. As a pure research reagent, Aladdin Scientific supplies trans-Urocanic Acid (SKU: T162071) as a white to off-white crystalline powder of defined purity suitable for cell-based assays, photochemistry experiments, and biochemical studies.

Biological Role and Research Significance

In normal human skin, trans-UCA is one of the most abundant small molecules in the stratum corneum. Its primary physiological role is thought to involve UV photoprotection through absorption of solar UVB radiation. However, the biological significance of trans-UCA extends well beyond simple UV screening.

Upon UVB exposure, trans-UCA is photoisomerized to cis-UCA, which has been shown in experimental models to suppress systemic immune responses, including delayed-type hypersensitivity reactions. This discovery established urocanic acid photoisomerization as a key mechanism in UV-induced immunosuppression, a phenomenon linked to photocarcinogenesis and the exacerbation of certain infectious diseases in sun-exposed individuals. Researchers use purified trans-UCA as a biochemical standard and experimental substrate to study isomerization kinetics, receptor interactions (notably with the serotonin receptor 5-HT_2A as proposed in some literature), and downstream immunological signaling. Understanding trans-UCA biology has implications for sunscreen development, skin cancer prevention, and immunodermatology.

Handling, Solubility, and Storage

Solubility: trans-Urocanic acid has limited solubility in cold water but dissolves more readily in hot water and dilute aqueous base (e.g., sodium hydroxide solution). It is sparingly soluble in common organic solvents such as ethanol and practically insoluble in non-polar solvents. For cell-based assays, stock solutions are typically prepared in dilute NaOH or phosphate-buffered saline adjusted to slightly alkaline pH, then diluted into the working buffer.

Storage: Store the solid compound at room temperature (15–25 °C) in a tightly sealed container, protected from light and moisture. Because trans-UCA is photosensitive and can isomerize to cis-UCA upon UV exposure, working solutions should be prepared fresh and handled under subdued or amber light conditions. Long-term storage of solutions is not recommended.

Safety: trans-Urocanic acid is not classified as a highly hazardous substance under standard laboratory reagent guidelines; however, standard laboratory precautions apply—wear appropriate PPE including gloves, lab coat, and eye protection. Consult the Safety Data Sheet (SDS) supplied with the product for full hazard information before use.

Common Experimental Applications

Researchers across dermatology, immunology, and photobiology routinely use trans-Urocanic Acid (T162071) in the following experimental contexts:

• UV Photoisomerization Studies: trans-UCA serves as the starting substrate for generating defined quantities of cis-UCA by controlled UVB irradiation, enabling studies of the kinetics and photochemistry of isomerization.
• Immunosuppression Models: In vitro and in vivo models use trans-UCA and its cis photoisomer to investigate mechanisms of UV-induced systemic immunosuppression, including effects on dendritic cells, T-cell responses, and cytokine profiles.
• Biochemical Assays: As an authentic chemical standard, trans-UCA is used to validate HPLC or mass spectrometry methods for quantifying urocanic acid isomers in biological matrices such as skin extracts, sweat, or tissue homogenates.
• Receptor Binding Studies: Some research groups use trans-UCA and cis-UCA to probe proposed receptor interactions, including platelet-activating factor receptor (PAFR) signaling pathways implicated in UV-induced immunosuppression.
• Sunscreen and Photoprotection Research: Because trans-UCA is a natural epidermal chromophore, it is included in studies evaluating the interplay between endogenous photoprotective molecules and exogenous sunscreen agents.

Frequently asked questions

What is the difference between trans-Urocanic Acid and cis-Urocanic Acid?

trans-Urocanic acid (trans-UCA) is the thermodynamically stable geometric isomer that predominates in human stratum corneum under normal conditions. Upon absorption of UVB radiation (~290–320 nm), it undergoes photoisomerization to cis-UCA, the metastable isomer. While trans-UCA itself is considered largely photoprotective, cis-UCA has been reported in experimental models to exert immunosuppressive effects, making the ratio of trans to cis isomers an important parameter in photoimmunology research.

How should I dissolve trans-Urocanic Acid for cell culture experiments?

trans-Urocanic acid has limited solubility in neutral aqueous buffers at room temperature. A practical approach is to dissolve the compound in a small volume of dilute sodium hydroxide solution (e.g., 0.1 M NaOH) to prepare a concentrated stock, then neutralize and dilute into your physiological buffer or cell culture medium to the desired working concentration. Prepare solutions fresh prior to each experiment and protect them from UV/visible light to prevent photoisomerization to the cis form during preparation.

Why does trans-Urocanic Acid need to be protected from light during storage and handling?

trans-UCA absorbs strongly in the UVB range and is susceptible to photoisomerization upon UV exposure, converting to the immunologically distinct cis-UCA isomer. Unintended photoisomerization during storage or sample preparation would compromise the purity and defined composition of your reagent, potentially confounding experimental results. Store the solid in a light-protected, sealed container and handle solutions under amber light or in a darkened environment to maintain the integrity of the trans isomer.

What analytical methods are used to confirm the purity and identity of trans-Urocanic Acid?

Identity and purity of trans-Urocanic Acid are typically confirmed by a combination of UV spectrophotometry (characteristic absorption maximum ~277 nm in aqueous solution for the trans isomer), HPLC with UV detection (to resolve trans and cis isomers and quantify impurities), and NMR spectroscopy (to confirm structural identity). Mass spectrometry provides additional molecular weight confirmation. Aladdin Scientific's Certificate of Analysis (CoA) for each lot details the specific analytical results; contact our technical support or your account manager to request a CoA for SKU T162071.

Can trans-Urocanic Acid be used for in vivo animal studies?

Yes, trans-UCA has been used in published in vivo research, typically administered topically or intradermally in murine models to study photoimmunology and UV-induced immune suppression. The reagent grade supplied by Aladdin Scientific (T162071) is intended for research use only and is not a pharmaceutical-grade or GMP-manufactured product. Researchers planning in vivo studies should consult their institutional animal ethics committee guidelines and may need to perform additional sterility or endotoxin testing of solutions prepared from this research-grade material.

References

1. Norval M. The mechanisms and consequences of ultraviolet-induced immunosuppression. Prog Biophys Mol Biol. 2006;92(1):108-118. doi:10.1016/j.pbiomolbio.2006.02.004
2. Gibbs NK, Norval M. Urocanic acid in the skin: a mixed blessing. J Invest Dermatol. 2011;131(1):14-17. doi:10.1038/jid.2010.289
3. Noonan FP, De Fabo EC. Immunosuppression by ultraviolet B radiation: initiation by urocanic acid. Immunol Today. 1992;13(7):250-254.

Products are supplied for research and development use only. Not for use in humans, animals, diagnosis, or therapy.