The Photochemical Isomerization Kinetics of Urocanic Acid and Their Effects upon the in vitro and in vivo Photoisomerization Action Spectra

Analytical tools for urocanic acid determination in human samples: A review

Urocanic acid is a chromophore found in the skin that has been identified as an important immunosuppressant and carcinogenesis mediator through its photoisomerization from trans to cis form induced by ultraviolet radiation. Research on analytical methods that explore urocanic acid isomerization is indispensable to fully understand the deleterious effects mediated by this biomarker. In this context, the current relevant analytical methods for determination of these isomers in human samples are summarized in this review. The methods presented here are applicable to human samples collected by noninvasive methods (or minimally invasive), encompassing an array of analytical techniques, including high-performance capillary electrophoresis, confocal Raman spectroscopy, gas chromatography, high-performance liquid chromatography, and mass spectrometry, among others. Developed high-performance liquid chromatography methods have proven to be advantageous, allowing noninvasive collections for in vivo analysis and the confocal Raman, specially, for real-time analysis. Among all these methods, high-performance liquid chromatography is the most investigated one with mass spectrometry or ultraviolet detector, and the mass spectrometry detector being the most studied in the last years, demonstrating high sensitivity, very low detection limits, and accurate identification, especially for clinical investigations.

New insight into the photoinduced wavelength dependent decay mechanisms of the ferulic acid system on the excited states

The ferulic acid (FA) is a kind of phenolic acid widely exists in nature plants. Apart from its medicinal values, the FA is also widely applied in cosmetic industry. Recently, it was found to have potential applications in commercial sunscreens for its strong photostability and photoprotection property from harmful UV rays. Such excellent property lies in the ultrafast decay process of the FA system when exposure to the UV light, but the underlying detailed relaxation pathway is still less clear-cut. In the current work, high-level ab initio electronic structure calculations and on-the-fly surface hopping dynamics simulations were employed to explore the photoinduced decay mechanism of the FA system both on the S₁ and S₃ states in the gas phase. The results provide a reasonable explanation for the wavelength dependent decay patterns of FA system. The S₁ state decay pathway is driven by a re-emission process to dissipate excess energy. While for the S₃ state deactivation process, the pathway is dominated by a non-adiabatic process driven by the internal conversion process through the conical intersection regions. A S₃-S₁-S₀ two step decay pattern is proposed, and the pathways are mainly driven by a puckering distortion motion of the aromatic ring and a twisting motion around the bridging double bond. The calculation results contribute to a better understanding of detailed dynamics behavior of the FA deactivation process, and provide theoretical guidance for further design of efficient and environmentally friendly sunscreens.

Marine diterpene glycosides

Marine diterpene glycosides (MDGs) respresent a small but highly significant group of the much larger class of marine diterpenes. The three well-studied examples of MDGs are eleutherobins, pseudopterosins and fuscosides, all of which exhibit extremely promising biological activity. The eleutherobins are potent anti-mitotic agents, and the pseudopterosins and fuscosides are potent anti-inflammatory agents. This review discusses the structures and biological activities of these compounds, as well as their biosynthesis and synthesis.

Theoretical investigation of the photosensitization mechanisms of urocanic acid

The photosensitization mechanisms of urocanic acid (UA), the main skin chromophores of ultraviolet (UV) light, are investigated by means of theoretical calculations. The results indicate that the direct photooxidative damage to DNA bases by triplet state UA through electron transfer reaction is not favorable on thermodynamic grounds. However, UA can photogenerate various reactive oxygen species (ROS, e.g., (1)O(2), O(2)(-)) theoretically and the ROS-generating mechanisms are illustrated as follows. Firstly, the (1)O(2)-generating pathway involves direct energy transfer between triplet state UA and (3)O(2). Secondly, UA gives birth to O(2)(-) through two pathways: (i) direct electron transfer between triplet state UA and (3)O(2); (ii) electron transfer between anion radical of UA (generated through autoionization reactions) and (3)O(2).

Some photophysical studies of cis- and trans-urocanic acid

Urocanic acid, an important human skin chromophore, undergoes a variety of photochemical transformations when exposed to the near-UV portion of sunlight and natural daylight, the principal reaction being the transformation from the stable trans- or (E)-form of the chromophore (trans-UA) to the biologically active cis- or (Z)-form (cis-UA), which is claimed to induce immunosuppression linked to the onset of skin cancer. This study is concerned with the comparative photophysical behaviour of the two urocanic acid isomers in aqueous solution using both continuous irradiation and pulsed irradiation techniques. The UV absorption maximum for both isomers occurs in the region of 270 nm with the absorption shape varying characteristically with pH, the cis-isomer showing a lower overall molar absorptivity. Both isomers exhibit weak fluorescence (quantum yields estimated to be less than 10(-4)) with each isomer showing small differences in the way in which pH and excitation wavelength influence the fluorescence emission characteristics. Pulsed nanosecond laser irradiation at 266 nm of aqueous solutions at pH 7 shows that both isomers undergo photo-ionisation with a quantum yield of 0.02 for the hydrated electron production, a quantum yield value comparable with that for photoisomerisation at this wavelength. Laser flash studies also show that the photo-ionised species reacts efficiently with oxygen (quenching rate kQ = 1.3 x 10(9) M(-1) s(-1)), while some preliminary experiments indicate that both cis- and trans-urocanic acids react with the semiquinone radical of L-3,4-dihydroxyphenylalanine (L-DOPA) with a fast reaction rate constant of approximately 5 x 10(7) M(-1) s(-1). The photophysical characteristics of trans-UA and cis-UA reported here are discussed in the context of other recent pulsed irradiation studies on urocanic acid over nanosecond and picosecond time scales, in an attempt to clarify the complex photo-behaviour of this interesting biomolecule.

The potential role for urocanic acid and sunlight in the immune suppression associated with protein malnutrition

Irradiation of skin by sunlight or ultraviolet B (UVB, 290-320 nm) brings about a downregulation of cell-mediated immunity. An action spectrum for photoimmune suppression in mice indicates that trans-urocanic acid absorbs UV photons and is isomerized to the cis-isomer in the stratum corneum. Cis-urocanic acid is subsequently shown to suppress cellular immunity in mice. When histidine is elevated in a mouse diet, a higher level of urocanic acid is detected in mouse skin. These mice are more susceptible to photoimmune suppression. There is evidence that humans and animals experiencing protein malnutrition have very high levels of urocanic acid and/or histidine. Urocanic acid is formed by deamination of histidine in one enzymatic step. We discuss the protein malnutrition of kwashiorkor patients. They experience suppressed immunity and disturbed histidine metabolism. Here, we present a testable hypothesis: one cause of the immune deficiency observed in humans with protein malnutrition is the photoconversion by UVB of increased levels of trans-urocanic acid in skin to cis-urocanic acid, which suppresses the cellular immune system.