TD-DFT Computational Insight into the Origin of Wavelength-Dependent E/Z Photoisomerization of Urocanic Acid

Urocanic acid as a novel scaffold for next-gen nature-inspired sunscreens: II. Time-resolved spectroscopy under solution conditions

In recent years the use of synthetic UV filters in commercial skincare formulations has come under considerable scrutiny. Urocanic acid is a naturally occurring UV filter that could serve as a scaffold for developing next-generation biomimetic UV filters. We have carried out time-resolved electronic and vibrational absorption studies on urocanic acid and modified variants in various solvents on timescales spanning eighteen orders of magnitude; from femtoseconds to hours. In combination with quantum chemical calculations these provide vital insight into the photochemical and photophysical properties of urocanic acid and how these are tuned by substitutions and solvents. Moreover, they solve the hitherto conundrum of the wavelength dependence of the photochemistry of trans-urocanic acid in an aqueous environment. Crucially, these studies - together with the accompanying article that reports high-resolution laser spectroscopic studies performed under isolated gas-phase conditions (https://doi.org/10.1039/D4CP02087A) open novel avenues for a rational design of urocanic acid-based UV filters.

High pressure Raman investigation on trans-urocanic acid

Trans-urocanic acid (t-UCA) is an important epidermal UV protector predominantly found in human skin. Exposure of UV radiation triggers photoisomerization of t-UCA into its other conformer, cis-urocanic acid (cis-UCA), which has been shown to be a mediator of UV-induced immune-suppression leading to skin cancer. In this report, we present the investigation of molecular changes of t-UCA under high pressures by in-situ high pressure Raman spectroscopy. The study indicates onset of ring opening polymerization of t-UCA at pressure above 1.4 GPa. At pressures beyond 5 GPa, a well discernible characteristic vibrational mode (CC stretch) accompanied by several other spectral features such as δ CO₂^- and δ NH modes of cis-UCA point towards the isomerization of residual t-UCA monomers into cis-UCA. The content of cis-UCA gradually increased with increase in pressure. On release to ambient conditions, the spectrum of the quenched sample showed Raman modes of polymer and cis-UCA indicating that the changes are irreversible.

Recent advances in urocanic acid photochemistry, photobiology and photoimmunology

Urocanic acid (UCA), produced in the upper layers of mammalian skin, is a major absorber of ultraviolet radiation (UVR). Originally thought to be a 'natural sunscreen', studies conducted a quarter of a century ago proposed that UCA may be a chromophore for the immunosuppression that follows exposure to UVR. With its intriguing photochemistry, its role in immunosuppression and skin cancer development, and skin barrier function, UCA continues to be the subject of intense research effort. This review summarises the photochemical, photobiological and photoimmunological findings regarding UCA, published since 1998.