Hydrogen bond dynamics governs the effective photoprotection mechanism of plant phenolic sunscreens

Spectroscopy and Excited-State Dynamics of Methyl Ferulate in Molecular Beams

The spectroscopic and dynamic properties of methyl ferulate─a naturally occurring ultraviolet-protecting filter─and microsolvated methyl ferulate have been studied under molecular beam conditions using resonance-enhanced multiphoton ionization spectroscopy in combination with quantum chemical calculations. We demonstrate and rationalize how the phenyl substitution pattern affects the state ordering of the lower excited singlet state manifold and what the underlying reason is for the conformation-dependent Franck-Condon (FC) activity in the UV-excitation spectra. Studies on microsolvated methyl ferulate reveal potential coordination sites and the influence of such coordination on the spectroscopic properties. Our quantum chemical studies also enable us to obtain a quantitative understanding of the dominant excited-state decay routes of the photoexcited ππ* state involving a ∼3 ns intersystem crossing pathway to the triplet manifold─which is much slower than found for coumarates─and a relatively fast intersystem crossing back to the ground state (∼30 ns). We show that a common T1/S0 crossing can very well explain the observation that T1 lifetimes are quasi-independent of the phenyl substitution pattern.

Mechanistic Aspects of the Photophysics of UVA Filters Based on Meldrum Derivatives

Skin photoprotection against UVA radiation is crucial, but it is hindered by the sparsity of approved commercial UVA filters. Sinapoyl malate (SM) derivatives are promising candidates for a new class of UVA filters. They have been previously identified as an efficient photoprotective sunscreen in plants due to their fast nonradiative energy dissipation. Combining experimental and computational results, in our previous letter (J. Phys. Chem. Lett. 2021, 12, 337-344) we showed that coumaryl Meldrum (CMe) and sinapoyl Meldrum (SMe) are outstanding candidates for UVA filters in sunscreen formulations. Here, we deliver a comprehensive computational characterization of the excited-state dynamics of these molecules. Using reaction pathways and excited-state dynamics simulations, we could elucidate the photodeactivation mechanism of these molecules. Upon photoexcitation, they follow a two-step logistic decay. First, an ultrafast and efficient relaxation stabilizes the excited state alongside a 90° twisting around the allylic double bond, giving rise to a minimum with a twisted intramolecular excited-state (TICT) character. From this minimum, internal conversion to the ground state occurs after overcoming a 0.2 eV barrier. Minor differences in the nonradiative decay and fluorescence of CMe and SMe are associated with an additional minimum present only in the latter.

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.

Molecular modeling as a design tool for sunscreen candidates: a case study of bemotrizinol

Sunscreen-based photoprotection is an important strategy to prevent photoaging and skin cancer. Among the effective and modern sunscreens, triazine compounds are known as an important class based on their physical-chemical properties, such as photostability and UV broad-spectrum absorption (UVA and UVB). Molecular modeling and quantum mechanical calculations approaches can be helpful to orientate the design of sunscreens. Herein, a case study is presented to demonstrate the importance of the molecular modeling as a design tool for promising sunscreen candidates based on the synthesis research previously described of bemotrizinol, a broad-spectrum photostable organic UV filter present in many sunscreens products. Time-dependent density functional theory (TD-DFT) calculations performed in gas phase on the isolated organic UV filters proved to reproduce the experimental UV absorption, guiding the choice of the most efficient candidate as sunscreen. The present work highlights the importance of molecular modeling as an effective tool to support synthesis research, increasing the possibility of obtaining promising compounds with reduced costs and effluent production. Graphical abstractA case study to demonstrate the importance of the molecular modeling as a design tool for promising sunscreen candidates is presented. The method proved to be a valuable tool to reproduce the experimental UV absorption and to determinate the most efficient molecule as sunscreen among the candidates.

Charge-Transfer Knowledge Graph among Amino Acids Derived from High-Throughput Electronic Structure Calculations for Protein Database

The charge-transfer coupling is an important component in tight-binding methods. Because of the highly complex chemical structure of biomolecules, the anisotropic feature of charge-transfer couplings in realistic proteins cannot be ignored. In this work, we have performed the first large-scale quantitative assessment of charge-transfer preference by calculating the charge-transfer couplings in all 20 × 20 possible amino acid side-chain combinations, which are extracted from available high-quality structures of thousands of protein complexes. The charge-transfer database quantitatively shows distinct features of charge-transfer couplings among millions of amino acid side-chain combinations. The overall distribution of charge-transfer couplings reveals that only one average or representative structure cannot be regarded as the typical charge-transfer preference in realistic proteins. This work provides us an alternative route to comprehensively understand the charge-transfer couplings for the overall distribution of realistic proteins in the foreseen big data scenario.

Photoprotection: extending lessons learned from studying natural sunscreens to the design of artificial sunscreen constituents

Evolution has ensured that plants and animals have developed effective protection mechanisms against the potentially harmful effects of incident ultraviolet radiation (UVR). Tanning is one such mechanism in humans, but tanning only occurs post-exposure to UVR. Hence, there is ever growing use of commercial sunscreens to pre-empt overexposure to UVR. Key requirements for any chemical filter molecule used in such a photoprotective capacity include a large absorption cross-section in the UV-A and UV-B spectral regions and the availability of one or more mechanisms whereby the absorbed photon energy can be dissipated without loss of the molecular integrity of the chemical filter. Here we summarise recent experimental (mostly ultrafast pump-probe spectroscopy studies) and computational progress towards unravelling various excited state decay mechanisms that afford the necessary photostability in chemical filters found in nature and those used in commercial sunscreens. We also outline ways in which a better understanding of the photophysics and photochemistry of sunscreen molecules selected by nature could aid the design of new and improved commercial sunscreen formulations.

Photophysics of a UV-B Filter 4-Methylbenzylidene Camphor: Intersystem Crossing Plays an Important Role

4-Methylbenzylidene camphor (4MBC) is a frequently used ultraviolet (UV) filter in commercial sunscreens, which is experimentally found to undergo efficient intersystem crossing to triplet manifolds followed by predominant radiationless decay to the ground state. However, its photophysical mechanism is unclear. Herein, we have employed combined CASPT2 and CASSCF methods to study the spectroscopic properties, geometric and electronic structures, conical intersections and crossing points, and excited-state deactivation channels of 4MBC. We have found that the V(¹ ππ*) state is populated with large probability in the Franck-Condon region. Starting from this state, there are two efficient nonradiative relaxation processes to populate the ³ ππ* state. In the first one, the V(¹ ππ*) state decays to the V'(¹ ππ*) state. The resultant V'(¹ ππ*) state further jumps to the ¹ nπ* state by internal conversion at the ¹ ππ*/¹ nπ* conical intersection. Then, the ¹ nπ* state hops to the ³ ππ* state through an efficient ¹ nπ*→³ ππ* intersystem crossing process. In the second one, the V(¹ ππ*) state can diabatically relax along the photoisomerization reaction coordinate. In this process, a ¹ ππ*/³ nπ* crossing point helps the ¹ ππ* system decay to the ³ nπ* state, which further decays to the ³ ππ* state through internal conversion at the ³ nπ*/³ ππ* conical intersection. Once the ³ ππ* state is formed, a nearly barrierless relaxation path drives the ³ ππ* system to hop to the S₀ state via the ³ ππ*/S₀ crossing point. Our current work not only rationalizes recent experimental observations but also enriches our photophysical knowledge of UV filters.

Spin crossover dynamics studies on the thermally activated molecular oxygen binding mechanism on a model copper complex

The theoretical description of the primary dioxygen (O₂) binding and activation step in many copper or iron enzymes, suffers from the instrinsically electronic non-adiabaticity of the spin flip events of the triplet dioxygen molecule (³O₂), mediated by spin–orbit couplings.

Investigating isomer specific photoprotection in a model plant sunscreen

Sinapate esters are used throughout the plant kingdom, for example in photoprotection from ultraviolet radiation.

Direct Learning Hidden Excited State Interaction Patterns from ab initio Dynamics and Its Implication as Alternative Molecular Mechanism Models

The excited states of polyatomic systems are rather complex, and often exhibit meta-stable dynamical behaviors. Static analysis of reaction pathway often fails to sufficiently characterize excited state motions due to their highly non-equilibrium nature. Here, we proposed a time series guided clustering algorithm to generate most relevant meta-stable patterns directly from ab initio dynamic trajectories. Based on the knowledge of these meta-stable patterns, we suggested an interpolation scheme with only a concrete and finite set of known patterns to accurately predict the ground and excited state properties of the entire dynamics trajectories, namely, the prediction with ensemble models (PEM). As illustrated with the example of sinapic acids, The PEM method does not require any training data beyond the clustering algorithm, and the estimation error for both ground and excited state is very close, which indicates one could predict the ground and excited state molecular properties with similar accuracy. These results may provide us some insights to construct molecular mechanism models with compatible energy terms as traditional force fields.

Ultrafast Barrierless Photoisomerization and Strong Ultraviolet Absorption of Photoproducts in Plant Sunscreens

Sunscreens are aimed at protecting skin from solar ultraviolet (UV) irradiation. By utilizing femtosecond transient absorption spectroscopy and time-dependent density functional theory, we explain nature's selection of sinapoyl malate rather than sinapic acid as the plant sunscreen molecule. In physiological pH conditions, the two molecules are deprotonated, and their excited ππ* states are found to relax to the ground states in a few tens of picoseconds via a barrierless trans-cis photoisomerization. After the cis-photoproduct is formed, the efficacy of sinapic acid is greatly reduced. In contrast, the efficacy of sinapoyl malate is affected only slightly because the cis-product still absorbs UV light strongly. In addition, protonated sinapic acid is found to be a good potential sunscreen molecule.