Conservation of ultrafast photoprotective mechanisms with increasing molecular complexity in sinapoyl malate derivatives

Integrated metabolomic and transcriptomic analysis reveals the regulatory mechanisms of flavonoid and alkaloid biosynthesis in the new and old leaves of Murraya tetramera Huang

BACKGROUND

Murraya tetramera Huang is a traditional Chinese woody medicine. Its leaves contain flavonoids, alkaloids, and other active compounds, which have anti-inflammatory and analgesic effects, as well as hypoglycemic and lipid-lowering effects, and anti-tumor effects. There are significant differences in the content of flavonoids and alkaloids in leaves during different growth cycles, but the synthesis mechanism is still unclear.

RESULTS

In April 2021, new leaves (one month old) and old leaves (one and a half years old) of M. tetramera were used as experimental materials to systematically analyze the changes in differentially expressed genes (DEGs) and differentially accumulated metabolites (DAMs) with transcriptomics and metabolomics technology. This was done to identify the signaling pathways of flavonoid and alkaloid synthesis. The results showed that the contents of total alkaloids and flavonoids in old leaves were significantly higher than those in new leaves. Thirteen flavonoid compounds, three isoflavone compounds, and nineteen alkaloid compounds were identified, and 125 and 48 DEGs related to flavonoid and alkaloid synthesis were found, respectively. By constructing the KEGG (Kyoto Encyclopedia of Genes and Genomes) network of DEGs and DAMs, it was shown that the molecular mechanism of flavonoid biosynthesis in M. tetramera mainly focuses on the "flavonoid biosynthetic pathway" and the "flavonoid and flavonol biosynthetic pathway". Among them, p-Coumaryl alcohol, Sinapyl alcohol, Phloretin, and Isoquercitrin were significantly accumulated in old leaves, the up-regulated expression of CCR (cinnamoyl-CoA reductase) might promote the accumulation of p-Coumaryl alcohol, upregulation of F5H (ferulate-5-hydroxylase) might promote Sinapyl alcohol accumulation. Alkaloids, including indole alkaloids, pyridine alkaloids, imidazole alkaloids, and quinoline alkaloids, were significantly accumulated in old leaves, and a total of 29 genes were associated with these substances.

CONCLUSIONS

These data are helpful to better understand the biosynthesis of flavonoids and alkaloids in M. tetramera and provide a scientific basis for the development of medicinal components in M. tetramera.

Understanding the Impact of Symmetrical Substitution on the Photodynamics of Sinapate Esters Using Gas-Phase Ultrafast Spectroscopy

Two model biomimetic systems, ethyl sinapate (ES) and its symmetrical analogue, diethyl 2-(4-hydroxy-3,5-dimethoxybenzylidene)malonate (or diethyl sinapate, DES), are stripped to their core fundamentals through gas-phase spectroscopy to understand the underlying photophysics of photothermal materials. Following photoexcitation to the optically bright S1(ππ*) state, DES is found to repopulate the electronic ground state over 3 orders of magnitude quicker than its nonsymmetrical counterpart, ES. Our XMS-CASPT2 calculations shed light on the experimental results, revealing crucial differences in the potential energy surfaces and conical intersection topography between ES and DES. From this work, a peaked conical intersection, seen for DES, shows vital importance for the nonradiative ground-state recovery of photothermal materials. This fundamental comparative study highlights the potential impact that symmetrical substitution can have on the photodynamics of sinapate esters, providing a blueprint for future advancement in photothermal technology.

Characterization of Solar Radiation-Induced Degradation Products of the Plant Sunscreen Sinapoyl Malate

Agricultural activities at lower temperatures lead to lower yields due to reduced plant growth. Applying photomolecular heater agrochemicals could boost yields under these conditions, but UV-induced degradation of these compounds needs to be assessed. In this study, we employ liquid chromatography-mass spectrometry (LC-MS) coupled with infrared ion spectroscopy (IRIS) to detect and identify the degradation products generated upon simulated solar irradiation of sinapoyl malate, a proposed photomolecular heater/UV filter compound. All major irradiation-induced degradation products are identified in terms of their full molecular structure by comparing the IRIS spectra obtained after LC fractionation and mass isolation with reference IR spectra obtained from quantum-chemical calculations. In cases where physical standards are available, a direct experimental-to-experimental comparison is possible for definitive structure identification. We find that the major degradation products originate from trans-to-cis isomerization, ester cleavage, and esterification reactions of sinapoyl malate. Preliminary in silico toxicity investigations using the VEGAHUB platform suggest no significant concerns for these degradation products' human and environmental safety. The identification workflow presented here can analogously be applied to break down products from other agrochemical compounds. As the method records IR spectra with the sensitivity of LC-MS, application to agricultural samples, e.g., from field trials, is foreseen.

Direct structural observation of ultrafast photoisomerization dynamics in sinapate esters

Sinapate esters have been extensively studied for their potential application in 'nature-inspired' photoprotection. There is general consensus that the relaxation mechanism of sinapate esters following photoexcitation with ultraviolet radiation is mediated by geometric isomerization. This has been largely inferred through indirect studies involving transient electronic absorption spectroscopy in conjunction with steady-state spectroscopies. However, to-date, there is no direct experimental evidence tracking the formation of the photoisomer in real-time. Using transient vibrational absorption spectroscopy, we report on the direct structural changes that occur upon photoexcitation, resulting in the photoisomer formation. Our mechanistic analysis predicts that, from the photoprepared ππ* state, internal conversion takes place through a conical intersection (CI) near the geometry of the initial isomer. Our calculations suggest that different CI topographies at relevant points on the seam of intersection may influence the isomerization yield. Altogether, we provide compelling evidence suggesting that a sinapate ester's geometric isomerization can be a more complex dynamical process than originally thought.

Radiationless mechanism of UV deactivation by cuticle phenolics in plants

Hydroxycinnamic acids present in plant cuticles, the interphase and the main protective barrier between the plant and the environment, exhibit singular photochemical properties that could allow them to act as a UV shield. Here, we employ transient absorption spectroscopy on isolated cuticles and leaf epidermises to study in situ the photodynamics of these molecules in the excited state. Based on quantum chemical calculations on p-coumaric acid, the main phenolic acid present in the cuticle, we propose a model in which cuticle phenolics display a photoprotective mechanism based in an ultrafast and non-radiative excited state deactivation combined with fluorescence emission. As such, the cuticle can be regarded as the first and foremost protective barrier against UV radiation. This photostable and photodynamic mechanism seems to be universal in land plants giving a special role and function to the presence of different aromatic domains in plant cuticles and epidermises.

Phenolics are abundant in plant cuticles. Here, via transient absorption spectroscopy and quantum chemical calculations, the authors propose a model by which cuticle phenolics provide photoprotection due to ultrafast and non-radiative excited state deactivation combined with fluorescence emission.

Theoretical Studies on the Excited-State Decay Mechanism of Homomenthyl Salicylate in a Gas Phase and an Acetonitrile Solution

Here, we employ the CASPT2//CASSCF and QM(CASPT2//CASSCF)/MM approaches to explore the photochemical mechanism of homomenthyl salicylate (HMS) in vacuum and an acetonitrile solution. The results show that in both cases, the excited-state relaxation mainly involves a spectroscopically "bright" S₁(¹ππ*) state and the lower-lying T₁ and T₂ states. In the major relaxation pathway, the photoexcited S₁ keto system first undergoes an essentially barrierless excited-state intramolecular proton transfer (ESIPT) to generate the S₁ enol minimum, near which a favorable S₁/S₀ conical intersection decays the system to the S₀ state followed by a reverse ground-state intramolecular proton transfer (GSIPT) to repopulate the initial S₀ keto species. In the minor one, an S₁/T₂/T₁ three-state intersection in the keto region makes the T₁ state populated via direct and T₂-mediated intersystem crossing (ISC) processes. In the T₁ state, an ESIPT occurs, which is followed by ISC near a T₁/S₀ crossing point in the enol region to the S₀ state and finally back to the S₀ keto species. In addition, a T₁/S₀ crossing point near the T₁ keto minimum can also help the system decay to the S₀ keto species. However, small spin-orbit couplings between T₁ and S₀ at these T₁/S₀ crossing points make ISC to the S₀ state very slow and make the system trapped in the T₁ state for a while. The present work rationalizes not only the ultrafast excited-state decay dynamics of HMS but also its low quantum yield of phosphorescence at 77 K.

Unravelling the Photoprotection Properties of Garden Cress Sprout Extract

Plants, as with humans, require photoprotection against the potentially damaging effects of overexposure to ultraviolet (UV) radiation. Previously, sinapoyl malate (SM) was identified as the photoprotective agent in thale cress. Here, we seek to identify the photoprotective agent in a similar plant, garden cress, which is currently used in the skincare product Detoxophane nc. To achieve this, we explore the photodynamics of both the garden cress sprout extract and Detoxophane nc with femtosecond transient electronic absorption spectroscopy. With the assistance of liquid chromatography-mass spectrometry, we determine that the main UV-absorbing compound in garden cress sprout extract is SM. Importantly, our studies reveal that the photoprotection properties of the SM in the garden cress sprout extract present in Detoxophane nc are not compromised by the formulation environment. The result suggests that Detoxophane nc containing the garden cress sprout extract may offer additional photoprotection to the end user in the form of a UV filter booster.

Quantum Mechanics/Molecular Mechanics Studies on the Photophysical Mechanism of Methyl Salicylate

Methyl salicylate (MS) as a subunit of larger salicylates found in commercial sunscreens has been shown to exhibit keto-enol tautomerization and dual fluorescence emission via excited-state intramolecular proton transfer (ESIPT) after the absorption of ultraviolet (UV) radiation. However, its excited-state relaxation mechanism is unclear. Herein, we have employed the quantum mechanics(CASPT2//CASSCF)/molecular mechanics method to explore the ESIPT and excited-state relaxation mechanism of MS in the lowest three electronic states, that is, S₀, S₁, and T₁ states, in a methanol solution. Based on the optimized geometric and electronic structures, conical intersections and crossing points, and minimum-energy paths combined with the computed linearly interpolated Cartesian coordinate paths, the photophysical mechanism of MS has been proposed. The S₁ state is a spectroscopically bright ¹ππ* state in the Franck-Condon region. From the initially populated S₁ state, there exist three nonradiative relaxation paths to repopulate the S₀ state. In the first one, the S₁ system (i.e., ketoB form) first undergoes an ESIPT path to generate an S₁ tautomer (i.e., enol form) that exhibits a large Stokes shift in experiments. The generated S₁ enol tautomer further evolves toward the nearby S₁/S₀ conical intersection and then hops to the S₀ state, followed by the backward ground-state intramolecular proton transfer (GSIPT) to the initial ketoB form S₀ state. In the second one, the S₁ system first hops through the S₁ → T₁ intersystem crossing (ISC) to the T₁ state, which then further decays to the S₀ state via T₁ → S₀ ISC at the T₁/S₀ crossing point. In the third path, the T₁ system that stems from the S₁ → T₁ ISC process via the S₁/T₁ crossing point first takes place a T₁ ESIPT to generate a T₁ enol tautomer, which can further decay to the S₀ state via T₁-to-S₀ ISC. Finally, the GSIPT occurs to back the system to the initial ketoB form S₀ state. Our present work could contribute to understanding the photophysics of MS and its derivatives.

Substitution effect on the nonradiative decay and trans → cis photoisomerization route: a guideline to develop efficient cinnamate-based sunscreens

Cinnamate derivatives are very useful as UV protectors in nature and as sunscreen reagents in daily life. They convert harmful UV energy to thermal energy through effective nonradiative decay (NRD) including trans → cis photoisomerization. However, the mechanism is not simple because different photoisomeirzation routes have been observed for different substituted cinnamates. Here, we theoretically examined the substitution effects at the phenyl ring of methylcinnamate (MC), a non-substituted cinnamate, on the electronic structure and the NRD route involving trans → cis isomerization based on time-dependent density functional theory. A systematic reaction pathway search using the single-component artificial force-induced reaction method shows that the very efficient photoisomerization route of MC can be essentially described as "1ππ* (trans) → 1nπ* → T1 (3ππ*) → S0 (trans or cis)". We found that for efficient 1ππ* (trans) → 1nπ* internal conversion (IC), MC should have the substituent at the appropriate position of the phenyl ring to stabilize the highest occupied π orbital. Substitution at the para position of MC slightly lowers the 1ππ* state energy and photoisomerization occurs via a slightly less efficient "1ππ* (trans) → 3nπ* → T1 (3ππ*) → S0 (trans or cis)" pathway. Substitution at the meta or ortho positions of MC significantly lowers the 1ππ* state energy so that the energy barrier of IC (1ππ* → 1nπ*) becomes very high. This substitution leads to a much longer 1ππ* state lifetime than that of MC and para-substituted MC, and a change in the dominant photoisomerization route to "1ππ* (trans) → C[double bond, length as m-dash]C bond twisting on 1ππ* → S0 (trans or cis)". As a whole, the "1ππ* → 1nπ*" IC observed in MC is the most important initial step for the rapid change of UV energy to thermal energy. We also found that the stabilization of the π orbital (i) minimizes the energy gap between 1ππ* and 1nπ* at the 1ππ* minimum and (ii) makes the 0-0 level of 1ππ* higher than 1nπ* as observed in MC. These MC-like relationships between the 1ππ* and 1nπ* energies should be ideal to maximize the "1ππ* → 1nπ*" IC rate constant according to Marcus theory.