Synthesis and Photoswitching Properties of Bioinspired Dissymmetric γ-Pyrone, an Analogue of Cyclocurcumin

Computational Characterization of the DAD Photoisomerization: Functionalization, Protonation, and Solvation Effects

Photoswitches are becoming increasingly popular in pharmacology due to the possibility of modifying their activity with light. Hence, it is crucial to understand the photophysics of these compounds to identify promising light-activated drugs. We focused our study on DAD, an azobenzene derivative that, according to a previous experimental investigation, can restore visual function in blind mice due to trans-cis photoisomerization upon light absorption. With the present computational study, we aim to characterize the absorption spectrum of DAD, and to understand its photoisomerization mechanism by means of conformational search analysis, quantum mechanical (QM) and hybrid QM/continuum calculations, and classical molecular dynamics simulations. Moreover, we explored the effect of the derivation (DAD vs azobenzene), the protonation (DAD vs DADH22+, the two possible protonation states) and the solvation (vacuum vs water) on the photoisomerization. Similarly to azobenzene, we showed that the photoisomerization of both protonation states of DAD begin with the population of the bright S2 state. Then, it crosses to the S1 surface and relaxes along the rotation of the azo dihedral to a S1/S0 crossing point. The latter is close to a transition state that connects the trans and cis geometries on the ground state. Finally, our results suggested that amino derivation, nonprotonation and water solvation could improve the quantum yield of the photoisomerization.

A Portrait of the Chromophore as a Young System-Quantum-Derived Force Field Unraveling Solvent Reorganization upon Optical Excitation of Cyclocurcumin Derivatives

The study of fast non-equilibrium solvent relaxation in organic chromophores is still challenging for molecular modeling and simulation approaches, and is often overlooked, even in the case of non-adiabatic dynamics simulations. Yet, especially in the case of photoswitches, the interaction with the environment can strongly modulate the photophysical outcomes. To unravel such a delicate interplay, in the present contribution we resorted to a mixed quantum-classical approach, based on quantum mechanically derived force fields. The main task is to rationalize the solvent reorganization pathways in chromophores derived from cyclocurcumin, which are suitable for light-activated chemotherapy to destabilize cellular lipid membranes. The accurate and reliable decryption delivered by the quantum-derived force fields points to important differences in the solvent's reorganization, in terms of both structure and time scale evolution.

Total Synthesis of (+)-Silybin A

We report the first total synthesis of silybin A (1). Key synthetic steps include the construction of the 1,4-benzodioxane neolignan skeleton, a modified Julia-Kocienski olefination reaction between m-nitrophenyltetrazole sulfone (m-NPT sulfone) 10 and aldehyde 21, the formation of the flavanol lignan skeleton 28 via a quinomethide intermediate under acidic conditions, and stepwise oxidation of the benzylic position of flavanol 29.

A (TD-)DFT study on photo-NHC catalysis: photoenolization/Diels-Alder reaction of acid fluorides catalyzed by N-heterocyclic carbenes

A comprehensive mechanistic study on the N-heterocyclic carbene (NHC) catalyzed photoenolization/Diels-Alder (PEDA) reaction of acid fluorides was performed in the framework of (time-dependent) density functional theory ((TD)-DFT). The 1,5-hydrogen atom transfer (1,5-HAT) during photoenolization of an ortho-toluoyl azolium salt was found to be feasible via, first, singlet excitation and photoenolization, and then, after crossing to the triplet manifold, populating a biradical dienol which allows for the formation of two ortho-quinodimethane (o-QDM) isomers due to a low rotational barrier. The (Z)-isomer is mostly unproductive through sigmatropic rearrangement back to the starting material while the (E)-isomer reacts in a subsequent concerted Diels-Alder reaction likely as the deprotonated dienolate. The experimentally observed diastereoselectivity is correctly predicted by theory and is determined by a more favorable endo trajectory in the cycloaddition step. These findings demonstrate that ortho-toluoyl azolium species exhibit similar photophysical properties as structurally related benzophenones, highlighting the unique ability of the NHC organocatalyst to transiently alter the excited state properties of an otherwise photoinactive carboxylic acid derivative, thereby expanding the scope of classical carbonyl photochemistry.

Perturbation of Lipid Bilayers by Biomimetic Photoswitches Based on Cyclocurcumin

The use of photoswitches which may be activated by suitable electromagnetic radiation is an attractive alternative to conventional photodynamic therapy. Here, we report all-atom molecular dynamics simulation of a biomimetic photoswitch derived from cyclocurcumin and experiencing E/Z photoisomerization. In particular, we show that the two isomers interact persistently with a lipid bilayer modeling a cellular membrane. Furthermore, the interaction with the membrane is strongly dependent on the concentration, and a transition between ordered and disordered arrangements of the photoswitches is observed. We also confirm that the structural parameters of the bilayer are differently affected by the two isomers and hence can be modulated through photoswitching, offering interesting perspectives for future applications.

Biomimetic Photo-Switches Softening Model Lipid Membranes

We report the synthesis and characterization of a novel photo-switch based on biomimetic cyclocurcumin analogous and interacting with the lipid bilayer, which can be used in the framework of oxygen-independent light-induced therapy. More specifically, by using molecular dynamics simulations and free energy techniques, we show that the inclusion of hydrophobic substituents is needed to allow insertion in the lipid membrane. After having confirmed experimentally that the substituents do not preclude the efficient photoisomerization, we show through UV-vis and dynamic light scattering measurements together with compression isotherms that the chromophore is internalized in both lipid vesicles and monomolecular film, respectively, inducing their fluidification. The irradiation of the chromophore-loaded lipid aggregates modifies their properties due to the different organization of the two diastereoisomers, E and Z. In particular, a competition between a fast structural reorganization and a slower expulsion of the chromophore after isomerization can be observed in the kinetic profiles recorded during E to Z photoisomerization. This report paves the way for future investigations in the optimization of biomimetic photoswitches potentially useful in modern light-induced therapeutic strategies.

2-(2-(Dimethylamino)vinyl)-4H-pyran-4-ones as Novel and Convenient Building-Blocks for the Synthesis of Conjugated 4-Pyrone Derivatives

A straightforward approach for the construction of the new class of conjugated pyrans based on enamination of 2-methyl-4-pyrones with DMF-DMA was developed. 2-(2-(Dimethylamino)vinyl)-4-pyrones are highly reactive substrates that undergo 1,6-conjugate addition/elimination or 1,3-dipolar cycloaddition/elimination followed by substitution of the dimethylamino group without ring opening. This strategy includes selective transformations leading to conjugated and isoxazolyl-substituted 4-pyrone structures. The photophysical properties of the prepared 4-pyrones were determined in view of further design of novel merocyanine fluorophores. A solvatochromism was found for enamino-substituted 4-pyrones accompanied by a strong increase in fluorescence intensity in alcohols. The prepared conjugated structures demonstrated valuable photophysical properties, such as a large Stokes shift (up to 204 nm) and a good quantum yield (up to 28%).

Synthesis of γ-Pyrones and N-Methyl-4-pyridones via the Au Nanoparticle-Catalyzed Cyclization of Skipped Diynones in the Presence of Water or Aqueous Methylamine

Supported Au nanoparticles on TiO₂ catalyze the hydration/6-endo cyclization of skipped diynones to γ-pyrones in aqueous dioxane, via triple bond activation. The isomeric 3(2H)-furanones which could be formed through a competing and often prevailing 5-exo cyclization pathway using homogeneous ionic Au(I) catalysts were not seen. The reaction does not proceed via the initial 1,3-transposition of the skipped diynones to their corresponding conjugated 1,3-diynone isomers. If aqueous methylamine is added, N-methyl-4-pyridones are exclusively formed in 69-79% yields via an analogous hydroamination/Au-catalyzed 6-endo cyclization pathway.

Don't help them to bury the light. The interplay between intersystem crossing and hydrogen transfer in photoexcited curcumin revealed by surface-hopping dynamics

Curcumin is a natural compound extracted from turmeric (Curcuma longa), which has shown remarkable anti-inflammatory, antibacterial, and possibly anticancer properties. The intense absorption in the visible domain and the possibility of intersystem crossing make curcumin attractive also for photodynamic therapy purposes. In the present contribution, we unravel, thanks to non-adiabatic surface hopping dynamics, the interplay between intersystem crossing and hydrogen transfer in the enol form, i.e. the most stable tautomer of curcumin. Most notably, we show that while hydrogen transfer is ultrafast and happens in the sub-ps regime, intersystem crossing is still present, as shown by the non-negligible population of the triplet state manifold after 2 ps. Hence, while the hydrogen transfer channel can act as a deactivating channel, curcumin, also in the red-shifted absorption enol form, can still be regarded as potentially favorable for photodynamic therapy applications.