In humans, vision is limited to a small fraction of the whole electromagnetic spectrum. One possible strategy for enhancing vision in deep-red or poor-light conditions consists of recruiting chlorophyll derivatives in the rod photoreceptor cells of the eye, as suggested in the case of some deep-sea fish. Here, we employ all-atom molecular simulations and high-level quantum chemistry calculations to rationalize how chlorin e6 (Ce6), widely used in photodynamic therapy although accompanied by enhanced visual sensitivity, mediates vision in the dark, shining light on a fascinating but largely unknown molecular mechanism. First, we identify persistent interaction sites between Ce6 and the extracellular loops of rhodopsin, the transmembrane photoreceptor protein responsible for the first steps in vision. Triggered by Ce6 deep-red light absorption, the retinal within rhodopsin can be isomerized thus starting the visual phototransduction cascade. Our data largely exclude previously hypothesized energy-transfer mechanisms while clearly lending credence to a retinal isomerization indirectly triggered by singlet oxygen, proposing an alternative mechanism to rationalize photosensitizermediated night vision.

The photoinduced cycloreversion of oxetanes has been thoroughly investigated in connection with the photorepair of the well-known DNA (6-4) photoproducts. In the present work, the direct photolysis of the two...

Lapatinib (LAP) is an anticancer drug, which is metabolized to the N-a nd O-dealkylated products (N-LAP and O-LAP,r espectively). In view of the photosensitizing potential of relatedd rugs, ac omplete experimentala nd theoretical study has been performedo nL AP, N-LAP and O-LAP, both in solution and upon complexationw ith human serum albumin (HSA). In organic solvents, coplanar locally excited (LE) emissive states are generated;t hey rapidly evolve towards twisted intramolecularc harge-transfer (ICT) states. By contrast, within HSA only LE states are detected. Accordingly,f emtosecond transient absorption reveals av ery fast switching (ca. 2ps) from LE (l max = 550 nm) to ICT states (l max = 480 nm) in solution, whereas within HSA the LE species become stabilized and live much longer(up to the ns scale). Interestingly,m olecular dynamics simulation studies confirm that the coplanar orientation is preferred for LAP (or to al esser extent N-LAP) within HSA, explaining the experimental results.

Azetidines are interesting compounds in medicine and chemistry as bioactive scaffolds and synthetic intermediates. However, photochemical processes involved in the generation and fate of azetidine-derived radical ions have scarcely been reported. In this context, the photoreduction of this four-membered heterocycle might be relevant in connection with the DNA (6-4) photoproduct obtained from photolyase. Herein, a stable azabipyrimidinic azetidine (AZT ), obtained from cycloaddition between thymine and 6-azauracil units, is considered to be an interesting model of the proposed azetidine-like intermediate. Hence, its photoreduction and photo-oxidation are thoroughly investigated through a multifaceted approach, including spectroscopic, analytical, and electrochemical studies, complemented by CASPT2 and DFT calculations. Both injection and removal of an electron result in the formation of radical ions, which evolve towards repaired thymine and azauracil units. Whereas photoreduction energetics are similar to those of the cyclobutane thymine dimers, photo-oxidation is clearly more favorable in the azetidine. Ring opening occurs with relatively low activation barriers (<13 kcal mol ) and the process is clearly exergonic for photoreduction. In general, a good correlation has been observed between the experimental results and theoretical calculations, which has allowed a synergic understanding of the phenomenon.

Advances (2016)(2017) in Quantum Chemistry of the Excited State (QCEX) are presented in this book chapter focusing firstly on developments of methodology and excited-state reaction-path computational strategies and secondly on the applications of QCEX to study light-matter interaction in distinct fields of biology, (nano)-technology, medicine and the environment. We highlight in this contribution developments of static and dynamic electron-correlation methods and methodological approaches to determine dynamical properties, recent examples of the roles of conical intersections, novel DNA spectroscopy and photochemistry findings, photo-sensitisation mechanisms in biological structures and the current knowledge on chemiexcitation mechanisms that give rise to light emission (in the chemiluminescence and bioluminescence phenomena).

Photocycloreversion plays a central role in the study of the repair of DNA lesions, reverting them into the original pyrimidine nucleobases. Particularly, among the proposed mechanisms for the repair of DNA (6-4) photoproducts by photolyases, it has been suggested that it takes place through an intermediate characterized by a four-membered heterocyclic oxetane or azetidine ring, whose opening requires the reduction of the fused nucleobases. The specific role of this electron transfer step and its impact on the ring opening energetics remain to be understood. These processes are studied herein by means of quantum-chemical calculations on the two azetidine stereoisomers obtained from photocycloaddition between 6-azauracil and cyclohexene. First, we analyze the efficiency of the electron-transfer processes by computing the redox properties of the azetidine isomers as well as those of a series of aromatic photosensitizers acting as photoreductants and photo-oxidants. We find certain stereodifferentiation favoring oxidation of the cis-isomer, in agreement with previous experimental data. Second, we determine the reaction profiles of the ring-opening mechanism of the cationic, neutral, and anionic systems and assess their feasibility based on their energy barrier heights and the stability of the reactants and products. Results show that oxidation largely decreases the ring-opening energy barrier for both stereoisomers, even though the process is forecast as too slow to be competitive. Conversely, one-electron reduction dramatically facilitates the ring opening of the azetidine heterocycle. Considering the overall quantum-chemistry findings, N,N-dimethylaniline is proposed as an efficient photosensitizer to trigger the photoinduced cycloreversion of the DNA lesion model.

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Research on excited-state chemistry using multiconfigurational quantum chemistry (MQC) allows deep analyses of the involved electronic-structure transformations. Overview of the published bibliography in the 2020 and 2021 period shows predominant MQC research on biomedicine and materials science. Significant work on method developments has been also carried out, and applications in atmospheric chemistry are gaining interest as compared to previous years. Topics that are representative of the progress, and are revised herein, are methodology developments for more realistic predictions of UV/Vis absorption and emission spectra, sunlight chemistry on atmospheric modelling, DNA photostability and photosensitisation, DNA reactivity by ionising radiation, chemiluminescence and self-illuminating photosensitisation, DNA repair mechanisms and quantum computing applications in photochemistry.