Photostability Mechanisms in Human γB-Crystallin: Role of the Tyrosine Corner Unveiled by Quantum Mechanics and Hybrid Quantum Mechanics/Molecular Mechanics Methodologies

The tyrosine corner is proposed as a featured element to enhance photostability in human γB-crystallin when exposed to UV irradiation. Different ultrafast processes were studied by multiconfigurational quantum chemistry coupled to molecular mechanics: photoinduced singlet–singlet energy, electron and proton transfer, as well as population and evolution of triplet states. The minimum energy paths indicate two possible UV photoinduced events: forward–backward proton-coupled electron transfer providing to the system a mechanism for ultrafast internal conversion, and energy transfer, leading to fluorescence or phosphorescence. The obtained results are in agreement with the available experimental data, being in line with the proposed photoinduced processes for the different tyrosine environments within γB-crystallin

Photoinduced Proton Transfer as a Possible Mechanism for Highly Efficient Excited-State Deactivation in Proteins

CASSCF//CASPT2 pathways for a two-glycine minimal model system show that photoinduced electron-driven forward and backward proton transfer could play an important role for the stability of proteins against damage by UV radiation, when a hydrogen bond is located between the two amino acids. The overall photoinduced process involves two electron and proton transfer processes (forward and backward) and results in the reformation of the initial closed-shell electronic structure of the system

Mechanochemical Tuning of Pyrene Absorption Spectrum Using Force Probes

Control of absorption spectra in chromophores is a fundamental aspect of many photochemical and photophysical processes as it constitutes the first step of the global photoinduced process. Here we explore the use of mechanical forces to modulate the light absorption process. Specifically, we develop a computational formalism for determining the type of mechanical forces permitting a global tuning of the absorption spectrum. This control extends to the excitation wavelength, absorption bands overlap, and oscillator strength. The determination of these optimal forces permits us to rationally guide the design of new mechano-responsive chromophores. Pyrene has been chosen as the case study for applying these computational tools because significant absorption spectra information is available for the chromophore as well as for different strained derivatives. Additionally, pyrene presents a large flexibility, which makes it a good system to test the inclusion of force probes as the strategy to exert forces on the system

Modulating Nitric Oxide Release by <i>S</i>-Nitrosothiol Photocleavage: Mechanism and Substituent Effects

The photochemistry and photophysics of a series of <i>S</i>-nitrosothiols (RSNOs) have been studied computationally. The photocleavage mechanism of the model compound CH₃SNO to release CH₃S· and ·NO was studied at the CASPT2 level resulting in a barrierless process when irradiating in the visible region (S₁), in the near UV region (S₂) and for photosensitized (T₁) reaction. The absorption energy required to initiate photocleavage was calculated at the CASPT2 and B3P86 levels showing the possibility of the modulation of NO release by RSNO photoactivation as a function of the substituent R. Good correlations between the wavelengths of the lowest energy ¹(<i>n</i>,π*) and ¹(π,π*) transitions of aryl <i>S</i>-nitrosothiols and the corresponding Hammett constants of the substituents have been obtained

Chiral Hydrogen Bond Environment Providing Unidirectional Rotation in Photoactive Molecular Motors

Generation of a chiral hydrogen bond environment in efficient molecular photoswitches is proposed as a novel strategy for the design of photoactive molecular motors. Here, the following strategy is used to design a retinal-based motor presenting singular properties: (i) a single excitation wavelength is needed to complete the unidirectional rotation process (360°); (ii) the absence of any thermal step permits the process to take place at low temperatures; and (iii) the ultrafast process permits high rotational frequencies

Study of Model Systems for Bilirubin and Bilin Chromophores: Determination and Modification of Thermal and Photochemical Properties

Bilin chromophores and bilirubin are involved in relevant biological functions such as light perception in plants and as protective agents against Alzheimer and other diseases. Despite their extensive use, a deep rationalization of the main factors controlling the thermal and photochemical properties has not been performed yet, which in turn hampers further applications of these versatile molecules. In an effort to understand those factors and allow control of the relevant properties, a combined experimental and computational study has been carried out for diverse model systems to understand the interconversion between <i>Z</i> and <i>E</i> isomers. In this study, we have demonstrated the crucial role of steric hindrance and hydrogen-bond interactions in thermal stability and the ability to control them by designing novel compounds. We also determined several photochemical properties and studied the photodynamics of two model systems in more detail, observing a fast relaxation of the excited state shorter than 2 ps in both cases. Finally, the computational study allowed us to rationalize the experimental evidence

Toward an Optomechanical Control of Photoswitches by Tuning Their Spectroscopical Properties: Structural and Dynamical Insights into Azobenzene

A new methodology to calculate efficiently the absorption spectrum of a single molecule when subjected to mechanical stress is presented. As example, the developed methodology was applied to <i>cis-</i> and <i>trans</i>-azobenzene, commonly used as photoswitch in a wide variety of applications. The results show that both ¹(<i>n</i>,π*) and ¹(π,π*) optical transitions can be efficiently modulated by applying an external force. A structural analysis was performed to evaluate the role of each internal coordinate in the excitation process, taking into account the application of external forces at different positions of azobenzene. Moreover, stress–strain curves were calculated in order to determine the maximum applicable forces within the elastic region, highlighting notable differences between the mechanical properties of <i>cis</i>- and <i>trans</i>-azobenzene conformers. The optomechanical work obtained by elongation and compression steps is calculated for a single azobenzene molecule and compared to available experimental data. Finally, the implications derived from the application of azobenzene as main chain component of a linear polymer acting as a photoinduced motor are discussed

C–H Functionalization of BN-Aromatics Promoted by Addition of Organolithium Compounds to the Boron Atom

Addition of an organolithium compound to a BN-phenanthrene with embedded B and N atoms is proposed to result in coordination of RLi to the boron atom. This coordination, supported by NMR spectroscopy and DFT calculations, increases the nucleophilicity of the system in the β position to the N atom and is therefore a useful tool for promoting regioselective C–H functionalization of BN aromatics

Mechanical Forces Alter Conical Intersections Topology

Photoreactivity can be influenced by mechanical forces acting over a reacting chromophore. Nevertheless, the specific effect of the external forces in the photoreaction mechanism remains essentially unknown. Conical intersections are key structures in photochemistry, as they constitute the funnels connecting excited and ground states. These crossing points are well known to provide valuable information on molecular photoreactivity, including crucial aspects as potential photoproducts which may be predicted by just inspection of the branching plane vectors. Here, we outline a general framework for understanding the effect of mechanical forces on conical intersections and their implications on photoreactivity. Benzene S₁/S₀ conical intersection topology can be dramatically altered by applying less than 1 nN force, making the peaked pattern of the intersection become a sloped one, also provoking the transition state in the excited state to disappear. Both effects can be related to an increase in the photostability as the conical intersection becomes more accessible, and its topology in this case favors the recovery of the initial reactant. The results indicate that the presence of external forces acting over a chromophore have to be considered as a potential method for photochemical reactivity control

Synthesis, Optical Properties, and Regioselective Functionalization of 4a-Aza-10a-boraphenanthrene

4a-Aza-10a-boraphenanthrene has been synthesized in only four steps from commercially available materials with a remarkable overall yield of 62%. In contrast to other BN-isosteres of phenathrene, this isomer is weakly fluorescent, which has been explained by means of computational studies that found a low energy conical intersection for the nonradiative deactivation of the excited state. Moreover, a completely regioselective functionalization of 4a-aza-10a-boraphenanthrene at C-1 by reaction with activated electrophiles has been achieved