Dynamics of Oxygen-Independent Photocleavage of Blebbistatin as a One-Photon Blue or Two-Photon Near-Infrared Light-Gated Hydroxyl Radical Photocage

Development of versatile, chemically tunable photocages for photoactivated chemotherapy (PACT) represents an excellent opportunity to address the technical drawbacks of conventional photodynamic therapy (PDT) whose oxygen-dependent nature renders it inadequate in certain therapy contexts such as hypoxic tumors. As an alternative to PDT, oxygen free mechanisms to generate cytotoxic reactive oxygen species (ROS) by visible light cleavable photocages are in demand. Here, we report the detailed mechanisms by which the small molecule blebbistatin acts as a one-photon blue light-gated or two-photon near-infrared light-gated photocage to directly release a hydroxyl radical (•OH) in the absence of oxygen. By using femtosecond transient absorption spectroscopy and chemoselective ROS fluorescent probes, we analyze the dynamics and fate of blebbistatin during photolysis under blue light. Water-dependent photochemistry reveals a critical process of water-assisted protonation and excited state intramolecular proton transfer (ESIPT) that drives the formation of short-lived intermediates, which surprisingly culminates in the release of •OH but not superoxide or singlet oxygen from blebbistatin. CASPT2//CASSCF calculations confirm that hydrogen bonding between water and blebbistatin underpins this process. We further determine that blue light enables blebbistatin to induce mitochondria-dependent apoptosis, an attribute conducive to PACT development. Our work demonstrates blebbistatin as a controllable photocage for •OH generation and provides insight into the potential development of novel PACT agents

Ground-State Intermolecular Proton Transfer of N₂O₄ and H₂O: An Important Source of Atmospheric Hydroxyl Radical?

To evaluate the significance of the generation of atmospheric hydroxyl radical from reaction of N₂O₄ with H₂O, CASPT2//CASSCF as well as CASPT2//CASSCF/Amber QM/MM approaches were employed to map the minimum-energy profiles of sequential reactions, NO₂ dimerization and ground-state intermolecular proton transfer of <i>trans</i>-ONONO₂ as well as the photolysis of HONO. A highly efficient ground-state intermolecular proton transfer of <i>trans</i>-ONONO₂ is found to dominate the generation of hydroxyl radical under atmospheric conditions. Although proton transfer occurs with high efficiency, the precursor reaction of dimerization producing <i>trans</i>-ONONO₂ has to overcome a 17.1 kcal/mol barrier and cannot compete with the barrierless channel of symmetric O₂N–NO₂ formation from isolated NO₂ monomers. Our computations reveal that the photolysis of HONO without a barrier definitely makes significant contributions to the concentration of the atmospheric hydroxyl radical, but its importance is influenced by the lack of <i>trans</i>-ONONO₂ isomer in the atmospheric environment

Nonadiabatic Curve-Crossing Model for the Visible-Light Photoredox Catalytic Generation of Radical Intermediate via a Concerted Mechanism

Photoredox catalysis relies on the excited-state single-electron transfer (SET) processes to drive a series of unique bond-forming reactions. In this work accurate electronic structure calculations at the CASPT2//CASSCF/PCM level of theory together with the kinetic assessment of SETs and intersystem crossing are employed to provide new insights into the SET initiation, activation, and deactivation by calculating the SET paths for a paradigm example of photoredox α-vinylation reaction mediated by iridium­(III) catalysts. The concerted photocatalysis mechanism described by the nonadiabatic curve-crossing model, in essence of Marcus electron transfer theory, is first applied for the mechanistic description of the SET events in visible-light photoredox catalysis. The C–C bond functionalization has been revealed to take place in a concerted manner along an energy-saving pathway, in which the generated α-amino radical is unlikely independent existence but strongly depends on the mutual interaction with different substrates. These mechanistic insights offer a plausible picture for the excited-state SET-mediated chemical transformations that should be applicable to further studies of photoredox catalysis in organic chemistry

<i>ON–OFF</i> Mechanism of a Fluorescent Sensor for the Detection of Zn(II), Cd(II), and Cu(II)Transition Metal Ions

An ab initio multiconfigurational (CASPT2//CASSCF) approach has been employed to map radiative and nonradiative relaxation pathways for a cyclam-methylbenzimidazole fluorescent sensor and its metal ion (Zn²⁺, Cd²⁺, and Cu²⁺) complexes to provide an universal understanding of <i>ON</i>–<i>OFF</i> fluorescent mechanisms for the selective identification of these metal ions. The photoinduced electron transfer (PET) between the receptor and the signaling unit is quantitatively attributed for the first time to a newly generated transition of S₀→S_CT(¹nπ*), which is a typical ¹nπ* excitation but exhibits a significant charge transfer character and zwitterionic radical configuration. The present study contributes the two theoretical models of the competitive coexistence of radiative/nonradiative decay channel in ¹ππ*/S_CT(¹nπ*) states for the detection of metal ions with d¹⁰ configuration (i.e., Zn²⁺, Cd²⁺, etc.) and a downhill ladder relaxation pathway through multi nona-diabatic relays for the probing of d⁹ cations (Cu²⁺, etc.). These computational results will establish a benchmark for <i>ON</i>–<i>OFF</i> mechanisms of a fluorescent sensor that coordinates various transition metal ions with different electron configuration and radius

Various collective motions under different combinations of <i>N</i> and <i>K</i>.

<p>((a) <i>N</i> = 15, <i>K</i> = 7; (b) <i>N</i> = 15, <i>K</i> = 8; (c) <i>N</i> = 16, <i>K</i> = 7; (d) <i>N</i> = 16, <i>K</i> = 10; (e) <i>N</i> = 15, <i>K</i> = 9; (f) <i>N</i> = 20, <i>K</i> = 11).</p

Distribution of the out-degrees of individuals for different steps and different combinations of <i>N</i> and <i>K</i>.

<p>((a) <i>N</i> = 16, <i>K</i> = 7, step = 1; (b) <i>N</i> = 16, <i>K</i> = 7, step = 3000; (c) <i>N</i> = 23, <i>K</i> = 6, step = 1; (d) <i>N</i> = 23, <i>K</i> = 6, step = 1020; (e) <i>N</i> = 23, <i>K</i> = 6, step = 1021; (f) <i>N</i> = 23, <i>K</i> = 6, step = 5000).</p

Two subgroups at initial locations under different combinations of <i>N</i> and <i>K</i>.

<p>((a) <i>N</i> = 15, <i>K</i> = 5; (b) <i>N</i> = 15, <i>K</i> = 7).</p

A simple example of interactions between individuals based on the rules in Eq (1).

<p>(<i>N</i> = 5, <i>K</i> = 2, <i>d</i>_<i>l</i> = 0, <i>d</i>_<i>e</i> = 5.2, <i>d</i>_<i>h</i> = 12, (<i>d</i>_<i>e</i> + <i>d</i>_<i>h</i>)/2 = 8.6).</p

Two kinds of fission behavior under different combinations of <i>N</i> and <i>K</i>.

<p>((a) <i>N</i> = 22, <i>K</i> = 5; (b) <i>N</i> = 23, <i>K</i> = 6).</p

The quantitative variation of various network properties under different combinations of <i>N</i> and <i>K</i>.

<p>((a) <i>N</i> = 16, <i>K</i> = 7; (b) <i>N</i> = 23, <i>K</i> = 6).</p