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

CG models of phospholipids.

(A) λ-shaped DMPC. (B) h-shaped DMPC. (C) h-shaped DMPG. (D) h-shaped DOPC. Red and black circles refer to head groups and hydrophobic tails, respectively.</p

Snapshots of (A) a constrained DMPC lipid and (B) a DPPC lipid at different locations relative to the center of the membrane.

Each color represents the configuration of the lipid at a different time, taken from the last 5 × 105 time steps and separated by 5 × 104 time steps. The overlaying of the configurations illustrates the flexibility and orientation of the constrained lipid molecule. Head groups and tails of the lipid are represented by thick and thin bonds, respectively.</p

Equilibrium bonds, angles and corresponding force constants for amino acids: B stands for backbone bead and S represents side-chain bead.

Equilibrium bonds, angles and corresponding force constants for amino acids: B stands for backbone bead and S represents side-chain bead.</p

Snapshots of a restrained (A) DMPC and (B) DPPC lipid at different locations relative to the membrane center.

At each location 11 independent frames in equilibrium states are presented. Different frames are represented by different colors. Thick bonds stand for head groups, while thin bonds stand for hydrophobic tails.</p

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

Snapshots of (A) fluid phase DMPC and (B) gel phase DPPC lipid bilayers at various <i>a</i>_prj.

Snapshots of (A) fluid phase DMPC and (B) gel phase DPPC lipid bilayers at various aprj.</p

Adiabatic and Nonadiabatic Bond Cleavages in Norrish Type I Reaction

One of the fundamental photoreactions for ketones is Norrish type I reaction, which has been extensively studied both experimentally and theoretically. Its α bond-cleavage mechanisms are usually explained in an adiabatic picture based on the involved excited-state potential energy surfaces, but scarcely investigated in terms of a nonadiabatic picture. In this work, the S1 α bond-cleavage reactions of CH3OC(O)Cl have been investigated by using the CASSCF and MRCI-SD calculations, and the ab initio based time-dependent quantum wavepacket simulation. The numerical results indicate that the photoinduced dissociation dynamics of CH3OC(O)Cl could exhibit strong nonadiabatic bond-fission characteristics for the S1 α C–Cl bond cleavage, while the dynamics of the S1 α C–O bond cleavage is mainly of adiabatic characteristics. This nonadiabatic mechanism for Norrish type I reaction of CH3OC(O)Cl is uncovered for the first time. The quantum wavepacket dynamics, based on the reduced-dimensional coupled potential energy surfaces, to some extent illustrates the significance of the nonadiabatic effect in the transition-state region on the dynamics of Norrish type I reaction

Equilibrium bonds, angles and corresponding force constants for DMPC, DMPG, DPPC, and DOPC lipids.

<p>The angle with the superscript a and b is for DPPC and DOPC lipid, respectively.</p

Snapshots of self-assemblies of DMPC lipids simulated by different scaling factor <i>s</i>.

<p>(A-I) <i>s</i> equals to 0.1–0.9. Cross-sectional views of half-cut micelles or vesicles are also given in (G-H).</p