6. Rehybridization of the Acceptor (RICT) 3908 7. Planarization of the Molecule (PICT) 3909 III. Fluorescence Spectroscopy 3909 A. Solvent Effects and the Model Compounds 3909 1. Solvent Effects on the Spectra 3909 2. Steric Effects and Model Compounds 3911 3. Bandwidths 3913 4. Isoemissive Points 3914 B. Dipole Moments 3915 C. Radiative Rates and Transition Moments 3916 1. Quantum Yields and Radiationless Deactivation Processes 3916

Structural Changes Accompanying Intramolecular Electron Transfer: Focus on Twisted Intramolecular Charge-Transfer States and Structures

Recent developments in the chemistry of dihydropyridines

The enthalpy changes of 47 five-membered heterocyclic compounds (ZH) [33 substituted 2,3-dihydro-2-phenylbenzo[d]imidazoles (1H-5H), 9 substituted 2,3-dihydro-2-phenylbenzo[d]thiazoles (6H), and 5 substituted 2,3-dihydro-2-phenylbenzo[d]oxazoles (7H)] as a class of very important organic hydride donors to release hydride anion were determined by using titration calorimetry. The result shows that the enthalpy change scale of the 47 ZH in acetonitrile ranges from 49.0 to 93.4 kcal/mol. Such a long energy scale evidently shows that the 47 ZH can construct a large and useful library of organic hydride donors, which can provide various organic hydride donors that the hydride-releasing enthalpies are known. The enthalpy changes of the 47 ZH to release hydrogen atom and the 47 ZH+• to release proton and hydrogen atom were also evaluated by using relative thermodynamic cycles according to Hess' law. The results show:  (1) the enthalpy change scale of the 47 ZH to release hydrogen atom covers a range from 71.8 to ...

Hydride, Hydrogen Atom, Proton, and Electron Transfer Driving Forces of Various Five-Membered Heterocyclic Organic Hydrides and Their Reaction Intermediates in Acetonitrile

Methods for the regeneration of nicotinamide coenzymes

Hypoxic tumours are a major problem for cancer photodynamic therapy. Here, we show that photoredox catalysis can provide an oxygen-independent mechanism of action to combat this problem. We have designed a highly oxidative Ir(III) photocatalyst, [Ir(ttpy)(pq)Cl]PF6 ([1]PF6, where 'ttpy' represents 4'-(p-tolyl)-2,2':6',2''-terpyridine and 'pq' represents 3-phenylisoquinoline), which is phototoxic towards both normoxic and hypoxic cancer cells. Complex 1 photocatalytically oxidizes 1,4-dihydronicotinamide adenine dinucleotide (NADH)-an important coenzyme in living cells-generating NAD• radicals with a high turnover frequency in biological media. Moreover, complex 1 and NADH synergistically photoreduce cytochrome c under hypoxia. Density functional theory calculations reveal π stacking in adducts of complex 1 and NADH, facilitating photoinduced single-electron transfer. In cancer cells, complex 1 localizes in mitochondria and disrupts electron transport via NADH photocatalysis. On light irradiation, complex 1 induces NADH depletion, intracellular redox imbalance and immunogenic apoptotic cancer cell death. This photocatalytic redox imbalance strategy offers a new approach for efficient cancer phototherapy.

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Targeted photoredox catalysis in cancer cells

Two mechanisms of the interconversion of NADH and NAD+ in the coenzyme itself and in its analogues are discussed:  a one-step hydride transfer and a stepwise electron−proton−electron transfer. Direct characterization of the transient species in the stepwise process and inversion of the stability order of the keto and enol tautomers is presented. The nonenzymatic and enzyme-mediated reactions of selected pyridinium salts that affect the NADH ⇆ NAD+ equilibrium are discussed in terms of their potential cytotoxicity.

Transient species in the stepwise interconversion of NADH and NAD

The radical cations generated from three NADH model compounds were investigated with particular emphasis on the possible formation of tautomeric enol radical cations. To a small extent, hydrogen transfer occurs upon ionization of all three compounds in argon matrixes. In the bicyclic seven-membered ring derivative enolization continues in a thermal reaction at 12 K (with highly dispersive kinetics), or upon visible irradiation. These processes are not observed in methylcyclohexane glasses at 77 K, which indicates that a cage effect is preventing the attainment of a reactive conformation in this medium. The assignment of electronic spectra of the primary (keto) cations and of their tautomeric (enol) forms is supported by CASSCF/CASPT2 and TD-B3LYP calculations.

Jerzy Gȩbicki

Papers

Transient species in the stepwise interconversion of NADH and NAD

Hydrogen-Transferred Radical Cations of NADH Model Compounds. 1. Spontaneous Tautomerization

Evidence for a ketene intermediate in the photochemical transformation of matrix-isolated o-nitrobenzaldehyde

Structural Aspects and Rearrangement of Radical Cations Generated from NADH Analogues

Electronic absorption spectra of aliphatic diamine radical cations. Conformation‐dependent charge delocalization