Design, synthesis, and evaluation of a biomimetic artificial photolyase model.

Two new artificial photolyase models that recognize pyrimidine dimers in protic and aprotic organic solvents as well as in water through a combination of charge and hydrogen-bonding interactions and use a mimic of the flavine to achieve repair through reductive photoinduced electron transfer are presented. Fluorescence and NMR titration studies show that it forms a 1:1 complex with pyrimidine dimers with binding constants of approximately 10(3) M(-1) in acetonitrile or methanol, while binding constants in water at pH 7.2 are slightly lower. Excitation of the complex with visible light leads to clean and rapid cycloreversion of the pyrimidine dimer through photoinduced electron transfer catalysis. The reaction in water is significantly faster than in organic solvents. The reaction slows down at higher conversions due to product inhibition.

Sterically crowded bicyclo[1.1.0]butane radical cations.

The variability of carbon-carbon single bonds by steric and electronic effects is probed by DFT calculations of sterically crowded bicyclo[1.1.0]butanes and their radical cations. The interplay of sterics and electronics on the gradual weakening and breaking of bonds was studied by investigating bridgehead substitution in 1,3-di-tert-butylbicyclo[1.1.0]butane and 2,2',4,4'-tetramethyl-1,3-di-tert-butylbicyclo[1.1.0]butane and geminal substitution in 2,2'-di-tert-butylbicyclo[1.1.0]butane and 2,2',4,4'-tetra-tert-butylbicyclo[1.1.0]butane. Bridgehead substitution leads to a lengthening of the central bond, whereas bisubstitution on the geminal carbon leads to a shortening of this bond due to a Thorpe-Ingold effect. Although the character of the central bond can be modulated by substitution and electron transfer over a range of 0.35 A, the state forbidden ring planarization does not occur. Sterically crowded bicyclo[1.1.0]butane radical cations are therefore promising candidates for the investigation of extremely long carbon-carbon single bonds.

Isotope effects and the mechanism of an electron-transfer-catalyzed Diels-Alder reaction.

The electron-transfer-catalyzed Diels-Alder reaction of indole and 1,3-cyclohexadiene was studied by a combination of experimental and theoretical methods. The (13)C kinetic isotope effects were determined at natural abundance by NMR methodology. B3LYP/6-31G* calculations allow for the first time a quantitatively accurate description of the different possible pathways and provide the basis for an analysis of the experimentally observed isotope effects. The computational results, in conjunction with experimental observations, show that the reaction has a stepwise mechanism that is initiated by attack of the diene into the 3-position of the indole. Numerical simulation of the experimentally observed isotope effects shows that the first step is rate-determining and that the electron exchange in the reactant contributes partially to the overall isotope effect. The combination of electronic structure theory, experimental isotope effects, and numerical simulation thus allows a detailed analysis of a complex reaction mechanism.