RESUMEN
Traditionally, cross-dehydrogenative coupling (CDC) leads to C-N bond formation under basic and oxidative conditions and is proposed to proceed via a two-electron bond formation mediated by carbenium ions. However, the formation of such high-energy intermediates is only possible in the presence of strong oxidants, which may lead to undesired side reactions and poor functional group tolerance. In this work we explore if oxidation under basic conditions allows the formation of three-electron bonds (resulting in "upconverted" highly-reducing radical-anions). The benefit of this "upconversion" process is in the ability to use milder oxidants (e. g., O2 ) and to avoid high-energy intermediates. Comparison of the two- and three-electron pathways using quantum mechanical calculations reveals that not only does the absence of a strong oxidant shut down two-electron pathways in favor of a three-electron path but, paradoxically, weaker oxidants react faster with the upconverted reductants by avoiding the inverted Marcus region for electron transfer.
RESUMEN
Low reorganization free energies are necessary for fast electron transfer (ET) reactions. Hence, rational design of redox proteins with lower reorganization free energies has been a long-standing challenge, promising to yield a deeper understanding of the underlying principles of ET reactivity and to enable potential applications in different energy conversion systems. Herein we report studies of the intramolecular ET from pulse radiolytically produced disulfide radicals to Cu(II) in rationally designed azurin mutants. In these mutants, the copper coordination sphere has been fine-tuned to span a wide range of reduction potentials while leaving the metal binding site effectively undisrupted. We find that the reorganization free energies of ET within the mutants are indeed lower than that of WT azurin, increasing the intramolecular ET rate constants almost 10-fold: changes that are correlated with increased flexibility of their copper sites. Moreover, the lower reorganization free energy results in the ET rate constants reaching a maximum value at higher driving forces, as predicted by the Marcus theory.
Asunto(s)
Azurina/química , Azurina/metabolismo , Azurina/genética , Cobre/química , Disulfuros/química , Espectroscopía de Resonancia por Spin del Electrón , Transporte de Electrón , Cinética , Modelos Moleculares , Mutagénesis Sitio-Dirigida , Ingeniería de Proteínas , Pseudomonas aeruginosa/genética , Pseudomonas aeruginosa/metabolismo , Radiólisis de Impulso , Proteínas Recombinantes/química , Proteínas Recombinantes/genética , Proteínas Recombinantes/metabolismo , TermodinámicaRESUMEN
The role of another excited pyrene molecule in the vicinity of an excited pyrene molecule undergoing electron transfer (ET) process with a N,N,-dimethylaniline molecule has been explored in non-polar and polar solvents where specifically solvent mediated nature of ET is prominent. Analytic expression has been derived for the dependence of ET kinetics on pyrene concentration in the experimental conditions normally encountered in sub-nanosecond time correlated single photon counting (TCSPC) apparatus which is mostly used. The life time data has been explained on the basis of present day understanding.