Here we report a computational method to improve efficiency of a de novo designed Kemp Eliminase enzyme KE15, by identifying mutations that enhance electric fields and chemical positioning of the substrate that contribute to free energy stabilization of the transition state.Starting from the design that has a kcat/KM of 27 M -1 s -1 , the most improved variant introduced 4 computationally targeted mutations to yield a kcat/KM of 403 M -1 s -1 , with almost all of the enzyme improvement realized through a 43-fold improvement in kcat, indicative of a direct impact on the chemical step. This work raises the prospect of computationally designing enzymes that achieve better efficiency with more minimal experimental intervention using electric field optimization as guidance. † authors contributed equally

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We study the kinetics of the lateral hole transfer occurring between dye molecules anchored at the surface of the metal oxide in Dye Sensitized Solar Cells (DSSC). We use Marcus' charge transfer rate equation for which we need the electronic coupling between two molecules (J) and the reorganization energy (λtot). In DSSC the medium surrounding the dyes is highly polar. This means that the contribution of the solvent to the reorganization energy cannot be neglected. Here we elaborate a method to calculate, from first principles, the total (i.e., inner- and outer-sphere) reorganization energy of hole exchange between ruthenium dyes. The influence of the solvent and of the ions in the solvent is incorporated. The inner-sphere reorganization energy depends on the nature of the dye, 0.1 eV for ruthenium dyes with CN ligands, 0.2 eV for ruthenium dyes with NCS ligands. In acetonitrile, the solvent reorganization energy contributes for at least 80% of the total giving a total reorganization energy of around 0.86 eV for ruthenium dyes with CN ligands and 0.95 eV for ruthenium dyes with NCS ligands. We use these results to estimate the rate of hole transfer within Marcus theory. We suggest that low diffusion coefficients observed experimentally may arise from the high polarity of the medium rather than by the chemical structure of the dye.

Charge recombination between oxidized dyes attached to mesoporous TiO2 and electrons in the TiO2 was studied in inert electrolytes using transient absorption spectroscopy. Simultaneously, hole transport within the dye monolayers was monitored by transient absorption anisotropy. The rate of recombination decreased when hole transport was inhibited selectively, either by decreasing the dye surface coverage or by changing the electrolyte environment. From Monte Carlo simulations of electron and hole diffusion in a particle, modelled as a cubic structure, we identify the conditions under which hole lifetime depends on the hole diffusion coefficient for the case of normal (disorder free) diffusion. From simulations of transient absorption and transient absorption anisotropy, we find that the rate and the dispersive character of hole transport in the dye monolayer observed spectroscopically can be explained by incomplete coverage and disorder in the monolayer. We show that dispersive transport in the dye monolayer combined with inhomogeneity in the TiO2 surface reactivity can contribute to the observed stretched electron-hole recombination dynamics and electron density dependence of hole lifetimes. Our experimental and computational analysis of lateral processes at interfaces can be applied to investigate and optimize charge transport and recombination in solar energy conversion devices using electrodes functionalized with molecular light absorbers and catalysts.

Measured hole diffusion coefficients in dye monolayers are larger than can be explained by a charge hopping model with a static distribution of parameters describing intermolecular hole transfer. We show that large amplitude fluctuations of the tethered dye configurations on the surface could explain the observed diffusion rates by enabling charges trapped in particular configurations to escape as the dye orientations change. We present a multiscale model of hole transport which includes the effect of dynamic rearrangement of the monolayer of anchored dyes. Conformations of pairs of indolene dye molecules (both D102 and D149) were generated by a rigid molecular packing algorithm and Car-Parrinello molecular dynamics to mimic the conformational and configurational disorder of a dye monolayer adsorbed to an anatase (101) titanium dioxide surface. The electronic coupling (J i j) for each pair of neighbouring dyes was calculated to build distributions representing the disorder in a real system. These