Although QM/MM calculations are the primary current tool for modeling enzymatic reactions, the reliability of such calculations can be limited by the size of the QM region. Thus, we examine in this work the dependence of QM/MM calculations on the size of the QM region, using the reaction of catechol-O-methyl transferase (COMT) as a test case. Our study focuses on the effect of adding residues to the QM region on the activation free energy, obtained with extensive QM/MM sampling. It is found that the sensitivity of the activation barrier to the size of the QM is rather limited, while the dependence of the reaction free energy is somewhat larger. Of course, the results depend on the inclusion of the first solvation shell in the QM regions. For example, the inclusion of the Mg2+ ion can change the activation barrier due to charge transfer effects. However, such effects can easily be included in semiempirical approaches by proper parametrization. Overall, we establish that QM/MM calculations of activation barriers of enzymatic reactions are not highly sensitive to the size of the QM region, beyond the immediate region that describes the reacting atoms.

Computer-aided enzyme design presents a major challenge since in most cases it has not resulted in an impressive catalytic power. The reasons for the problems with computational design include the use of nonquantitative approaches, but they may also reflect other difficulties that are not completely obvious. Thus, it is very useful to try to learn from the trend in directed evolution experiments. Here we explore the nature of the refinement of Kemp eliminases by directed evolution, trying to gain an understanding of related requirements from computational design. The observed trend in the directed evolution refinement of KE07 and HG3 are reproduced, showing that in the case of KE07 the directed evolution leads to ground-state destabilization, whereas in the case of HG3 the directed evolution leads to transition-state stabilization. The nature of the different paths of the directed evolution is examined and discussed. The present study seems to indicate that computer-aided enzyme design may require more than calculations of the effect of single mutations and should be extended to calculations of the effect of simultaneous multiple mutations (that make a few residues preorganized effectively). However, the analysis of two known evolution paths can still be accomplished using the relevant sequences and structures. Thus, by comparing two directed evolution paths of Kemp eliminases we reached the important conclusion that the more effective path leads to transition-state stabilization.

One of the leading goals in contemporary chemical catalysis is to render improved efficiency to existing catalytic protocols. A few pertinent trends can readily be noticed from the current literature encompassing both catalysis development and applications. First, there has been an unprecedented growth in the use of metal-free organocatalytic methods toward realizing a plethora of synthetic targets. In parallel, the availability of newer and more efficient transition metal catalytic methods for the synthesis of complex molecules has become a reality over the years. The most recent developments indicate the emergence of multicatalytic approaches under one-pot reaction conditions, wherein the complementary attributes of two or more catalysts are made to work together. This domain, known as cooperative catalysis, is showing signs of immense promise. The mechanistic underpinnings of both of these forms of catalysis have been investigated by using a range of computational chemistry tools. With the availability of improved accuracy in computational methods aided by ever increasing computing technologies, the exploration of potential energy surfaces relating to complex cooperative catalytic systems has become more affordable. In this review, we have chosen a select set of examples from the emerging domain of cooperative catalysis to illustrate how computational methods have been effectively used toward gaining vital molecular insights. Emphasis is placed on mechanistic details, energetics of reaction, and, more importantly, on transition states that are responsible for stereoselectivity in asymmetric cooperative catalytic reactions.

Rational enzyme design presents a major challenge that has not been overcome by computational approaches. One of the key challenges is the difficulty in assessing the magnitude of the maximum possible catalytic activity. In an attempt to overcome this challenge, we introduce a strategy that takes an active enzyme (assuming that its activity is close to the maximum possible activity), design mutations that reduce the catalytic activity, and then try to restore that catalysis by mutating other residues. Here we take as a test case the enzyme haloalkane dehalogenase (DhlA), with a 1,2-dichloroethane substrate. We start by demonstrating our ability to reproduce the results of single mutations. Next, we design mutations that reduce the enzyme activity and finally design double mutations that are aimed at restoring the activity. Using the computational predictions as a guide, we conduct an experimental study that confirms our prediction in one case and leads to inconclusive results in another case with 1,2-dichloroethane as substrate. Interestingly, one of our predicted double mutants catalyzes dehalogenation of 1,2-dibromoethane more efficiently than the wild-type enzyme.

Cooperative multicatalytic methods are steadily gaining popularity in asymmetric catalysis. The use of chiral Brønsted acids such as phosphoric acids in conjunction with a range of transition metals has been proven to be effective in asymmetric synthesis. However, the lack of molecular-level understanding and the accompanying ambiguity on the role of the chiral species in stereoinduction continues to remain an unresolved puzzle. Herein, we intend to disclose some novel transition state models obtained through DFT(B3LYP and M06) computations for a quintessential reaction in this family, namely, palladium-catalyzed asymmetric Tsuji-Trost allylation of aldehydes. The aldehyde is activated as an enamine by the action of a secondary amine (organocatalysis), which then adds to an activated Pd-allylic species (transition metal catalysis) generated through the protonation of allyic alcohol by chiral BINOL-phosphoric acid (Brønsted acid catalysis). We aim to decipher the nature of chiral BINOL-phosphates and their role in creating a quaternary chiral carbon atom in this triple catalytic system. The study reports the first transition state model capable of rationalizing chiral counterion-induced enantioselectivity. It is found that the chiral phosphate acts as a counterion in the stereocontrolling event rather than the conventional ligand mode.

Increasing number of reports in the most recent literature convey the use of palladium and Brønsted acids as cooperative catalytic partners. However, the mechanistic understanding of several such cooperative catalytic reactions and the origin of cooperativity continue to remain limited. In transition metal catalysis, it is typically assumed that the native ligands, such as the acetates in palladium acetate, are retained throughout the catalytic cycle. Herein, we convey the significance of invoking ligand exchanges in transition metal catalysis by using the mechanism of a representative cooperative dual-catalytic reaction. Density functional theory (M06 and B3LYP) computations have been employed to decipher the mechanism of Pd(II)-Brønsted acid catalyzed migratory ring expansion reaction of an indenyl cyclobutanol to a spirocyclic indene bearing a quaternary carbon. The molecular role of water, benzoquinone and phosphoric acid has been probed by computing the energetics using several combinations of all these as ligands on palladium. Of the two key mechanistic possibilities examined, a Wacker-type pathway (involving a semipinacol ring expansion of cyclobutanol followed by a reductive elimination) is found to be energetically more preferred over an allylic pathway wherein the ring expansion in a Pd-π-allyl intermediate occurs subsequent to the initial allylic C-H activation. The Gibbs free energies of the transition states with the native palladium acetate are much higher than a Pd-bis-phosphate species generated through ligand exchanges.

The first catalytic enantioselective Fischer indolization of prochiral diketones containing enantiotopic carbonyl groups is developed and shown to proceed through dynamic kinetic resolution (DKR). Catalyzedb yt he combination of as pirocyclic chiral phosphoric acid and ZnCl 2 (Lewis acid assisted Brønsted acid), this direct approach combines 2,2disubstituted cyclopentane-1,3-diones with N-protected phenylhydrazines to furnish cyclopenta[b]indole derivatives containing an all-carbon quaternary stereocenter with good to excellent enantioselectivities. Scheme 1. Catalytic enantioselective Fischer indolizations.

Insights into chiral induction for an asymmetric sulfoxidation reaction involving a single oxygen atom transfer are gained through analyzing the stereocontrolling transition states. The fitting of the substrate into the chiral cavity of a new class of imidodiphosphoric Brønsted acids, as well as weak CH⋅⋅⋅π and CH⋅⋅⋅O noncovalent interactions, are identified as responsible for the observed chiral induction.