Locating Minimum Energy Crossings of Different Spin States Using the Fragment Molecular Orbital Method

Abstract

Spin-dependent processes involving nonadiabatic transitions between electronic states with different spin multiplicities play important roles in the chemistry of complex systems. The rates of these processes can be predicted based on the molecular properties at the minimum energy crossing point (MECP) between electronic states. We present the development of the MECP search technique within the fragment molecular orbital (FMO) method applicable to large complex systems. The accuracy and scalability of the new method is demonstrated on several models of the metal–sulfur protein rubredoxin. The effect of the model size on the MECP geometry and relative energy is discussed. The fragment energy decomposition and spin density delocalization analyses reveal how different protein residues and solvent molecules contribute to stabilization of the spin states. The developed FMO-MECP method can help to clarify the role of nonadiabatic spin-dependent processes in complex systems and can be used for designing mutations aimed at controlling these processes in metalloproteins, including spin-dependent catalysis and electron transfer.

DpgC‐Catalyzed Peroxidation of 3,5‐Dihydroxyphenylacetyl‐CoA (DPA‐CoA): Insights into the Spin‐Forbidden Transition and Charge Transfer Mechanisms**

“…It requires the characterization of all the concurrent potential energy surfaces (PESs), for at least two different spin multiplicities. For these reactions, the minimum energy crossing point (MECP) between the two PESs of different spin multiplicities plays the role of the effective barrier [44–49] . To determine the rate coefficient of a spin‐forbidden reaction, it is also necessary to calculate the SOC.…”

“…For typical values of SOC, the hopping probability lies between 0.001 and 0.1, which is equivalent to an increase in the activation energy of 1–4 kcal mol −1 at room temperature [47, 49] . Similar methodologies have been successfully applied to the study of some enzymatic reactions, [15, 28–33, 44, 52, 53] including glucose oxidase [32, 33] and p ‐hydroxyphenylacetate hydroxylase, [28] for which O 2 reacts with flavin, as well as the cofactorless oxygenases (1 H )‐3‐hydroxy‐4‐oxoquinaldine 2,4‐dioxygenase (HOD) [29, 31] and nogalamycin monoxygenase, [30] some of them using small active site models.…”

DpgC-Catalyzed Peroxidation of DPA-CoA: Insights onto the Spin-Forbidden Transition and Charge Transfer Mechanisms

“…For these reactions, the minimum energy crossing point (MECP) between the two PESs of different spin multiplicities plays the role of the effective barrier. [44][45][46][47][48][49] To determine the rate coefficient of a spin-forbidden reaction, it is also necessary to calculate the SOC. In the non-adiabatic transition state theory, the transmission coefficient is approximated by the hopping probability, which depends on the magnitude of the SOC, and also on the difference in slope of the PESs along the reac-tion coordinate in the crossing point.…”

“…50 For typical values of SOC, the hopping probability lies between 0.001 and 0.1, which is equivalent to an increase of the activation energy of 1-4 kcal/mol at room temperature. 47,49 Similar methodologies have been successfully applied to the study of some enzymatic reactions, 15,[28][29][30][31][32][33]44,[51][52][53] including glucose oxidase 32,33 and p-hydroxyphenylacetate hydroxylase, 28 for which O 2 reacts with flavin, and the cofactorless oxygenases HOD 29,31 and Nogalamycin Monoxygenase, 30 some of them using small active site models.…”

Modeling Spin-Crossover Dynamics

“…These theories can account for quantum effects, such as tunneling and zero-point vibrational energy (10), and are ideally suited to study kinetics of slow spin-crossovers in large complex systems for which long molecular dynamics simulations are not feasible. NASTs require electronic structure information only at very few nuclear geometries, making these theories compatible with the high-level electronic structure methods (10,11) and molecular fragmentation techniques (12).…”