| Quantum tunneling of atoms has a major impact on chemical reactions, in particular on reactions including hydrogen transfers. Glutamate mutase (GluM) is an enzyme catalyzing a radical reaction with two different types of reaction steps. The first are hydrogen transfers, whereas the second includes a carbon-skeleton rearrangement. GluM is investigated with QM/MM calculations based on density functional theory. We studied the reaction mechanism of glutamate mutase and found the hydrogen transfers to exhibit the highest barriers [1]. Experimental investigations of glutamate mutase resulted in kinetic isotope effects ranging from 4.1 to 35 which suggests hydrogen tunnelling. An improved instanton approach, based on path-integrals (oscillatory integrals), is used to figure out how the enzyme modulates these effects [2]. Instantons (pseudoparticles) have a close relation to gauge theory. The derivation of a stringent instanton rate formula includes a zero mode renormalisation via the socalled Faddeev-Popov trick. The calculation of tunneling rates and kinetic isotope effects in systems with several hundred degrees of freedom like enzymes requires to locate the instanton via an optimization [3]. A variable step size formulation of the instanton rate theory allows to distribute sampling points more evenly along the instanton path. Especially at the beginning and ending of the instanton less sampling points are necessary which significantly reduces the computational effort [4]. |
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