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Excited state dynamics can be used to probe donor-acceptor distances for H-tunneling reactions catalyzed by flavoproteins.

ABSTRACT: In enzyme systems where fast motions are thought to contribute to H-transfer efficiency, the distance between hydrogen donor and acceptor is a very important factor. Sub-ångstrom changes in donor-acceptor distance can have a large effect on the rate of reaction, so a sensitive probe of these changes is a vital tool in our understanding of enzyme function. In this study we use ultrafast transient absorption spectroscopy to investigate the photoinduced electron transfer rates, which are also very sensitive to small changes in distance, between coenzyme analog, NAD(P)H4, and the isoalloxazine center in the model flavoenzymes morphinone reductase (wild-type and selected variants) and pentaerythritol tetranitrate reductase (wild-type). It is shown that upon addition of coenzyme to the protein the rate of photoinduced electron transfer is increased. By comparing the magnitude of this increase with existing values for NAD(P)H4-FMN distances, based on charge-transfer complex absorbance and experimental kinetic isotope effect reaction data, we show that this method can be used as a sensitive probe of donor-acceptor distance in a range of enzyme systems.

SUBMITTER: Hardman SJ

PROVIDER: S-EPMC3853081 | biostudies-literature | 2013 Dec

REPOSITORIES: biostudies-literature

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Excited state dynamics can be used to probe donor-acceptor distances for H-tunneling reactions catalyzed by flavoproteins.

Hardman Samantha J O SJ Pudney Christopher R CR Hay Sam S Scrutton Nigel S NS

Biophysical journal 20131201 11

In enzyme systems where fast motions are thought to contribute to H-transfer efficiency, the distance between hydrogen donor and acceptor is a very important factor. Sub-ångstrom changes in donor-acceptor distance can have a large effect on the rate of reaction, so a sensitive probe of these changes is a vital tool in our understanding of enzyme function. In this study we use ultrafast transient absorption spectroscopy to investigate the photoinduced electron transfer rates, which are also very ...[more]

PMID: 24314085

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Project description:A series of three perylenemonoimide-p-oligophenylene-dimethylaniline molecular dyads undergo photoinduced charge separation (CS) with anomalous distance dependence as a function of increasing donor-acceptor (DA) distances. A comprehensive experimental and computational investigation of the photodynamics in the donor-bridge-acceptor (DBA) chromophores reveals a clear demarcation concerning the nature of the CS accessed at shorter (bridgeless) and longer DA distances. At the shortest distance, a strong DA interaction and ground-state charge delocalization populate a hot excited state (ES) with prominent charge transfer (CT) character, via Franck-Condon vertical excitation. The presence of such a CT-polarized hot ES enables a subpicosecond CS in the bridgeless dyad. The incorporation of the p-oligophenylene bridge effectively decouples the donor and the acceptor units in the ground state and consequentially suppresses the CT polarization in the hot ES. Theoretically, this should render a slower CS at longer distances. However, the transient absorption measurement reveals a fast CS process at the longer distance, contrary to the anticipated exponential distance dependence of the CS rates. A closer look into the excited-state dynamics suggests that the hot ES undergoes ultrafast geometry relaxation (τ < 1 ps) to create a relaxed ES. As compared to a decoupled, twisted geometry in the hot ES, the geometry of the relaxed ES exhibits a more planar conformation of the p-oligophenylene bridges. Planarization of the bridge endorses an increased charge delocalization and a prominent CT character in the relaxed ES and forms the origin for the evident fast CS at the longest distance. Thus, the relaxation of the hot ES and the concomitantly enhanced charge delocalization adds a new caveat to the classic nature of distance-dependent CS in artificial DBA chromophores and recommends a cautious treatment of the attenuation factor (β) while discussing anomalous CS trends.

Dataset Information

Excited state dynamics can be used to probe donor-acceptor distances for H-tunneling reactions catalyzed by flavoproteins.

Publications

Excited state dynamics can be used to probe donor-acceptor distances for H-tunneling reactions catalyzed by flavoproteins.

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