On the Elementary Chemical Mechanisms of Unidirectional Proton Transfers: A Nonadiabatic Electron-Wavepacket Dynamics Study

Abstract

We propose a set of chemical reaction mechanisms of unidirectional proton transfers, which may possibly work as an elementary process in chemical and biological systems. Being theoretically derived based on our series of studies on charge separation dynamics in water splitting by Mn oxides, the present mechanisms have been constructed after careful exploration over the accumulated biological studies on cytochrome c oxidase (CcO) and bacteriorhodopsin. In particular, we have focused on the biochemical findings in the literature that unidirectional transfers of approximately two protons are driven by one electron passage through the reaction center (binuclear center) in CcO, whereas no such dissipative electron transfer is believed to be demanded in the proton transport in bacteriorhodopsin. The proposed basic mechanisms of unidirectional proton transfers are further reduced to two elementary dynamical processes, namely, what we call the coupled proton and electron-wavepacket transfer (CPEWT) and the inverse CPEWT. To show that the proposed mechanisms can indeed be materialized in a molecular level, we construct model systems with possible molecules that are rather familiar in biological chemistry, for which we perform the ab initio calculations of full-dimensional nonadiabatic electron-wavepacket dynamics coupled with all nuclear motions including proton transfers.

Suppression of Charge Recombination by Auxiliary Atoms in Photoinduced Charge Separation Dynamics with Mn Oxides: A Theoretical Study

“…The second one is an efficient (yet natural) control of coherence and decoherence among the composite subsystems collaborating together in charge transfer. It has been shown theoretically that such appropriate switching-on and -off of coherence can make it possible for a one-way transfer (unidirectional without back flow) of protons and electrons to be realized in practice [ 55 ]. The very long time scale of electron transport in PSII suggests [ 59 ] that such on-and-off switching should be materialized by the large amplitude motion of (large) protein molecules and/or collision (contact) frequency that triggers the initiation of coherent process of CPEWT [ 55 ].…”

“…It has been shown theoretically that such appropriate switching-on and -off of coherence can make it possible for a one-way transfer (unidirectional without back flow) of protons and electrons to be realized in practice [ 55 ]. The very long time scale of electron transport in PSII suggests [ 59 ] that such on-and-off switching should be materialized by the large amplitude motion of (large) protein molecules and/or collision (contact) frequency that triggers the initiation of coherent process of CPEWT [ 55 ]. Such an example is seen in the water splitting catalytic cycle by Mn 4 CaO 5 in photosystem II [ 24 ].…”

“…In this sense our discussion covers only an ideal state in the coherent dynamics. For the role of decoherence in the electron wave-packet dynamics and charge separation, see references [ 24 , 55 ].…”

Charge separation and successive reconfigurations of electronic and protonic states in a water-splitting catalytic cycle with the Mn4CaO5 cluster. On the mechanism of water splitting in PSII

“…(2) Then another ground-state nonadiabatic dynamics follows, where an electron wavepacket jumps from EPD (a Mn oxide) to EA, along with the return (thus making a reciprocal motion) of a proton from PA to EA. 31 (3) Then proton relay-transfer occurs from EPD to PA, leaving electronic polarization in EPD. (4) PA captures a proton, while EA0 adopts an electron, and thereby charge separation is completed.…”

“…It turned out incidentally that the reciprocal motion of a proton between EA and PA plays a crucial role to maintain the electric neutrality of EPD and EA and furthermore this type of motion is physically relevant to one-way (non-reversible) transfer of electrons. 31 In what follows, our dynamical calculations presented in Fig. 2 begin with stage (2) of Fig.…”