Abstract
A variety of electron transfer (ET) reactions in biological systems occurs at short distances and is ultrafast. Many of them show behaviors that deviate from the predictions of the classic Marcus theory. Here, we show that these ultrafast ET dynamics highly depend on the coupling between environmental fluctuations and ET reactions. We introduce a dynamic factor, γ (0 ≤ γ ≤ 1), to describe such coupling, with 0 referring to the system without coupling to a “frozen” environment, and 1 referring to the system’s complete coupling with the environment. Significantly, this system’s coupling with the environment modifies the reaction free energy, ΔGγ, and the reorganization energy, λγ, both of which become smaller. This new model explains the recent ultrafast dynamics in flavodoxin and elucidates the fundamental mechanism of nonequilibrium ET dynamics, which is critical to uncovering the molecular nature of many biological functions.
Highlights
A variety of electron transfer (ET) reactions in biological systems occurs at short distances and is ultrafast
It is widely known that, when environmental fluctuations are much faster than the ET reaction, the ET dynamics can be described by a single exponential decay, QðtÞ 1⁄4 eÀt=τET ; ð1Þ
When dealing with ET reactions in equilibrium (Fig. 1b III), it is shown that the new model is consistent with the classic Marcus theory (Eq 2)
Summary
The model assumed a fixed curvature of the free energy surface for the solvent coordinate. In contrast to the free energy defined in equilibrium, in which the density of state, exp(−H/kBT), is integrated over all possible values of qi, i=1, ..., N, the above definition assumes motions that are slower than the ET reaction (|ω|
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