Figure 5.

Schematic diagram explaining how donor-acceptor (DA) coupling fluctuations could wash out DA coupling structural differences with increasing DA distance. The diagram shows possible T_DA probability densities for two pairs of different electron-transfer (ET) species, each pair having the same average values of R_DA. σ₁ and σ₂ represent the root-mean-squared coupling fluctuations σ_TDA of each species in the pair. (a) For R_DA < r_crit, coupling fluctuations are small and do not wash out structural differences in $\langle T_{\mathit{\text{DA}}}^2\rangle$, i.e., $S_{R_{\mathit{\text{DA}}}<r_{\mathit{\text{crit}}}}=({\langle T_{\mathit{\text{DA}}}^2\rangle }_2-{\langle T_{\mathit{\text{DA}}}^2\rangle }_1)/({\langle T_{\mathit{\text{DA}}}^2\rangle }_2+{\langle T_{\mathit{\text{DA}}}^2\rangle }_1)~1$. (b) For R_DA > r_crit, the increase in coupling fluctuations could wash out structural differences in $\langle T_{\mathit{\text{DA}}}^2\rangle$, i.e., S_RDA>rcrit ≪ 1, leading to an average barrier limit. Surprisingly, our simulations do not observe this second regime (b) even though coupling fluctuations are large (45).