Tunneling is a quantum mechanical effect. It is only important for small particles such as protons and electrons. Standard transition state theory can be successfully applied to most reactions, except for those which transfer small particles (protons or electrons). In reactions that transfer protons or electrons tunneling can make reaction rates appear faster than would be expected from standard transition state theory. This is because the proton or electron can avoid going over the standard transition state barrier and can instead go beneath it to directly appear on the other side of the barrier. Thi is possible because small particles are strongly affected by the uncertainty principle and there is a nonzero probability that the particle can be found at a position where the potential energy exceeds the particle's total energy. For an electron transfer reaction the wave function describing the position and energy of the electron can reach beyond (or beneath) the higher energy barrier for electron transfer. There is a finite probability that the electron will appear on the other side of the barrier, without ever going over the barrier, because its wave function dictates that there is a finite probability that it can be there. Inelastic tunneling, the main topic of the proposed research for this FIRCA, includes the motion of the matrix (it doesn't make the Condon assumption). For inelastic tunneling there is the possibility that nuclear motions can be coupled with the electron transfer process to facilitate the electron transfer This gives additional energy to the electron, through electron-phonon coupling to help it complete the transfer. Figure 2 on page 21 of the proposal illustrates how the inelastic tunneling model (including effects of nuclear motion) is superior to elastic tunneling model (not including effects of nuclear motion) in describing the rate of long-range electron transfers in Ru-modified cytochromes. In summary, it appears that inelastic tunneling (including effects of nuclear motion) is important for long-range biological electron transfers such as thos in photosynthesis and electron transport in oxidative metabolism.
