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I was recently at a lecture given by Dr. Harry Gray, a biophysical chemist, where he talked about how proteins (specifically those involved in photosynthesis) are able to use various phenomena, like superexchange and tunneling, to move electrons through them, coupling redox reactions despite the extreme distances (~20 Å). By using tunneling/superexchange, various redox proteins are able to move 300-2000 electrons/second over this distance, while the reverse rate is effectively zero.

I asked about why this barrier is one-way, but I don't have enough base knowledge to understand his reply; something about how the injected hole is chased through a series of several bonds (sigma and hydrogen)...

In any case, I'm curious for a simpler case and explanation of this 'diode' effect about how electrons can be shuttled one way with ease but not the other

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I'm curious too.. the only way I can think would be with a potential as a function of time. Which might be the case, maybe the electron changes the structure of the molecule(s) causing them to move, changing the initial potential acting on the electron. –  Diego Sep 27 '11 at 1:43
    
My hunch would be that these are correlation effects and that a single-electron picture breaks down, which goes in the same direction as Diego's comment. –  Lagerbaer Sep 27 '11 at 3:43
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up vote 7 down vote accepted

The ratcheting in photosynthesis is due to energy loss at each step. If you go from a high energy to a lower energy state, where the difference exceeds kT by several multliples, you cannot go back, because the reverse process requires a thermodynamic conspiracy to take the energy out of many modes and transfer it back to one electron, and that's just not going to happen. The absorbed photon energy is about 20kT, so there is a lot of room for one-way processes.

The cases which are more mysterious are where you don't have an energetic photon supplying many kT's of energy--- this is the case during translation. If you preferentially bind a complementary anticodon to a codon during translation, you don't have an energy cost which is far enough above kT to guarantee fidelity of translation. So there are error correcting mechanisms, which cost energy, which repeat the binding several times, and use ATP at each step. This was predicted theoretically before it was observed by Hopfield, and is called Kinetic proofreading.

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More generally, irreversibility requires free energy dissipation. Enthalpy and entropy can both do the job. For example if a protein tunnels from a rigid (low entropy) configuration to a flexible (high entropy) configuration of the same enthalpy, it will reverse-tunnel at a much lower rate. –  Steve B Nov 10 '11 at 14:09
    
@Steve B: yes, of course. You can say "free energy" wherever I say "energy" above to make the answer correct for entropic polymers. I was thinking of electron transfer chain, where the motion is electronic and purely energetic. –  Ron Maimon Nov 10 '11 at 15:41
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