This follows up on a previous post about measurements of the rate at which electron transfer occurs in a photosynthetic protein. I noted several deviations of the experimental results from what is predicted by Marcus-Hush electron transfer theory. This is not necessarily surprising because one is not in quite in the right parameter regime.

In principle [at least to me] this should be described by a spin boson model which has the Hamiltonian

and the spectral density contains all the relevant information about the protein dynamics,

So the question I have is: if one has the correct parameters and spectral density can one actually describe all the experiments? Below is the spectral density found by Parson and Warshel in molecular dynamics simulations.

Your question raises other important questions, namely how well does the spin-boson model describe the system? Said another way, how does one calculate "the correct parameters and spectral density"? The bias, epsilon, and the electronic coupling, Delta, are not truly constant ... a normal mode approximation/linear coupling to the environment is surely not exact ... and furthermore one needs a technique to solve the spin-boson model, with no analytic solution existing, e.g. Redfield (weak-coupling) approaches, or influence functional/path integral schemes ...

ReplyDeleteNonetheless, I certainly agree that a spin-boson approach is surely better than e.g. Marcus theory when one expects non-Markovian effects and/or quantum coherence.