Role of protein fluctuation correlations in electron transfer in photosynthetic complexes

White and Colored Noises as Driving Forces of Electron Transfer: The Photolyase Repair Mechanism as a Test Case

We introduce a model to investigate electron transfer where the explicit temporal propagation of the electronic wave function is modified by white and colored noises. Atomic energies are perturbed randomly to determine an electron transfer where the periodic electronic oscillations are greatly smothered and the transfer rates can reach up to the experimental time scale. Application to the photolyase enzyme that repairs the DNA lesions shows that the optimal conditions to reproduce the experimental lifetime are equivalent to a red or Brownian noise acting every 80 fs, that is, of ∼400 cm^-1. Two-state model calculations show that the results of the quantum dynamics are robust and intrinsic to the use of noise in the simulations.

Vibration-Assisted and Vibration-Hampered Excitonic Quantum Transport

The interplay between excitons and vibrations is considered to be a key factor in determining the exciton-transfer properties in light-harvesting complexes. Here we study this interplay theoretically in a model for exciton transport, composed of two chromophores coupled to an exciton source and sink in the presence of vibrations. We consider two cases that show qualitatively distinct transport features. In the first, the vibrations are global and affect the two chromophores simultaneously. In the second case, the vibrations are localized on each chromophore. For global vibrations, the current exhibits antiresonances as a function of the chromophore energy difference, which are due to exciton-polaron interference. For local vibrations, on the contrary, the currents show tunneling resonances at multiples of the vibration energy. Counterintuitively, both effects increase with increasing temperature. Our results demonstrate that an environment can either assist or hamper exciton transport and is in accord with the current understanding of energy transfer in natural exciton-transfer complexes.

Gain and loss in open quantum systems

Photosynthesis is the basic process used by plants to convert light energy in reaction centers into chemical energy. The high efficiency of this process is not yet understood today. Using the formalism for the description of open quantum systems by means of a non-Hermitian Hamilton operator, we consider initially the interplay of gain (acceptor) and loss (donor). Near singular points it causes fluctuations of the cross section which appear without any excitation of internal degrees of freedom of the system. This process occurs therefore very quickly and with high efficiency. We then consider the excitation of resonance states of the system by means of these fluctuations. This second step of the whole process takes place much slower than the first one, because it involves the excitation of internal degrees of freedom of the system. The two-step process as a whole is highly efficient, and the decay is biexponential. We provide, if possible, the results of analytical studies, otherwise characteristic numerical results. The similarities of the obtained results to light harvesting in photosynthetic organisms are discussed.

Decoherence and spin echo in biological systems

The spin-echo approach is extended to include biocomplexes for which the interaction with dynamical noise, produced by the protein environment, is strong. Significant restoration of the free induction decay signal due to homogeneous (decoherence) and inhomogeneous (dephasing) broadening is demonstrated analytically and numerically for both an individual dimer of interacting chlorophylls and for an ensemble of dimers. Our approach does not require the use of small interaction constants between the electron states and the protein fluctuations. It is based on an exact and closed system of ordinary differential equations that can be easily solved for a wide range of parameters that are relevant for bioapplications.