O'Reilly, EJ; (2014) Quantum traits in the dynamics of biomolecular systems. Doctoral thesis , UCL (University College London).

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Abstract

The majority of biology can be adequately described by classical laws, yet there are suggestions that a variety of organisms may harness non-trivial quantum phenomena to gain a biological advantage. This thesis is concerned with the light induced dynamics in photosynthetic light harvesting antennae. Quantum coherences persisting on picosecond time-scales have been repeatedly observed in a variety of species. This ran contrary to the prevailing theories of energy transfer dynamics in these systems. A consensus has emerged that a delicate competition between electronic and vibrational interactions is responsible for prolonging coherences between electronic states of chromophores. In particular, interactions with specific under-damped vibrational modes are known to play a fundamental role. This thesis demonstrates that room temperature, efficient vibration-assisted energy transfer in a biologically relevant exciton-vibration dimers can manifest and benefit from non-classical fluctuations of collective pigment motions. The inadequacy of a classical description of selected vibrations is further illustrated by identifying features of electronic dynamics that are enhanced by quantum properties. A quan\-tum-thermo\-dynamical form\-alism describing heat and work fluxes between partitions of a closed quan\-tum system is extended to open quan\-tum systems in the non-per\-turb\-ative regime. This reveals non-trivial relations between the electronic interactions among chromophores and the relative contribution of work- and heat-like energy fluxes between electronic and vibrational motions. This in turn highlights relations between structure and energy transformations in photosynthetic complexes. Finally, the thesis investigates energy transfer within and between antennae of purple bacteria acclimated to different illumination conditions. The protein composition is altered depending on the light levels. Consequently, the electronic energy landscape is modified to accelerate intra-complex energy transfer without detriment to inter-complex transfer, thereby promoting or diminishing resonances with specific vibrational motions. This suggests that acclimation may serve to exploit non-trivial quantum phenomena.

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