The effect of electron phonon coupling in light-harvesting complexes: an application to Peridinin Chlorophyll Protein

Vincenzo Mascolia,b, Lorenzo Cupellinia, Sandro Jurinovicha, Benedetta Mennuccia
a
Dipartimento di Chimica e Chimica Industriale, University of Pisa, Via G. Moruzzi 3, I-56124 Pisa, Italy; bScuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy

The extraordinary efficiency of photosynthesis is mostly due to the complex architecture of antenna systems, which carry out light-harvesting. In such photosynthetic complexes, structure, dynamics and function are inextricably linked. Absorbing pigments are generally arranged in close proximity to one another, supported by the surrounding protein scaffold [1]. This implies that excited states are not localized on single chromophores: the excitation is often shared by many pigments to form delocalized electronic states (excitons). Exciton creation, migration and trapping are central to the functionality of a photosynthetic apparatus. All these processes are characterized by a high degree of tunability which allow the organism, for instance, to manage with different illumination conditions. The driving force in exciton migration and light-harvesting tunability resides in the interaction between the electronic degrees of freedom and the vibrational environment that is associated with either the pigments or the protein [2].

We will show a global description of electron-phonon coupling by means of a single, valuable ingredient, the so called spectral density, and we will also extend the use of this tool to describe electronic spectroscopies of multichromophoric systems. If for single chromophores a spectral density is enough to reproduce the correct lineshape, for light-harvesting complexes, spectral densities of chromophores will be only one of the ingredients that enter the exciton dynamic equations [3]. We therefore need to couple the computation of spectral densities of antenna pigments to an intermediate level of exciton dynamics theory, adopting multiscale QM/MM(Pol) strategies [4] to account for the influence of the environment. The application of this procedure to the description of optical spectroscopies and dynamic properties of the Peridinin Chlorophyll Protein (PCP) light-harvesting complex will be presented.

References

[1] Van Amerongen, H., Valkunas, L. and Van Grondelle, R. (2000). Photosynthetic excitons. Singapore: World Scientific.
[2] Huelga, S. F. and Plenio, M. B. (2013). Vibrations, quanta and biology. Contemporary Physics, 54(4), 181–207.
[3] Novoderezhkin, V. I. and van Grondelle, R. (2010). Physical origins and models of energy transfer in photosynthetic light-harvesting. Physical Chemistry Chemical Physics, 12 (27),7352–65.
[4] Curutchet, C., Muñoz-Losa, A., Monti, S., Kongsted, J., Scholes, G. D. and Mennucci, B. (2009). Electronic energy transfer in condensed phase studied by a polarizable QM/MM model. Journal of Chemical Theory and Computation, 5(7), 1838–1848.