DICHOTOMOUS MODEL FOR SYSTEM-ENVIRONMENT
INTERACTION IN PHOTOSYNTHETIC COMPLEX
Vytautas Bubilaitis, Olga Rancova, Darius Abramavicius
Department of Theoretical Physics, Faculty of Physics, Vilnius University, Lithuania
[email protected]
Photosynthesis is an exceptional process with high excitation transfer efficiency. Light harvesting 2 (LH2) complexes are responsible for turning solar energy into excitations in purple bacteria. In LH2 absorption spectrum of purple
bacteria’s Rhodoblastus acidophilus B850 band peak moves to higher frequencies when temperature rises [1]. To describe
this unusual behaviour the dichotomous model was developed [2]. In this model it is assumed that proteins have two
conformational states and conformation occupancy propability depends on temperature.
In present work dichotomous model is further expanded to include homogeneous spectral line broadening dependency on temperature and to get agreement with two-dimensional electronic spectroscopy (2DES) spectra of LH2 [3].
Fig. 1. Modeled LH2 B850 absorption spectrum peak
position dependency on temperature.
Fig. 2. Modeled LH2 B850 absorption spectrum FWHM
dependency on temperature.
Dichotomous model is expanded with an additional third conformation. Each conformation has specific parameters
for slow and fast fluctuations. Conformation occupancy propability for expanded dichotomous model:
pi (T ) =
ni exp (−Eai /kT )
.
1 + n2 exp (−Ea2 /kT ) + n3 exp (−Ea3 /kT )
(1)
The agreement with experimental data was achieved for simulated LH2 absorption spectra with dominating homogeneous disorder using expanded dichotomous model. Modeled LH2 B850 absorption spectra, with expanded dichomous
model, has very good agreement with experimental spectra’s FWHM (Fig. 2) and good agreement with band peak position dependency (Fig. 1) on temperature. The analysis of the spectral band shapes and spectral cross-sections showed
that in calculated LH2 2DES spectra there is homogeneous broadening dependency on temperature, because of expanded
dichotomous model.
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[2] J. Meldaikis, O. Zerlauskienė, D. Abramavičius, L. Valkūnas, Manifestation of protein conformations in B850 absorption band of light-harvesting
complex LH2, Chem. Phys. 423, 9–14 (2013).
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rhodobacter sphaeroides, J. Phys. Chem. Lett. 4, 1404–1409 (2013).