Temperature dependence of electron-vibronic spectra of photosynthetic systems. Computer simulations and comparison with experiment Original Research Article

Absorption bands of photosynthetic and other biological pigment-protein complexes undergo dramatic variations in position and shape upon changing the temperature. In this paper we have reviewed a calculation algorithm to simulate the temperature dependence of electronic spectra coupled to any number of vibrations with arbitrary coupling strength. A number of computer simulations is carried out in order to clarify the physics behind the spectral changes. Several schemes which link the theory with the experiment are presented, which indicate that in general the temperature dependence of the simple numerical characteristics of a spectral band (width, moment) are not enough to draw unambiguous conclusions on the microscopic level (electron-phonon coupling strength, phonon density of states). Finally we apply this algorithm to the elementary building block of the core antenna of the purple bacteria, the so called B820 complex. A good correspondence between model and experiment is demonstrated. The spectrum of the B820 is found to be dominated by an electron-phonon coupling parameter S ≈ 0.5, while the distribution of phonons coupled to the optical transition is broad and peaks at about 100 cm−1.