Phonon assisted exciton transitions on LHC-II complexes - a long wavelength absorption mechanism by cooperative action of photons and protein vibrations

Analyzing recent experimental data of the temperature dependence of absorpt ion on light-harvesting II-complexes of higher plants in the range from 5 K to room temperature it is shown that the long wavelength absorption tail c an be well explained by phonon-assisted exciton transitions described as a process of second order perturbation theory, a mechanism well known from th e semiconductor physics. The phonons involved in this absorption process ar e found to have a density of states ranging from some 10 to 350 cm(-1). The main argument in favour of the proposed mechanism is the strong redistribu tion of oscillator strength with temperature only to longer wavelength. It is shown that up to a critical temperature T-cr the population of all analy zed modes can be very well described by the Bose-Einstein distribution func tion. Also the strong deviations from this law above T-cr can be well expla ined by a failure of the linear theory of normal mode analysis and the domi nance of anharmonicity effects in the magnitudes and time scales for the pr otein motion, as it is suggested by the surprisingly good accordance of our experimental results with a corresponding full molecular dynamic simulatio n for a alpha-helix including anharmonicity of the protein motion [19]. Fro m T-cr we estimated a critical mean elongation for the onset of anharmonici ty effects a(cr) greater than or equal to 0.3 Angstrom. Finally we found po wer dependencies of the heat capacity in an extended temperature range with exponents between 1.1 and 1.7 which are in good agreement with correspondi ng values of other proteins.