The fluorescence excitation spectrum of a single chromophore molecule in a photosynthetic pigment-protein complex is known to change in time at liquid helium temperature. The spectral change reflects a conformational change of the protein to which the chromophore binds. This work follows the temporal behavior of the spectrum of a single chromophore in the temperature range between 5 adn 18 K. The temperature dependence reveals two types of conformational change of the protein, i.e., thermally activated motions over a potential barrier of ca. 0.1 kJ/mol and temperature-independent motions of tunneling of a proton.
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