How Deep Is the Potential Well Confining a Protein in a Specific Conformation? A Single-Molecule Study on Temperature Dependence of Conformational Change between 5 and 18 K

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.