Determination of the spectral diffusion kernel of a protein by single-molecule spectroscopy.

The spectral dynamics of individual bacterial light-harvesting-2 pigment-protein complexes have been studied at 1.4 K. The data provided the spectral diffusion kernel of the optical transitions of the embedded B800 bacteriochlorophyll a pigments. This kernel can be described by either a single Gaussian function or a superposition of Gaussian functions. Moreover, we found that the chromophores interact with two classes of TLSs that can be distinguished by their distance from the chromophore and are most likely located outside (class 1) and inside (class 2) the protein matrix.