On-time photoblinking statistics at liquid helium temperature for individual Atto647N molecules coupled to PASc (a) and coupled to streptavidin via biotin (b). Histograms of the detected number of photons per fluorophore for the PASc domain dimer (c) and the streptavidin molecule (d). The histograms show the detected number of photons per fluorophore on a protein complex. Since in the case of streptavidin there are possible intensity combinations, each molecule is less often observed alone. This reduces the average total number of detected photons per fluorophore during the same observation time.