Proteins are complex systems that share essential properties with viscous liquids and polymers. Structure, dynamics, and function of proteins are strongly interrelated, and structural and dynamic aspects can be revealed by studying protein function. Here we report measurements of the kinetics of intramolecular electron transfer (ET) from the primary quinone (Q
A) to the special pair (P) of reaction center (RC) proteins from Rhodobacter sphaeroides as a function of temperature (5 to 300 K) and illumination protocol (cooled in the dark and under illumination from 280 K). From the data, information about structural heterogeneity, relaxations and fluctuations of these molecules is obtained.