Single electron transfer events and dynamical heterogeneity in the small protein azurin from .

Monitoring the fluorescence of single-dye-labeled azurin molecules, we observed the reaction of azurin with hexacyanoferrate under controlled redox potential yielding data on the timing of individual (forward and backward) electron transfer (ET) events. Change-point analysis of the time traces demonstrates significant fluctuations of ET rates and of mid-point potential . These fluctuations are a signature of dynamical heterogeneity, here observed on a 14 kDa protein, the smallest to date.