Dynamic Distance Disorder in Proteins Is Caused by Trapping

Dynamic disorder in proteins, as demonstrated by variations in single-​mol. electron transfer rates, is investigated by mol. dynamics simulations. The potential of mean force for the fluctuating donor-​acceptor distance is calcd. for the NAD(P)​H:flavin oxidoreductase (Fre) complex with FAD and is found to be in agreement with that estd. from electron transfer expts. The calcd. autocorrelation function of the distance fluctuations has a simple exponential behavior at low temps. and stretched exponential behavior at higher temps. on femtosecond to nanosecond time scales. This indicates that the calcd. dynamic disorder arises from a wide range of trapping times in potential wells on the protein energy landscape and suggests a corresponding origin for the stretched exponential behavior obsd. exptl. on longer time scales.