Probing protein dynamics using Fluorescence Resonance Energy Transfer with donors of different lifetimes

Fluorescence resonance energy transfer (FRET), using nanosecond dyes, and its derivative, Lanthanide-based resonance energy transfer (LRET), using millisecond-lifetime lanthanide chelates, are methods to measure distances on the 2-10 nm length-scale. It has been found that in certain systems energy transfer efficiency E for FRET and LRET measurements can be dramatically different [Chakrabarty et al., PNAS, 99: 6011-6016 (2002)]. Here we develop a theoretical model that shows that the dramatic difference can be explained by the presence of intrinsic dynamics of the system. Furthermore, we quantitatively investigate how information about the time-scale and distance-scale associated with the intrinsic dynamics can be inferred, by comparison of FRET and LRET results.