Subpopulation-resolved photon statistics of single-molecule energy transfer dynamics

We present a technique that combines the power of single-molecule spectroscopy to separate subpopulations in a heterogeneous ensemble with submicrosecond correlation spectroscopy based on a Hanbury Brown and Twiss detection scheme. The use of four detectors allows such measurements to be performed with the spectral separation necessary for Pirster resonance energy transfer (FRET), which has become an important tool to study biomolecular structure and dynamics in single-molecule experiments. Our approach avoids the common limitations caused by the dead times of detectors and counting electronics in conventional single-molecule FRET experiments, and thus, allows access to dynamics down to the picosecond range. We illustrate the technical aspects of the method with recent measurements of the rapid chain dynamics in the unfolded state of a small protein.