Ultrafast dynamics of resonance energy transfer in myoglobin: probing local conformation fluctuations

We report here our systematic characterization of resonance energy transfer between intrinsic tryptophan and the prosthetic heme group in myoglobin in order to develop a novel energy-transfer pair as a molecular ruler in heme proteins to study local conformation fluctuations. With site-directed mutagenesis, we designed four tryptophan mutants along the A-helix of myoglobin and each mutant contains only a single tryptophan-heme energy-transfer pair. With femtosecond resolution, we observed, even at separation distances of 15-25 A, ultrafast energy transfer in tens to hundreds of picoseconds. On these time scales, the donor and acceptor cannot be randomized and the orientation factor in Forster energy transfer is highly restricted. Thus, direct measurement of the orientation-factor changes at different mutation sites reveals relative local structure flexibility and conformation fluctuations as particularly demonstrated here for positions of tryptophan 7 and 14. More importantly, the local environment relaxation occurs on the similar time scales of the energy transfer dynamics, resulting in a nonequilibrium dynamic process. With femtosecond- and wavelength-resolved fluorescence dynamics, we are able to determine the time scales of such nonequilibrium energy-transfer dynamics and elucidate the mechanism of the nonexponential energy-transfer dynamics caused by local dynamic heterogeneity and/or local environment relaxation.