Picosecond fluorescence decay of tryptophans in myoglobin.

The fluorescence decay characteristics of Mb, MbCO, metMb (sperm whale), metMb (yellowfin tuna), and their apo derivatives were determined by using a picosecond streak camera and time-correlated single photon counting. The emission is dominated by tryptophans that transfer their energy to the heme on a subnanosecond time scale. Sperm whale Mb and derivatives have two tryptophans and their decays can be interpreted mainly as two exponentials, one of ca. 20 ps and the other of 130 ps, whereas tuna Mb has one tryptophan and its emission is nonexponential but dominated by one component of 31 ps. These results along with Förster energy transfer calculations allow us to assign the ca. 30-ps emission to Trp-14 and the 130-ps emission to Trp-7 in Mb. The streak camera was modified to determine the decay of the fluorescence anisotropy. In metMb (tuna) the fluorescence anisotropy decays in 100 ps, which is postulated to result from rapid motion of the Trp-14. Because energy transfer was used to gate the anisotropy, the fast motion of Trp-14 is proposed to correspond to only 10% of the equilibrium distribution of molecules.