Molecular dynamics simulations of fluorescence polarization of tryptophans in myoglobin.

The fluorescence of heme proteins is influenced by energy transfer from the excited tryptophan to the heme. Molecular dynamics simulations of the tryptophan and heme motions in sperm whale myoglobin were used to calculate the fluorescence intensity and anisotropy decays. The side chains underwent both small rapid orientational fluctuations and large infrequent transitions between conformations. The predicted motions of the tryptophans and the heme produce large fluctuations in the instantaneous rate of energy transfer, but no stable conformations in which energy transfer is suppressed were found. The calculated fluorescence anisotropies exhibited a large subpicosecond decay, corresponding to nondiffusive side-chain motions. The calculations adequately predict the observed fluorescence decay curve for myoglobin and the total anisotropy decay at 16-ps time resolution. The subnanosecond decays of anisotropy for tryptophan-14 in tuna myoglobin are not reproduced by the calculation.