What causes the depolarization of trypsin and trypsinogen fluorescence: Intramolecular mobility or non-radiative energy transfer?

Analysis of protein data bank information about the coordinates of definite atoms of protein macromolecules provides an opportunity to evaluate the efficiency of non-radiative resonance energy transfer within the model of fixed, strictly oriented oscillators. Such evaluations for trypsin and trypsinogen (and also for trypsin complex with a pancreatic inhibitor) show that the efficiency of energy transfer among each pair of tryptophan residues is negligibly small. It is also shown that a fairly effective energy transfer from tyrosine to tryptophan residues is possible. The conclusions have been made that the Tyr-Trp energy transfer is one of the factors determining the shape of the trypsin polarization spectrum, and that upon fluorescence excitation at the long-wavelength edge of the absorption spectrum, the depolarization of trypsin fluorescence in aqueous solution at ambient temperature - compared to model compounds (tryptophan, N-acetyltryptophan, glycyltryptophan, etc.), under the conditions of infinite viscosity - is due to the Brownian rotational motion of the macromolecules as a whole as well as the intramolecular mobility. The differences in the level and character of intramolecular mobility of trypsin and trypsinogen are discussed.