Fluorescence depolarization and rotational modes of tyrosine in bovine pancreatic trypsin inhibitor.

The fluorescence of bovine pancreatic trypsin inhibitor (BPTI) is due to one or more of its four tyrosine residues. Observations of the stationary polarization of the fluorescence over a large range of temperatures and viscosities permit the demonstration of at least three modes of tyrosine rotation, and perhaps an ultrafast fourth one. The slowest mode is one of motion of the whole molecule; the second, a much faster motion limited to an amplitude of 11 degrees, is not changed by quenching of the fluorescence through addition of citrate and is therefore ascribed to the motion of internal tyrosines of BPTI. The third mode of motion is faster still; it has an amplitude similar to that of the second and, being sensitive to citrate quenching, is attributed to the rotation of the external tyrosine residue. A residual depolarization corresponding to a rotational amplitude of 22 degrees is deduced by comparison of the polarizations of BPTI and tyrosine dissolved in 80% glycerol-water at -40 degrees C. It is in accord in amplitude with the picosecond tyrosine rotations predicted by Karplus and collaborators from molecular dynamics computer simulations, but it could also originate, in whole or in part, from electronic energy transfer among the tyrosines.