Reorientation of tryptophan and simple peptides: Onset of internal flexibility

The absorption anisotropies of tryptophan (Trp) and some simple peptides are reported with a time resolution of 1 ps. The results are compared with those from fluoresence anisotropy, molecular dynamics simulations and Debye-Stokes-Einstein hydrodynamics. The rotational correlation time of tryptophan in aqueous solution (pH 7.0, 20°C) is 35 ps, obtained using time-resolved absorption anisotropy, which agrees with previous fluorescence anisotropy data. However, the rotational correlation time of Trp obtained from molecular dynamics simulations is three times faster than the experimental value, which indicates that the force field used in the simulation requires refinement. The data also show that polar interactions retard the reorientational motion significantly, which is qualitatively consistent with the results obtained from Debye-Stokes-Einstein hydrodynamic theory. Within the time resolution of the experiment (~1 ps), all dipeptides show single exponential anisotropy decays with rotational correlation times ranging from 40 to 60 ps. This suggests that the low initial value of the fluorescence anisotropy is attributable either to vibronic coupling or to one or more ultrafast processes in the excited state.