Reorientation of tryptophan and simple peptides: Onset of internal flexibility and comparison with molecular dynamics simulation

Absorption anisotropy decays were recorded for tryptophan and a series of dipeptides with a time resolution of less than 1 ps. Fluorescence anisotropies for tryptophan and several of its derivatives were also recorded. The data show that polar interactions retard the reorientational motion significantly. Molecular dynamics simulations of the reorientation, using SPC (single point charge) model water, give reorientation times about 4 times shorter than the experimental values. By studying fragments of the hormone ACTH, we find that internal flexibility of the peptide becomes detectable at a length of six residues, implying that the motion of longer peptides can be modeled by considering the motion of units larger than single residues.