Time-resolved fluorescence anisotropy (FA) measurements are reported for 5 helical bilayer-spanning henicosapeptides, each contg. 1 tryptophan at sequence position 1, 6, 11, 16, or 21. The FA decay reflects 2 mol. processes in all cases: local internal fluctuations of the tryptophan side chain with a relaxation time of 200-500 ps, and motions of the whole polypeptide mol. with a relaxation time of 9-10 ns. The amplitudes of the fast fluctuation are largest at the helix ends and decrease toward the center of the helix. A similar observation was made for the helical polypeptides dissolved in org. solvents showing that the mobility gradient along the polypeptide sequence is an inherent property of the polypeptide helix and not due to the lipid bilayer. However, the amplitudes of the fast fluctuations can be modulated by the phys. state of the lipid bilayer. With decreasing temp., the internal mobility of the tryptophan in all positions decreases with an abrupt change at the lipid phase transition. Concomitant mol. dynamics calcns. indicate a correlation between the fast FA decay and conformational fluctuations within the helix. According to the simulation, the conformation of the polypeptide is on av. predominantly helical, but actually the mol. can fluctuate between a variety of different substructures. The conformational fluctuations are largest at the helix ends and provide the free space required for rotation of the indole ring around the tryptophan side chain bonds. [on SciFinder (R)]