Ultrafast polarized fluorescence measurements on monomeric and self-associated melittin

The anisotropic and magic-angle fluorescence decay of the single tryptophan (Trp) residue of melittin, a bee venom peptide, was investigated by time-resolved fluorescence anisotropy using a streak camera setup. The peptide was dissolved either in distilled water or in Hepes/NaOH buffer containing low (10 mM) or high (2 M) concentrations of NaCl, the latter resulting in tetramerized melittin. For melittin in distilled water and low NaCl concentration, two anisotropy decay times were found in the order of ∼50 and ∼800 picoseconds, reflecting local and overall peptide dynamics, respectively, and for tetramerized melittin, two anisotropy decay times of ∼200 and ∼5500 picoseconds were found. Decay-associated spectra of the isotropic fluorescence decay show three time components in the range of ∼20 picoseconds, ∼500 picoseconds, and ∼3500 picoseconds, respectively. The relative amplitudes of the latter two change upon the self-association of melittin. This change can be explained by the existence of different rotamers of Trp in melittin, of which one is more favored in the melittin tetramer than in the melittin monomer.