Tryptophan mutants of human C5a anaphylatoxin: A fluorescence anisotropy decay and energy transfer study

Three mutants of the anaphylatoxin C5a were prepared with positions 2, 64 and 70, respectively, substituted by tryptophan. The last mutant was additionally labelled at Cys 27 for fluorescence energy transfer (FET) measurements. The structural integrity and biological activity of the molecules were not affected. Fluorescence anisotropy decay (FAD) measurements showed that the rotational correlation time for tryptophan decreases in the order: [Trp 2]rhC5a > [Trp 64]rhC5a > [Trp 70]rhC5a, indicating an increasing mobility of the side chain. Measurements of the fluorescence energy transfer from Trp 70 to the 1,5-AEDANS group at Cys 27 yielded a distance distribution of 2.4 ± 0.8 nm. This value is compatible with the C-terminal chain being arranged as a slightly stretched helix pointing away from the body of the molecule.