| Abstract: | The oligomerization of melittin with increasing ionic strength and protein concentration was investigated using the methods of decomposition of its tryptophan fluorescence spectra into "elementary" log-normal components. At high ionic strength (up to 2 M KCl), the emission spectra of tetrameric melittin are well described as the sum of two log-normal components, suggesting the presence of tryptophan residues in two sorts of environment with greatly differing polarity. Measurements of fluorescence spectra by iodide showed that these two spectral components possess different Stern-Volmer constants, that is, the tryptophans emitting them have different solvent accessibility, which does not correlate with the crystallographic structure of tetrameric melittin. Moreover, in the oligomerization transition induced by ionic strength, the tetrameric intermediate is formed, which has log-normal spectral components with relative contributions differing from those in 2 M KCl. |
