[Structural-dynamic properties of the tryptophan residue environment in melittin]

The structural dynamics of the environment of the single tryptophan residue in melittin was studied by site-selective red-edge-excitation fluorescence spectroscopy. The dependence of the spectral shift on transition from excitation in a maximum (at 280 nm) to long-wavelength edge (305 nm) was studied as a function of temperature. It was shown, that for melittin at high ionic strength (tetramer), the three regions of temperature dependence of the red-edge effect are observed: retarded relaxation (up to +30 degrees C), relaxational changes of spectra (from +30 to +50 degrees C) and thermal changes of structure (above +50 degrees C). The dipolar-re-orientational relaxation time and activation energy of orientation motions in the environment of indolic ring in the tetrameric melittin structure were estimated. Extrapolation from relaxational region to room temperature results in relaxation time 40 ns. In monomeric melittin (at low ionic strength) the red-edge shift of spectra is absent. The distinct differences in character of thermal quenching of fluorescence between monomeric and tetrameric forms of melittin are observed. It follows, that the short-wave-length fluorescence shift on monomer-tetramer transition is due to both the reduction of polarity, and the increase in rigidity of tryptophan environment, the absence of relaxation motions at nanosecond times.