Temperature-dependent shift of fluorescence spectra without conformational changes in protein; studies of dipole relaxation in the melittin molecule.

When studying bee venom melittin in an ordered tetrameric form we found a shift of the fluorescence spectrum to a longer wavelength with a rise in temperature above 25 degrees C. The application of the methods of circular dichroism, temperature-perturbation difference spectrophotometry, gel filtration, ionic quenching and polarization of Trp-19 fluorescence argues against the possibility of dissociation and change in conformation with the rise in temperature. The spectral shifts are, probably, caused by dipole-orientational structural relaxation of the tryptophanyl environment in the excited state at nanosecond times. The dependence of the fluorescence spectrum on the excitation wavelength was found to be a function of temperature. This function was applied to determine the dipole-orientational relaxation times.