Log-normal description of fluorescence spectra of organic fluorophores

The smooth fluorescence bands of various organic fluorophores of different classes (e.g. indole, tryptophan, tyrosine, phthalimides, quinine sulfate, aminopyridines, acetylanthracenes) in different ionization states of their substituents and dissolved in various solvents can be accurately fitted on the frequency (wavenumber) scale by the four-parametric log-normal distribution curves up to the far spectral wings, This fact suggests that the lognormal function is a good analytical description for smooth emission spectra of so-called complex molecules, Examination of log-normal parameters of 126 spectra of several tryptophan derivatives measured in different ionization states in various solvents revealed a straight-linear relation between three shape parameters of the lognormal function, namely, the positions of spectral maxima and of the two half-maximal amplitudes, This fact indicates that only one parameter is needed to describe the band shape for the set of tryptophan derivatives, A similar linearity was also observed for the series of spectra of 1- and 2-acetylanthracenes or 3-amino-N-methylphthalimides when varying the solvent, As a result, the spectral maximum position can entirely determine the shape of emission spectra of a series of a fluorophore derivatives in any environmental condition, Such a simplified mathematical representation significantly facilitates the problem of analytical resolution of composite fluorescence spectra, for instance, those of proteins.