UDC 535.376The electron energy loss spectra (EELS) of indole, 3-indolyl propionic acid, 3-indolealdehyde, 3-dimethylaminomethylindole, tryptophan, and N-acetyl-L-tryptophan in the gas phase upon excitation by monokinetic electrons with an energy of E 0 = 11-50 eV are obtained. The structure of EELS is determined in the main by the indole chromophore; the side groups, except for the C=O group of 3-indolealdehyde, exert an insignificant influence. The energy of the lower triplet level 3 L a is 3.3 eV for indole and its derivatives and 3.2 eV for tryptophan and N-acetyl-L-tryptophan. Four singlet transitions in the region of 4.4-7.2 eV have been identified. The molecules studied, except for tryptophan, fluoresce in the gas phase on excitation by electrons. At low values of E 0 (10-25 eV), the fluorescence spectra are similar and are due to the indole fluorophore. Just as in the case of optical excitation, fluorescence on excitation by electrons is associated with the 1 L b -S 0 transitions. An increase in the energy E 0 up to 60-80 eV leads to dissociation of a portion of the indole molecules and to the appearance of additional bands in the fluorescence spectrum. Keywords: electron, electron energy loss spectrum, fluorescence spectrum, triplet and singlet energy levels. Introduction. The processes of transformation of the energy of electrons by molecules are of great interest in physics, chemistry, and biology. In recent years, interactions of the molecules of a number of classes of organic compounds (polyphenyls and polyacenes [1], carbazole and furans [2], oxazoles and oxadiazoles [3], and fluorine and fluorenones [4]) with electrons have been studied.The present work is devoted to the investigation of interactions, with a monokinetic beam of electrons, of free biologically active molecules of indole, 3-indolylpropionic acid, 3-indolealdehyde (formyl), 3-dimethylaminomethylindole (gramine), tryptophan, and N-acetyl-L-tryptophan, which manifest themselves in electron energy loss and fluorescence spectra. Tryptophan -the most intensely fluorescing amino acid -is widely applied as a fluorescing mark in studying peptides and proteins, and it also serves as the main center used in investigating intracellular inclusions and the cells themselves. The electronic spectra of these molecules on optical excitation in condensed media were widely used and reflected in [5,8].Experimental. To measure electron energy loss spectra in interaction of electrons with the molecules studied, an electron spectrometer [9] with an energy resolution of 0.3 eV was used. The vapor pressure of the above-indicated compounds in the chamber did not exceed 10 -3 Torr, which ensured a single collision of molecules with electrons. The electrons scattered at an angle of 90 o relative to the incident monokinetic beam were analyzed by a HughesRozhanskii 127-deg electrostatic analyzer. The current of an electron beam was not higher than 10 µA. The optical absorption spectra were measured on an SP-700A spectrophotometer (Great Britain), and fluorescence ...
