Spectral and kinetic properties of fluorescence of immunoactive steroids in the gas phase

Abstract

We have used steady-state fluorescence spectroscopy and pulsed kinetic fluorimetry with high time resolution to experimentally study the spectral and temporal characteristics of luminescence of immunoactive 8-aza-D-homogona-1,3,5(10),13-tetraene-12,17a-dione (8-aza-D-homogonane) molecules in the gas phase. We have established the existence of several centers emitting long-wavelength and short-wavelength fluorescence. The fluorescence of high-temperature 8-aza-D-homogonane vapor (T = 550 K) when excited by radiation with λ ex = 266 nm is represented by two spectrally separated emission bands with maxima λ max fl = 465 nm and λ max fl = 365 nm. Analysis of the distribution of fluorescence decay times for 8-aza-D-homogonane showed that in the spectral range of the emission wavelengths 360-590 nm, the fluorescence decay kinetics contains three components with average lifetimes <200 psec, 2.8 nsec and 13.5 nsec. The multicenter nature of luminescence of 8-aza-D-homogonane was confirmed by direct measurements of instantaneous fluorescence spectra in different stages of luminescence decay.Introduction. Biologically active organic compounds of the 8-azasteroid series, depending on their structure, can stimulate or suppress immunity in living organisms [1]. Studies of the spectral luminescence properties of 8-azasteroids are important from the standpoint of establishing the electronic structure of these compounds and determining possible correlations between their spectroscopic characteristics and biological activity.It was shown earlier that luminescence of 8-azasteroids is observed in different aggregate states: in the gas phase [2], in solutions [2, 3], and in microcrystals [4]. Based on analysis of the absorption, fluorescence, fluorescence excitation spectra and the duration of the excited states in aqueous solution (pH 7.4) for 2,3,-dimethoxy-8-azagona-1,3,5(10),13-tetraene-12,17-dione, it has been established that two centers exist that emit long-wavelength and shortwavelength fluorescence [5]. In the neutral solvent hexane, the fluorescence of 2,3-dimethoxy-8-azagona-1,3,5(10), 13-tetraene-12,17-dione and 8-aza-D-homogona-1,3,5(10),13-tetraene-12,17a-dione even appears as three luminescence centers [6].Of special interest are studies of the luminescence of 8-azasteroids in the gas phase, where there is no effect from the surrounding medium and relaxation processes appear in pure form. N. A. Brisevich et al. [7] comprehensively studied a number of immunoactive compounds of the 8-azasteroid class in the gas phase, and the presence of three absorption and fluorescence centers was observed. In this work, these results were confirmed by direct measurements of the kinetics of the fluorescence spectra in the early stages of luminescence decay. We have analyzed in detail the spectral and kinetic properties of fluorescence of 8-aza-D-homogona-1,3,5(10),13-tetraene-12,17a-dione molecules (called 8-aza-D-homogonane in the following):