Effect of micellar and sol-gel media on the spectral and kinetic properties of tetracycline and its complexes with Mg2+

The spectroscopic and photophysical properties of the broad-spectrum antibiotic tetracycline (TC) and its Mg2+ complexes were studied in organized media attained by means of three iso-structural quaternary ammonium surfactants able to self-assemble in water at low c. m. c. values, thus giving spherical micelles and sol-gel media upon increasing the concentration. Specific protonated forms of TC and its complexes were introduced in these micro-heterogeneous environments and then investigated through steady-state (both in absorption and emission) and pulsed (up to femtosecond resolution) spectroscopic techniques. Free TC showed minor spectral and kinetic variations while complexes remained unchanged in the presence of spherical micelles, meaning that TC is likely to be placed at the interface between the micelle and the bulk aqueous solution, without altering its bioactivity. Ultrafast transient absorption spectroscopy proved to be a powerful tool to gain deep insight into the distribution of the investigated species between the heterogeneous structure of sol-gel media. In fact, according to the polarity and net charge of free TC and its complexes, these species can be mostly found in the hydrophobic (intertwined worm-like micelles) or in the hydrophilic domains (basically aqueous pools) that the sol-gel is made up of. In the first case, the properties are dramatically altered (highly enhanced fluorescence and lengthened lifetime of the first singlet excited state up to the nanosecond time scale), leading to the improved traceability of the drug.