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

Abstract

The spectroscopic and photophysical properties of the broad-spectrum antibiotic tetracycline (TC) and its Mg(2+) 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.

Inclusion of push–pull N-methylpyridinium salts within surfactant hydrogels: is their excited state intramolecular charge transfer mediated by twisting?

“…Fluorescence quantum yields increase by a factor 5 and one order of magnitude for 1 in pDoAO gel and Gemini gel, respectively, while the emission intensity of 2 is enhanced by nearly 70% in both hydrogels, irrespective of the surfactant structure. 24,27,28 Since N-methylpyridinium salts show pseudoamphiphilic features, having a polar cationic portion and a hydrophobic aromatic moiety, their distribution between the two domains can be envisaged. However, the remarkable shift in the fluorescence spectrum and the non-negligible increase of f F are clear signs of at least partial permeation.…”

“…The molecular structure of the single chain and Gemini surfactants forming the viscoelastic solutions comprises one or two aromatic rings, respectively. 24,25 This feature may favor the interaction with the push-pull compound by p-p stacking, in spite of the electrostatic repulsion between the positive charge of the methylpyridinium ring and the likewise positive charge localized on the surfactant head group. The pDoAO and Gemini hydrogels, formed from aqueous solutions of the two surfactants in deionized water, have also been previously morphologically characterized by means of SEM measurements, 23 which revealed a porous surface super-arrangement of interconnected lamellae and a non-uniform distribution of pores with an average diameter of some micrometres.…”

Acid–base strength and acido(fluoro)chromism of three push–pull derivatives of 2,6-distyrylpyridine

“…The experimental setup for ultrafast transient absorption experiments has previously been reported [29,[51][52][53]. The 400 nm excitation pulses (ca.…”

Probing the structural features and the micro-heterogeneity of various deep eutectic solvents and their water dilutions by the photophysical behaviour of two fluorophores

“…The experimental setups for femtosecond transient absorption measurements have been widely described elsewhere [59][60][61]. In particular, the 400 nm excitation pulses of ca.…”