The Role of pH in Modulating the Electronic State Properties of Minocycline Drug and Its Inclusion within Micellar Carriers.

J Phys Chem A

Department of Chemistry, Biology and Biotechnology and Center of Excellence on the Innovative Nanostructured Materials (CEMIN), University of Perugia, via Elce di Sotto 8, 06123 Perugia, Italy.

Published: July 2016

AI Article Synopsis

  • This study investigates the properties of minocycline (MC) in water, examining how different pH levels, solvent polarities, and micellar surfactants affect its behavior.
  • It highlights the unique effects of an extra dimethylamino group in MC's structure compared to other tetracyclines, identifying key equilibrium constants related to its deprotonation reactions.
  • Advanced techniques like pump-probe spectroscopy reveal dynamic behavior of the drug's excited states and show that pH is crucial for determining MC's charge and interactions with cationic micelles, impacting its biological activity.

Article Abstract

A detailed investigation of the spectral and photophysical properties of minocycline (MC) in water at different pHs, solvents of different polarity, and micellar surfactant solutions was carried out in this study. An unusual behavior was highlighted with respect to other tetracyclines due to the presence of an additional dimethylamino group in the MC molecular structure. In particular, four equilibrium constants associated with mono-deprotonation reactions were characterized by steady-state spectroscopy. Femtosecond time-resolved pump-probe and fluorescence up-conversion measurements allowed the dynamics of the lowest excited singlet state of the five different acid-base species of MC to be characterized in terms of lifetimes and transient spectra. Two emissive species associated with keto-enol tautomerism resulting from excited-state intramolecular proton transfer (ESIPT) were revealed with time constants of a few and tens of picoseconds. TD-DFT quantum mechanical calculations were also performed to define the state order and nature of the differently protonated species, together with their absorption spectra. The role of pH proved to be fundamental in modulating the drug charge and therefore the interaction with cationic micelles where the neutral form of MC, that is the biologically active one, resulted efficiently included.

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http://dx.doi.org/10.1021/acs.jpca.5b12707DOI Listing

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