Chitosan biofilms: Insights for the selective electromembrane extraction of fluoroquinolones from biological samples

A selective electromembrane extraction procedure for the extraction of Enrofloxacin, Marbofloxacin and Flumequine, usually employed as antibiotic in veterinarian use, is proposed by using a chitosan biofilm, composed by 60% (w/w) chitosan and 40% (w/w) Aliquat®336, as active biopolymeric support. The interaction mechanism occurring between the target drugs and the biopolymer has been deeply studied using the Quantum Theory of Atoms in Molecules. The obtained results show the interaction between the extracted fluoroquinolones and the biomembrane is stabilized by two hydrogen bonds formed between both the carboxyl and keto groups of the drugs with both the amine and hydroxyl groups of glucosamine in the biopolymer. The energetic results agree with the high extraction efficiency obtained for Marbofloxacin, Enrofloxacin and Flumequine in terms of enrichment factors (83, 82 and 58, respectively) in presence of other fluoroquinolones. Under optimum conditions, the proposed electromembrane extraction method exhibits wide linear ranges of 4.2-200 μg L^-1, 5.6-200 μg L^-1 and 5.1-200 μg L^-1, respectively; low limits of detection close to 1.3 μg L^-1 and appropriate repeatability (relative standard deviation values 4-7%).

Hydrogen Bonds and n → π* Interactions in the Acetylation of Propranolol Catalyzed by Candida antarctica Lipase B: A QTAIM Study

Enzyme-substrate interactions play a crucial role in enzymatic catalysis. Quantum theory of atoms in molecules (QTAIM) calculations are extremely useful in computational studies of these interactions because they provide very detailed information about the strengths and types of molecular interactions. QTAIM also provides information about the intramolecular changes that occur in the catalytic reaction. Here, we analyze the enzyme-substrate interactions and the topological properties of the electron density in the enantioselective step of the acylation of (R,S)-propranolol, an aminoalcohol with therapeutic applications, catalyzed by Candida antarctica lipase B. Eight reaction paths (four for each enantiomer) are investigated and the energies, atomic charges, hydrogen bonds, and n → π* interactions of propranolol, the catalytic triad (composed of D187, H224, and S105), and the oxyanion hole are analyzed. It is found that D187 acts as an electron density reservoir for H224, and H224 acts as an electron density reservoir for the active site of the protein. It releases electron density when the tetrahedral intermediate is formed from the Michaelis complex and receives it when the enzyme-product complex is formed. Hydrogen bonds can be grouped into noncovalent and covalent hydrogen bonds. The latter are stronger and more important for the reaction than the former. We also found weak n → π* interactions, which are characterized by QTAIM and the natural bond orbital (NBO) analysis.

Electrophilic reactivity of tetrabromorhodamine 123 is bromine induced: convergent interpretation through complementary molecular descriptors

Nucleophilic addition of water and of methanol to 3,6-diamino-2,4,5,7-tetrabromo-9-[2-(methoxycarbonyl) phenyl]-9H-xanthen-9-ylium, 4BrR123, yields respectively 2-(3,6-diamino-2,4,5,7-tetrabromo-9-hydroxy-9H-xanthen-9-yl)xanthyl benzoate, HO4BrR123 and 2-(3,6-diamino-2,4,5,7-tetrabromo-9-methoxy-9H-xanthen-9-yl)xanthyl benzoate, MeO4BrR123. The novel experimental results are addressed theoretically. The linear free energy relationship, LFER, second-order perturbation theory analysis of the natural bond orbital, NBO, and quantum theory of atoms in molecules, QTAIM, lead to the same conclusion: the electron-withdrawing effect of bonded Br atoms in 4BrR123 extremely enhances the molecular electrophilicity, as compared to 3,6-diamino-9-[2-(methoxycarbonyl) phenyl]-9H-xanthen-9-ylium, R123. The reactivity of these diaminoxanthylium cations is discussed in the context of local and global softness in extended conjugated systems.

Spectral and photophysical properties of α-carboline (1-azacarbazole) in aqueous solutions

The absorption and fluorescence spectra of α-carboline, 9H-pyrido[2,3-b]indole, AC, in organic aprotic solvents containing different water proportions and in acid/base aqueous solutions inside and outside the pH range have been examined. In the organic aprotic solvents, the addition of increasing concentrations of water sequentially quenches and shifts to the red the fluorescence spectra of AC. These spectral changes have been rationalized assuming the formation, at the lower water concentrations, of a discrete ground state non-cyclic weakly fluorescent AC hydrate emitting at 376 nm that, upon increasing the water concentrations, evolves to a higher order AC poly hydrate emitting at 397 nm. The changes of the AC absorption spectra in aqueous acid/basic solutions indicate the existence of three ground state prototropic species; the pyridinic protonated cation, C (pK(a) = 4.10 ± 0.05), the neutral, N (pK(a) = 14.5 ± 0.2), and the pyrrolic deprotonated anion, A. Conversely, the changes of the AC fluorescence spectra in these media indicate the existence of four excited state species emitting at 376 nm, 397 nm, 460 nm and 465 nm. Since the emissions at 376 nm and 397 nm satisfactorily match those of the hydrates observed in the organic-water mixtures, they were consistently assigned to two differently hydrated ground state N species. The remaining emissions at 460 nm and 465 nm have been assigned without ambiguity, on the basis of their excitation spectra, to the C and A species, respectively. The excited-state pK(a)s of the prototropic species of AC have been estimated by using the Förster-Weller cycle.