PNA-mediated efflux inhibition as a therapeutic strategy towards overcoming drug resistance in Mycobacterium smegmatis.

The emergence of antibiotic-resistant strains of Mycobacterium tuberculosis and the decelerating development of new and effective antibiotics has impaired the treatment of tuberculosis (TB). Efflux pump inhibitors (EPIs) have the potential to improve the efficacy of existing anti-TB drugs although with toxicity limitations. Peptide nucleic acids (PNAs), oligonucleotide mimics, by virtue of their high nucleic acid binding specificity have the capability to overcome this drawback. We, therefore, investigated the efflux pump inhibitory properties of a PNA designed against an efflux pump of Mycobacterium smegmatis. LfrA, an efflux pump found in M. smegmatis, is majorly involved in conferring innate drug resistance to this strain and, therefore, was selected as a target for gene silencing via PNA. qRT-PCR and EtBr assays confirmed the EPI activity of the anti-lfrA PNA. On testing the effect of the anti-lfrA PNA on the bactericidal activity of a fluoroquinolone, norfloxacin, we observed that 5 µM of anti-lfrA PNA in combination with norfloxacin led to an enhanced killing of up to 2.5 log-fold against wild-type and a lab-generated multidrug resistant strain, exemplifying its potential in countering resistance. Improved efficacy was also observed against intra-macrophage mycobacteria, where the drug-PNA combination enhanced bacterial clearance by 1.3 log-fold. Further, no toxicity was observed with PNA concentrations up to 4 times higher than the efficacious anti-lfrA PNA concentration. Thus, PNA, as an adjuvant, presents a novel and viable approach to rejuvenate anti-TB therapeutics.

Effect of π-System Extension on the Ionization Energy of the Planar Blatter Radical: Experimental and Theoretical Studies.

Fusion of benzene, naphthalene, and phenalene rings with the D ring of the planar Blatter radical leads to extension of the π-system and increased spin delocalization. The effect of this π-extension and the position of the ring fusion on the electronic structure of the radicals was investigated by UV-photoelectron spectroscopy and DFT CAM-B3LYP/6-311G(d,p) method. The experimental data obtained for 3 out of 8 derivatives were correlated with DFT-derived ionization energies.

UV-photoelectron spectroscopy of stable radicals: the electronic structure of planar Blatter radicals as materials for organic electronics.

The electronic structure of Blatter radicals and a series of C(10)-substituted derivatives of 2-phenyl-3H-[1,2,4]triazino[5,6,1-kl]phenoxazin-3-yl (planar Blatter radicals) containing H, F, Cl, Br, CN, CF3 and OMe substituents was investigated by gas phase UV-photoelectron spectroscopy. The energy of the SOMO of the radicals, determined to be about 6.5 eV, was correlated with their electrochemical oxidation potentials, E0/+11/2, relative to the Fc/Fc+ couple in CH2Cl2 giving the correction of 6.60(1) eV. The optical band gap Eoptg ∼ 1.7 eV of the radicals yielded the electronic transport gap, Eelg, of about 2.1 eV, which is similar to the electronic parameters of pentacene. The radicals were analyzed by EPR spectroscopy and single crystal XRD methods, and all experimental data were compared to DFT computational results obtained at the CAM-B3LYP/6-311G(d,p) level of theory.