The catalytic activities and magnetic behaviours of rare µ3-chlorido and µ1,1,1-azido bridged defective dicubane tetranuclear Mn(ii) complexes.

Reaction of Mn(ii) salts with a flexidentate Mannich base ligand, N,N'-dimethyl-N,N'-bis(2-hydroxy-3-methoxy-5-methylbenzyl)ethylenediamine (H2L) in the presence of chloride or azide ions yielded two new tetranuclear complexes, [Mn4L2(µ3-Cl)2Cl2] (1) and [Mn4L2(µ1,1,1-N3)2(N3)2] (2). Single crystal X-ray structural analyses reveal that these two discrete tetranuclear Mn(ii) complexes possess defective dicubane cores with two µ3-Cl (for 1) or two µ1,1,1-N3 bridges (for 2). One of the triply bridging groups is situated above and the other one below the plane of the four Mn(ii) ions. In both complexes, all Mn(ii) ions are in a hexa-coordinated distorted octahedral environment. Both complexes 1 and 2 exhibit catecholase like activity and phenoxazinone synthase-like activity under ambient conditions. The turnover numbers (kcat) for the aerobic oxidation of 3,5-di-tert-butylcatechol are 1492.4 and 1431.2 h-1 and those of o-aminophenol are 2265.5 and 2132.2 h-1 for complexes 1 and 2, respectively. Mass spectral analyses were performed to get an idea about the probable mechanistic pathway for these catalytic reactions. Variable temperature magnetic susceptibility measurements of a solid sample reveal that both the complexes are antiferromagnetically coupled. The coupling constants are calculated to be J1 = -0.19 cm-1, J2 = -6.87 cm-1 and J3 = -0.70 cm-1 for 1 and J1 = 0.11 cm-1, J2 = -0.64 cm-1 and J3 = 0.11 cm-1 for 2 and these values are rationalized by DFT calculations.

Biochemical activity of a fluorescent dye rhodamine 6G: Molecular modeling, electrochemical, spectroscopic and thermodynamic studies.

Interaction of CT DNA with Rhodamine 6G (R6G) has been studied using molecular docking, electrochemical, spectroscopic and thermodynamic methods. From the study, it was illustrated that Rhodamine 6G binds to the minor groove of CT DNA. The binding was cooperative in nature. Circular voltametric study showed significant change in peak current and peak potential due to complexation. All the studies showed that the binding constant was in the order of 106M-1. Circular dichroic spectra showed significant conformational change on binding and DNA unwind during binding. Thermodynamic study showed that binding was favored by negative enthalpy and positive entropy change. From thermodynamic study it was also observed that several positive and negative free energies played significant role during binding and the unfavorable conformational free energy change was overcame by highly negative hydrophobic and salt dependent free energy changes. The experimental results were further validated using molecular docking study and the effect of structure on binding has been studied theoretically. From docking study it was found that the hydrophobic interaction and hydrogen bonds played a significant role during binding. The dye was absorbed by cell and this phenomenon was studied using fluorescent microscope. Cell survivability test showed that the dye active against Human Breast Cancer cells MDA-MB 468. ROS study showed that the activity is due to the production of reactive oxygen.

Thermodynamic study of rhodamine 123-calf thymus DNA interaction: determination of calorimetric enthalpy by optical melting study.

In this paper, the interaction of rhodamine123 (R123) with calf thymus DNA has been studied using molecular modeling and other biophysical methods like UV-vis spectroscopy, fluoremetry, optical melting, isothermal titration calorimetry, and circular dichroic studies. Results showed that the binding energy is about -6 to -8 kcal/mol, and the binding process is favored by both negative enthalpy change and positive entropy change. A new method to determine different thermodynamic properties like calorimetric enthalpy and heat capacity change has been introduced in this paper. The obtained data has been crossed-checked by other methods. After dissecting the free-energy contribution, it was observed that the binding was favored by both negative hydrophobic free energy and negative molecular free energy which compensated for the positive free energies due to the conformational change loss of rotational and transitional freedom of the DNA helix.

Kinetics and mechanism of the oxidation of hydroxylamine by a {Mn3O4}4+ core in aqueous acidic media.

In this work we report the kinetics of oxidation of hydroxylamine by a trinuclear Mn(IV) oxidant, [Mn(3)(µ-O)(4)(phen)(4)(H(2)O)(2)](4+) (1, phen = 1,10-phenanthroline), in aqueous solution over a pH range 2.0-4.0. The trinuclear Mn(IV) species (1) deprotonates in aqueous solution at physiological pH: 1 ⇌ 2 + H(+); pK(1) = 4.00 (± 0.15) at 25.0 °C, I = 1.0 (M) NaNO(3). Both 1 and 2 are reactive oxidants reacting with the conjugate acid of hydroxylamine, viz. NH(3)OH(+) where the deprotonated oxidant 2 reacts faster. This finding is in contrast to a common observation and belief that protonated oxidants react quicker than their deprotonated analogues. Mn(IV)(3) to Mn(II) transition in the present reaction proceeds through the intervention of a spectrally detected mixed-valent Mn(III)Mn(IV) dimer that quickly collapses to Mn(II). The rate of the reaction was found to be lowered in D(2)O-enriched media in comparison to that in pure H(2)O media. An initial one electron one proton transfer to Mn(IV)(3) (electroprotic; 1e, 1H(+)) could be mechanistically conceived as the rate step. We also demonstrate by means of high level DFT studies that, among the two sets of Mn(IV) atoms in the trinuclear oxidant, the unique one that is coordinated with two phen ligands and two oxo-bridges is reduced to Mn(III) at the rate step. This is explained based on energetic and spin density calculations. Moreover, this result agrees with the charge distribution on the Mn atoms of the trinuclear complex.

Mechanistic studies on the oxidation of ascorbic acid and hydroquinone by a {Mn4O6}4+ core in aqueous media.

Described in this work is the kinetics of oxidation of ascorbic acid and hydroquinone by a tetranuclear Mn(IV) oxidant, [Mn(4)(µ-O)(6)(bipy)(6)](4+) (1(4+), bipy =2,2(/)-bipyridine), in aqueous solution over a wide pH range 1.5-6.0. In particular, below pH 3.0, protonation on the oxo-bridge of 1(4+) results in the formation of [Mn(4)(µ-O)(5)(µ-OH)(bipy)(6)](5+) (1H(5+)) as an additional oxidant over 1(4+). Both ascorbic acid and ascorbate whereas only hydroquinone and none of its protolytic species were found to be reactive reducing agents in these reactions. Analysis of the rate data clearly established that the oxo-bridge protonated oxidant 1H(5+) is kinetically far more superior to 1(4+) in oxidizing ascorbic acid and hydroquinone. Rates of these reactions are substantially lowered in D(2)O-enriched media in comparison to that in H(2)O media. An initial one electron one proton transfer electroprotic rate step could be mechanistically conceived. DFT studies established that among the two sets of terminal and central Mn(IV) atoms in the tetranuclear oxidant, one of the two terminal Mn(IV) is reduced to Mn(III) at the rate step that we can intuitively predict considering the probable positive charge distribution on the Mn(IV) atoms.