Amrit Sarmah

Understanding the preferential binding interaction of aqua-cisplatins with nucleobase guanine over adenine: a density functional reactivity theory based approach

The binding interaction of hydrolyzed cisplatin with the nucleobases guanine and adenine has been investigated using density functional reactivity theory (DFRT). The energy changes involved in the interaction of both mono-aqua and di-aqua cisplatins with these two purine bases are explored on the basis of different reactivity parameters of DFRT. As observed by earlier experimental and theoretical studies, the present approach also reveals that interaction of the cisplatin di-aqua complex is stronger than the corresponding mono-aqua complex and, more importantly, interactions of both mono and di-aqua complexes of cisplatin are stronger with guanine than with adenine. Interestingly, all these observations are based on the energy components and charge transfer quantities which depend solely on the electronic properties of the isolated aqua-cisplatins and nucleobases (i.e., not on the electronic properties of the adducts). Moreover, the sign of the energy components and charge transfer values clearly demonstrate the electron donor and acceptor nature of the purine bases and aqua cisplatins, respectively. The results are also consistent across all three methods (both ab initio and DFT) adopted in this study.

Effect of Geometrical Distortion on the Electronic Structure: Synthesis and Characterization of Monoradical-Coordinated Mononuclear Cu(II) Complexes

Ligand H3Sami(Mixed(tBu)) was composed of two different compartments, a redox-active 2-aminophenol and a salen salicylidene. Both compartments were linked via a benzyl linker. The ligand reacted with CuCl2·2H2O under air in the presence of Et3N and provided the corresponding monoradical-coordinated mononuclear Cu(II) complex (1). Complex 1, in solution, reacted with air and provided complex 2 via ligand-centered oxygenation at the benzyl-CH2 position. Both complexes were characterized via IR, mass spectrometry, X-ray single-crystal diffraction, variable-temperature magnetic susceptibility, cyclic voltammograms (CVs), and UV-vis/NIR spectroscopic techniques. X-ray crystallographic analyses clearly showed almost equally distorted square planar geometry around the Cu(II) atom in both complexes. However, the bending of the radical-containing C6 ring compared to the N1-Cu1-O1 plane was different in both complexes. While complex 1 was paramagnetic and showed a ferromagnetic coupling between the d(x(2)-y(2)) magnetic orbital of Cu(II) ion and the p(z) orbital of coordinated π-radical, complex 2 was diamagnetic by experiencing a strong antiferromagnetic coupling between the two magnetic orbitals. UV-vis/NIR spectra of the complexes were dominated by charge-transfer transitions. CVs of the complexes showed two reversible one-electron oxidations and one reversible one-electron reduction. E(1/2)(ox2) and E(1/2)(red1) potentials were different in both complexes, while E(1/2)(ox1) values were almost the same and the process corresponded to the formation of phenoxyl radical. Theoretical studies were also performed to understand the magnetic coupling phenomena, and TD-DFT calculations were employed for the assignment of charge-transfer absorption bands.

Encapsulation of multi-stimuli AIE active platinum(II) complex: a facile and dry approach for luminescent mesoporous silica

Luminescent materials have great potential in diverse applications in their solid state. Because these materials are subject to the aggregation-caused quenching (ACQ) effect, increasing attention is focused on synthesizing aggregation-induced emission (AIE) active materials to avoid the ACQ effect. Herein a new class of AIE active, excimeric platinum(II) complex, [Pt(C^N)(L1)(Cl)], 3 [C^N = 2-phenylpyridine; L1 = N1-tritylethane-1,2-diamine] is reported. The complex 3 exhibited mechanofluorochromism (MFC) and thereby transformed into an orange-emitting complex, 3a, upon grinding. Crushing of 3 (or 3a) with meso-structured silica produced a luminescent composite material, 3b, and thereby the AIE Pt(II) complex moved into the mesopores and the process signaled with a drastic change of emission color (yellow → green). The solid-state luminescent behaviour of these complexes was thoroughly studied. The photophysical properties were also supported by TD-DFT based theoretical study.

Non-covalent control of spin-state in metal-organic complex by positioning on N-doped graphene

Nitrogen doping of graphene significantly affects its chemical properties, which is particularly important in molecular sensing and electrocatalysis applications. However, detailed insight into interaction between N-dopant and molecules at the atomic scale is currently lacking. Here we demonstrate control over the spin state of a single iron(II) phthalocyanine molecule by its positioning on N-doped graphene. The spin transition was driven by weak intermixing between orbitals with z-component of N-dopant (pz of N-dopant) and molecule (dxz, dyz, dz2) with subsequent reordering of the Fe d-orbitals. The transition was accompanied by an electron density redistribution within the molecule, sensed by atomic force microscopy with CO-functionalized tip. This demonstrates the unique capability of the high-resolution imaging technique to discriminate between different spin states of single molecules. Moreover, we present a method for triggering spin state transitions and tuning the electronic properties of molecules through weak non-covalent interaction with suitably functionalized graphene.Molecules can change their electronic properties when they are adsorbed on substrates, which can be useful for sensing and catalysis. Here, the authors use atomic force microscopy to show that the spin state of an iron complex can be changed upon displacing the molecule to different sites of a nitrogen-doped graphene

A density functional reactivity theory (DFRT) based approach to understand the interaction of cisplatin analogues with protecting agents.

In the present study some new insights are put into one of the major concern of cisplatin therapy and that is on the reduction of various cytotoxic and nephrotoxic side-effects of cisplatin analogues in cancer treatment. A better understanding of the interaction between different cisplatin analogues with various protecting agents can be achieved from the descriptors generated by density functional reactivity theory based comprehensive decomposition analysis of stabilization energy (Bagaria et al. in Phys Chem Chem Phys 11:8306–8315, 2009) scheme. Taking into account of three types of interactions i.e., of (1) Cisplatin analogues with DNA bases and base pairs (2) Cisplatin analogues with protecting agents and (3) Protecting agents with DNA bases, it is possible to develop a strategy (albeit qualitative) that suggests the best possible combinations of these drugs with protecting agents which can cause reduction in the toxic side-effects of cisplatin therapy. The sample set comprises of 96 pairs of cisplatin analogues and rescue agents and the generated data confirms the predictive power of the adopted strategy.Graphical abstractSulfur containing protecting agents are capable of inhibiting the toxicity induced by the cisplatin analogues in cancer treatment. Thermodynamic and kinetic parameters generated by DFRT based CDASE scheme are found to be helpful in predicting the best possible protecting agent against a particular cisplatin analogue.

Understanding the spin-dependent electronic properties of symmetrically far-edge doped zigzag graphene nanoribbon from a first principles study

We have systematically elucidated the electronic and spin dependent behavior of far-edge doped zigzag graphene nanoribbons from a DFT based first principles study. The relative changes in the electronic environment due to an increment in the impurity concentration as well as the size of the hexagonal carbon network have been thoroughly assayed. Although the substitutional doping is most favorable at the edges of the nanoribbons, our DFT simulations envision that far-edge doping also induced some tunable spin-dependent properties in the zigzag graphene nanoribbons. The total magnetic moment of the systems seems to have some sequential dependencies on the doping concentration along with the relative growth in the crystal lattice. It is observed that the impurity (boron or nitrogen) doping at the far-edge sites can break the electronic degeneracy in the up and down spin channels. Interestingly, in both cases spin-up electrons are found to be metallic or semi-metallic and the spin-down electrons demonstrate an insulating nature.

An Iminosemiquinone-Coordinated Oxidovanadium(V) Complex: A Combined Experimental and Computational Study

Ligand H4Sar(AP/AP) contained two terminal amidophenolate units that were connected by a disulfane bridge. The ligand reacted with VOSO4·5H2O in the presence of Et3N under air and provided a mononuclear octahedral oxidovanadium complex (1). X-ray crystal structure analysis of complex 1 revealed that the oxidation state of the V ion was V and the VO3+ unit was coordinated to an iminosemiquinone radical anion. An isotopic signal at g = 1.998 in the X-band electron paramagnetic resonance (EPR) spectrum and the solution magnetic moment μeff = 1.98 μB at 298 K also supported the composition. The formation of complex 1 preceded through the initial generation of a diamagnetic VO2+-iminoisemiquinone species, as established by time-dependent UV-vis-near-IR (NIR), X-band EPR, and density functional theory studies. The UV-vis-NIR spectrum of complex 1 consisted of four ligand-to-metal charge-transfer transitions in the visible region, while an intervalence ligand-to-ligand charge transfer appeared at 1162 nm. The cyclic voltammogram of the complex showed four oxidation waves and one reduction wave. Spectroelectrochemical studies at fixed potentials revealed that the oxidation and reduction processes were ligand-based.

Sequential BN-doping induced tuning of electronic properties in zigzag-edged graphene nanoribbons: a computational approach

We employed first-principles methods to elaborate doping induced electronic and magnetic perturbations in one-dimensional zigzag graphene nanoribbon (ZGNR) superlattices. Consequently, the incorporation of alternate boron and nitrogen (hole–electron) centers into the hexagonal network instituted substantial modulations to electronic and magnetic properties of ZGNR. Our theoretical analysis manifested some controlled changes to electronic and magnetic properties of the ZGNR by tuning the positions (array) of impurity centers in the carbon network. Subsequent DFT based calculations also suggested that the site-specific alternate electron–hole (B/N) doping could regulate the band-gaps of the superlattices within a broad range of energy. The consequence of variation in the width of ZGNR in the electronic environment of the system was also tested. The systematic analysis of various parameters such as the structural orientations, spin-arrangements, the density of states (DOS), band structures, and local density of states envisioned a basis for the band-gap engineering in ZGNR and attributed to its feasible applications in next generation electronic device fabrication.

A New Bis(aquated) High Relaxivity Mn(II) Complex as an Alternative to Gd(III)-Based MRI Contrast Agent

Disclosed here are a piperazine, a pyridine, and two carboxylate groups containing pentadentate ligand H2pmpa and its corresponding water-soluble Mn(II) complex (1). DFT-based structural optimization implied that the complex had pentagonal bipyramidal geometry where the axial positions were occupied by two water molecules, and the equatorial plane was constituted by the ligand ON3O donor set. Thus, a bis(aquated) disc-like Mn(II) complex has been synthesized. The complex showed higher stability compared with Mn(II)-EDTA complex [log KMnL = 14.29(3)] and showed a very high r1 relaxivity value of 5.88 mM-1 s-1 at 1.41 T, 25 °C, and pH = 7.4. The relaxivity value remained almost unaffected by the pH of the medium in the range of 6-10. Although the presence of 200 equiv of fluoride and bicarbonate anions did not affect the relaxivity value appreciably, an increase in the value was noticed in the presence of phosphate anion due to slow tumbling of the complex. Cell viability measurements, as well as phantom MR images using clinical MRI imager, consolidated the possible candidature of complex 1 as a positive contrast agent.