Structural and magnetic characterization of two tetranuclear Cu(II) complexes with closed‐cubane‐like core framework

Two novel tetranuclear Cu(II) complexes [Cu4(L1)4]·3(H2O) (1) and [Cu4(H2L2)4(H2O)4] (2) ( H2L1 = (E)-2-((1-hydroxybutan-2-ylimino)methyl)phenol; H4L2 = 2-((2-hydroxy-3-methoxybenzylidene)amino)-2-hydroxymethylpropane-1,3-diol) were synthesized from the self-assembly of copper(II) perchlorate and the tridentate Schiff base ligands. Both complexes crystallize in the tetragonal system with space group I 41/a and form tetranuclear species with closed-cubane like core framework. Both the complexes possess a S4 axis but of different stereochemistry due to the different arrangement of the ligands about the copper ions. Variable temperature magnetic susceptibility measurements indicate an overall weak antiferromagnetic exchange coupling in 1, while ferromagnetic exchange coupling in 2. In agreement with their closed-cubane structure, the magnetic behavior of the two complexes have been studied by employing the isotropic spin Hamiltonian of type H = J1 (S1S3 + S1S4 + S2S3 + S2S4) - J2 (S1S2 + S3S4) (J1 describes the magnetic exchange coupling between the four Cu(II) pairs with short Cu···Cu distances, while J2 characterizes the magnetic exchange coupling between the remaining two intermetallic pairs with long distances). The PHI program was used to study their magnetic behavior. A good agreement between the experimental and fitted curves was found with the following parameters: g = 2.14, J1 = -20.3 cm-1 and J2 = 0 cm-1 for 1 and g = 2.10, J1 = 101.1 cm-1 and J2 = -51.5 cm-1 for 2

Designing reversible multi-level resistance states in a half-doped manganite

We design reversible multi-level resistance states in a half-doped charge-ordered manganite, $\textrm{Sm}_{0.5}\textrm{Ca}_{0.25}\textrm{Sr}_{0.25}\textrm{MnO}_3$ (SCSMO). By exploiting the electronic phase separation in SCSMO at 10 K, we show that the system can be stabilized into several metastable states, against thermal cycling, up to 62 K. The magnetization and the resistivity remain unaltered in each metastable states during the thermal cycling. Monte Carlo calculations using a two-band double-exchange model, including super-exchange, electron-phonon coupling, and quenched disorder, show that the system freezes into a phase-separated metastable state due to the disorder during the thermal cycling. We outline pathway to control the multi-level switching between four reversible metastable states. Our results may open the door for future investigations to engineer memory devices based on multi-level resistance switching using phase-separated transition metal oxides

Functional alteration of a dimeric insecticidal lectin to a monomeric antifungal protein correlated to its oligomeric status.

BackgroundAllium sativum leaf agglutinin (ASAL) is a 25-kDa homodimeric, insecticidal, mannose binding lectin whose subunits are assembled by the C-terminal exchange process. An attempt was made to convert dimeric ASAL into a monomeric form to correlate the relevance of quaternary association of subunits and their functional specificity. Using SWISS-MODEL program a stable monomer was designed by altering five amino acid residues near the C-terminus of ASAL.Methodology/principal findingsBy introduction of 5 site-specific mutations (-DNSNN-), a β turn was incorporated between the 11(th) and 12(th) β strands of subunits of ASAL, resulting in a stable monomeric mutant ASAL (mASAL). mASAL was cloned and subsequently purified from a pMAL-c2X system. CD spectroscopic analysis confirmed the conservation of secondary structure in mASAL. Mannose binding assay confirmed that molecular mannose binds efficiently to both mASAL and ASAL. In contrast to ASAL, the hemagglutination activity of purified mASAL against rabbit erythrocytes was lost. An artificial diet bioassay of Lipaphis erysimi with mASAL displayed an insignificant level of insecticidal activity compared to ASAL. Fascinatingly, mASAL exhibited strong antifungal activity against the pathogenic fungi Fusarium oxysporum, Rhizoctonia solani and Alternaria brassicicola in a disc diffusion assay. A propidium iodide uptake assay suggested that the inhibitory activity of mASAL might be associated with the alteration of the membrane permeability of the fungus. Furthermore, a ligand blot assay of the membrane subproteome of R. solani with mASAL detected a glycoprotein receptor having interaction with mASAL.Conclusions/significanceConversion of ASAL into a stable monomer resulted in antifungal activity. From an evolutionary aspect, these data implied that variable quaternary organization of lectins might be the outcome of defense-related adaptations to diverse situations in plants. Incorporation of mASAL into agronomically-important crops could be an alternative method to protect them from dramatic yield losses from pathogenic fungi in an effective manner