Magnetic Modulation in Heteroallene and Heterocumulene Based tert-butyl Nitroxide Diradicals: Spin Delocalization and Conformation Play Crucial Roles

We have theoretically investigated the magnetic properties of heteroallene (>C=C=X−) and heterocumulene (>C=C=C=X−) based tert-butyl nitroxide diradicals (X is P/As). Calculation of magnetic exchange coupling constant (J) shows ferromagnetic interaction in heteroallene based diradicals. Whereas, in heterocumulene based diradicals, tuning of J value from antiferro- to ferro-magnetic state is observed from Z- to E- isomer. Delocalization of spin density from radical site to the coupler (in planar arrangement) is observed in spin distribution analysis which is also advocated by molecular orbital analysis. The typical feature of tert-butyl nitroxide radical creates spin delocalization along with spin polarization within the coupler. The J values of all the diradicals strongly depend on the dihedral angle between radical center and coupler. Magneto-structural correlation shows that the change in dihedral angle tunes the magnetic property for both the Z- and E- isomers of heterocumulenes depending on the spin accumulation on two nearby magnetic centers. The extent of spin delocalization and conformation of spin centers on the molecular axis are important for the different J values observed in our designed systems.     

On the Control of Magnetic Anisotropy through an External Electric Field

The effect of an external electric field on the magnetic anisotropy of a single‐molecule magnet has been investigated, for the first time, with the help of DFT. The application of an electric field can alter the magnetic anisotropy from “easy‐plane” to “easy‐axis” type. Excitation analysis performed through time‐dependent DFT predicts that the external electric field facilitates metal to π‐acceptor ligand charge transfer, leading to uniaxial magnetic anisotropy and concomitant spin Hall effect in a single molecule.     

Nickel(II)-Mediated Reversible Thiolate/Disulfide Conversion as a Mimic for a Key Step of the Catalytic Cycle of Methyl-Coenzyme M Reductase

According to the well-accepted mechanism, methyl-coenzyme M reductase (MCR) involves Ni-mediated thiolate-to-disulfide conversion that sustains its catalytic cycle of methane formation in the energy saving pathways of methanotrophic microbes. Model complexes that illustrate Ni-ion mediated reversible thiolate/disulfide transformation are unknown. In this paper we report the synthesis, crystal structure, spectroscopic properties and redox interconversions of a set of Ni^II complexes comprising a tridentate N₂S donor thiol and its analogous N₄S₂ donor disulfide ligands. These complexes demonstrate reversible Ni^II-thiolate/Ni^II-disulfide (both bound and unbound disulfide-S to Ni^II) transformations via thiyl and disulfide monoradical anions that resemble a primary step of MCR's catalytic cycle.     

Investigation of the solid phase synthesis of tyrosine‐derived diphenol monomers with resin‐bound carbodiimide coupling reagents

Tyrosine‐derived pseudo‐polypeptides have recently been established as potential biomaterials. The bulk production of these polymers is dependant upon the convenient synthesis of tyrosine‐derived diphenolic monomers. Suitable solution phase methods for the synthesis of such monomers, with transient activated esters, have been reported previously by Kohn, J.; Langer, R. In Biomaterials Science: An Introduction to Materials in Medicine; Ratner, B. D.; Hoffman, A. S.; Schoen, F. J.; Lemons, J. E., Eds.; Academic Press: New York, 1996; pp 64–72. However, for all the methods reported by them, purification and isolation of the diphenol monomer involves rigorous extraction, column purification, or an extensive aqueous workup. These may lead to the incorporation of solvent‐based as well as inorganic impurities and also may lead to difficulties in the process scale‐up. In this article, an alternate and relatively more convenient method for the synthesis of tyrosine‐based diphenol monomers is reported. This involved the investigation of polymer resin‐bound carbodiimide in the solid‐phase synthesis of L‐tyrosine‐based diphenolic monomers. This method was found to eliminate the need for rigorous purification processes and was found to maintain reasonable yield as well as maintain purity of the final monomer product. The monomer was able to produce polymers of a reasonably high‐molecular‐weight and a narrow polydispersity. The amide bond formation by such a polymer‐tethered reagent can be described to follow a reverse‐Merrifield sense and the method is relatively convenient to scale up. The L‐tyrosine‐based diphenolic monomeric compound formed by this method was analyzed by NMR, Fourier transform infrared, and elemental microanalysis techniques for chemical structure and composition. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4906–4915, 2004     

Benzopyrans. Part 42. Reactions of 4‐Oxo‐4H‐1‐benzopyran‐3‐carbaldehyde with some active methylene compounds in the presence of ammonia

The title aldehyde 1 in the presence of ammonia gives the pyridine derivatives 9‐11 respectively with acetylacetone, diethyl malonate and ethyl cyanoacetate, and ethyl (or methyl)‐l‐benzopyrano[4,3‐b]pyri‐dine‐3‐carboxylate 22 (or 23 ) with ethyl (or methyl) acetoacetate. Acetylacetone pretreated with ammonia condenses with 1 giving the fused pyridine 24 . Ammonia converts the ester 6 to the pyridine 13 or 14 . Chromic acid oxidation of 22 and 23 affords the coumarinopyridines 25 and 26 , respectively.     

Structure, interactions, and antibacterial activities of MSI-594 derived mutant peptide MSI-594F5A in lipopolysaccharide micelles: role of the helical hairpin conformation in outer-membrane permeabilization

Lipopolysaccharide (LPS) provides a well-organized permeability barrier at the outer membrane of Gram-negative bacteria. Host defense cationic antimicrobial peptides (AMPs) need to disrupt the outer membrane before gaining access to the inner cytoplasmic membrane or intracellular targets. Several AMPs are largely inactive against Gram-negative pathogens due to the restricted permeation through the LPS layer of the outer membrane. MSI-594 (GIGKFLKKAKKGIGAVLKVLTTG) is a highly active AMP with a broad-spectrum of activities against bacteria, fungi, and virus. In the context of LPS, MSI-594 assumes a hairpin helical structure dictated by packing interactions between two helical segments. Residue Phe5 of MSI-594 has been found to be engaged in important interhelical interactions. In order to understand plausible structural and functional inter-relationship of the helical hairpin structure of MSI-594 with outer membrane permeabilization, a mutant peptide, termed MSI-594F5A, containing a replacement of Phe5 with Ala has been prepared. We have compared antibacterial activities, outer and inner membrane permeabilizations, LPS binding affinity, perturbation of LPS micelles structures by MSI-594 and MSI-594F5A peptides. Our results demonstrated that the MSI-594F5A has lower activities against Gram-negative bacteria, due to limited permeabilization through the LPS layer, however, retains Gram-positive activity, akin to MSI-594. The atomic-resolution structure of MSI-594F5A has been determined in LPS micelles by NMR spectroscopy showing an amphipathic curved helix without any packing interactions. The 3D structures, interactions, and activities of MSI-594 and its mutant MSI-594F5A in LPS provide important mechanistic insights toward the requirements of LPS specific conformations and outer membrane permeabilization by broad-spectrum antimicrobial peptides.     

N–H···π Interactions in Two Isomers of an Amino Group Containing <Emphasis Type="Italic">bis</Emphasis>-Phenol

Abstract  

Crystal structures of two bis-phenols namely bis-(3,5-dimethyl-2-hydroxyphenyl)(3-amino phenyl)methane 1 and bis-(3,5-dimethyl-4-hydroxyphenyl)(3-aminophenyl) methane 2 are determined. The compound 1 crystallises in monoclinic P2₁/c with a = 12.2579(16) ?, b = 16.0906(19) ?, c = 10.6664(13) ?, β = 115.417(7)°, V = 1900.2(4) ?³ whereas 2 crystallizes in monoclinic C2/c, a = 9.2538(2) ?, b = 18.6579(4) ?, c = 23.2725(5) ?, β = 98.796(2)°, V = 3970.89(15) ?³. The crystal lattice of both the compounds shows presence of N–H···π interactions but no O–H···π interactions.