A linear Cu-Gd-Cu-Gd complex derived from the assembly reaction of [NaCuH3L] and [Gd(thd)3(H2O)2] (H3L = meso-1,2-diphenyl-1-(2-oxybenzamido)-2-(2-oxy-3-ethoxybenzylideneamino)ethane and Hthd = 2,2,6,6-tetra-methyl-3,5-heptanedione)

A linear tetranuclear Cu^II-Gd^III-Cu^II-Gd^III complex [Cu^IIL^dpen(meso)Gd^III(thd)₂(H₂O)]₂ was synthesized from the reaction of [NaCu^IIL^dpen(meso)(DMF)] with [Gd^III(thd)₃(H₂O)₂], and the structures and magnetic properties were investigated, where H₃L^dpen(meso) = meso-1,2-diphenyl-1-(2-hydroxybenzamido)-2-(2-hydroxy-3-ethoxybenzylideneamino)ethane and Hthd = 2,2,6,6-tetramethyl-3,5-heptanedione. The Cu^II complex component [NaCu^IIL^dpen(meso)(DMF)] has a one-dimensional (1D) chain structure, in which the Na⁺ ion is coordinated by two phenoxo and an ethoxy oxygen atoms of a Cu^II complex and an amido oxygen atom of the adjacent Cu^II unit to produce the 1D structure, in which the diphenylethylenediamine moieties have the array of {(1R,2S)-Na-(1S,2R)}_1∞. The assembly reaction of the Cu^II and Gd^III components gave a linear complex with the array of Cu(1)-Gd(1)-Cu(2)-Gd(2), in which two diphenylethylenediamine moieties have the same chirality of (1R,2S)-(1R,2S) or (1S,2R)-(1S,2R). Two linear Cu(1)-Gd(1)-Cu(2)-Gd(2) units are linked by hydrogen bonds through two water molecules to give a cyclic structure with a center of symmetry. The temperature dependence of the magnetic susceptibilities and field-dependent magnetization revealed the ferromagnetic interaction between the Cu^II and Gd^III ions within the linear chain.     

Crystal structures and Mössbauer spectra of spin-crossover iron(III) complexes of quinquedentate ligands

Magnetic properties, Mössbauer spectra and crystal structures of spin-crossover iron(III) complexes with a quinquedentate ligand [FeLX]BPh₄ are reported. X and L denote a unidentate ligand and a quinquedentate ligand, respectively. [Fe(mbpN)(im)]BPh₄ shows spin-crossover behavior in an appropriate organic solvent, and [Fe(mbpN)(lut)]BPh₄, [Fe(bpN)(py)]BPh₄ and [Fe(salten)X]BPh₄ (X=4me-py or 2me-im) show spin-crossover behavior in a solid and in an organic solvent. It was found that the ligand field strength of salten was stronger than that of mbpN. The rates of spin-state interexchange in the complexes are as fast as the inverse of the lifetime (1×10^–7 s) of the Mössbauer nuclear level. The Mössbauer spectroscopic behavior of [Fe(mbpN)(lut)]BPh₄ and [Fe(bpN)(py)]BPh₄ is different to that of [Fe(salten)X]BPh₄ (X=4me-py or 2me-im). The difference was ascribed to the different geometrical positions of the corresponding anions.     

Design and characterization of endosomal-pH-responsive coiled coils for constructing an artificial membrane fusion system

A weakly acidic pH-responsive polypeptide is believed to have the potential for an endosome escape function in a polypeptide-triggered delivery system. For constructing a membrane fusion device with pH-responsiveness, we have designed novel polypeptides that are capable of forming an α2 coiled coil structure. Circular dichroism spectroscopy reveals that a polypeptide, AP-LZ(EH5), with a Glu and His salt-bridge pair at a staggered position in the hydrophobic core forms a stable coiled coil structure only at endosomal pH values (pH 5.0 to 5.5). On the basis of their endosomal-pH responsiveness, a boronic acid/polypeptide conjugate (BA-H5-St) was also designed as a pilot molecule to construct a pH-responsive, one-way membrane fusion system with a sugarlike compound (phosphatidylinositol: PI)-containing liposome as a target. Membrane fusion behavior was characterized by lipid-mixing, inner-leaflet lipid-mixing, and contents-mixing assays. These studies reveal that membrane fusion is clearly observed when the pH of the experimental system is changed from 7.4 (physiological condition) to 5.0 (endosomal condition).     

Endocytosis-like uptake of surface-modified drug nanocarriers into giant unilamellar vesicles

We had previously developed surface-modified poly (D,L-lactide-co-glycolide) (PLGA) nanoparticles (NPs) for use as a cellular drug delivery system. The cellular uptake of PLGA-NPs was mediated predominantly by endocytosis, and this uptake was increased by surface modifications with polymers, such as chitosan (CS) and polysorbate 80 (P80). In the present study, we prepared a cell-sized giant unilamellar vesicle (GUV) that mimics a cell membrane to investigate the interaction between cell membranes and NPs. Endocytosis-like uptake of NPs into a GUV was observed when the NPs were modified with nonionic surfactant P80 probably due to change in viscoelasticity and enhanced fusion activity of the membrane induced by P80. In contrast, unmodified NPs and those modified with CS were not internalized into a GUV. These results suggest that surface properties of PLGA-NPs are an important formulation parameter for their interaction with lipid membranes.