Modification of proteins with azobenzene crosslinkers using reversible covalent bonds.

Thiol-reactive reagents designed for the chemical modification of proteins cannot, in general, be used directly for the modification of intracellular targets because the presence of millimolar concentrations of glutathione inside cells effectively outcompetes reaction with target thiols. Here we report an equilibrium, entropic strategy for achieving target selectivity using a cyanoacrylate-based thiol-reactive cross-linker (BCNA) with two reactive sites. This compound exhibits ≳200-fold selectivity for reaction with target peptides and proteins containing appropriately spaced pairs of thiols, versus reaction with mono-thiols. Photo-isomerization of the azobenzene moiety of the cross-linker can be used to affect the conformation of the target peptide or protein. This approach suggests a general strategy for the chemical modification of intracellular peptide and protein targets.

Synthesis, crystal structure and determination of the pKa value of 2,6-dimethoxycyclohexa-2,5-diene-1,4-dione 1-[2-(quinolin-8-yl)hydrazone].

The azo-coupling reaction between 8-aminoquinoline and 3,5-dimethoxyphenol produces 2,6-dimethoxycyclohexa-2,5-diene-1,4-dione 1-[2-(quinolin-8-yl)hydrazone], C17H15N3O3. Crystallization from methanol and strong alkaline solutions produced nonsolvated and solvated crystals, respectively. The crystal structure analysis and 1H NMR spectroscopy studies revealed that the compound exists only as the hydrazone form. A UV-Vis spectroscopic titration study revealed that the hydrazone compound has a relatively high pKa value of 10.0.

Synthesis, crystal structure and photophysical properties of chlorido-[(E)-3-hy-droxy-2-methyl-6-(quinolin-8-yldiazen-yl)phenolato]copper(II) monohydrate.

The reaction between copper(II) chloride dihydrate and the (E)-2-methyl-4-(quinolin-8-yldiazen-yl)benzene-1,3-diol ligand in aceto-nitrile leads to the formation of the title compound, [Cu(C16H12N3O2)Cl]·H2O. The ligand is deprotonated and coordinates with three donor atoms (tridentate) to the CuII ion. Individual mol-ecules of the CuII complex are connected by chloride bridges, forming a one-dimensional coordination polymer. No photoisomerization to the cis isomer of the azo ligand was observed upon irradiation with UV light.

Luminescence switch based on the acid/base induced reversibility of covalent bonds in lanthanide(III) complexes.

We report an exceptional example of Tb(III) luminescence switching using a reversible covalent bond. The antenna and quencher moieties attached to a ligand of a Tb(III) complex undergo acid/base-driven exchange based on the reversible formation of a hemiaminal ether structure to achieve on-off regulation of luminescence.

Spin Ice-like Magnetic Relaxation of a Two-dimensional Network based on Manganese(III) Salen-type Single-Molecule Magnets.

Spin ice is an exotic type of magnetism displayed by bulk rare-earth pyrochlore oxides. We discovered a spin ice-like magnetic relaxation of [{Mn(saltmen)}4 {Mn(CN)6 }](ClO4 )⋅13 H2 O (saltmen2- =N,N'-(1,1,2,2-tetramethylethylene)bis(salicylideneiminate)). This magnetic system can be considered as a two-dimensional network of MnIII salen-type single-molecule magnets (SMMs) in which each SMM unit (ST =4) has two orthogonally oriented axial anisotropies and is connected ferromagnetically through the [Mn(CN)6 ]3- unit (S=1). This work illustrates that a two-dimensional SMM network with competition between the ferromagnetic interaction and local noncollinear magnetic anisotropies on SMMs is a new type of magnetic system exhibiting slow relaxation of magnetization with a Davidson-Cole-type broad distribution of the relaxation time.

Crystal structures of two isotypic lanthanide(III) complexes: tri-aqua-[2,6-di-acetyl-pyridine bis-(benzoyl-hydrazone)]methano-llanthanide(III) trichloride methanol disolvates (LnIII = Tb and Dy).

The title lanthanide complexes, [Ln(DAPBH2)(CH3OH)(H2O)3]Cl3·2CH3OH [LnIII = Tb and Dy; DAPBH2 = 2,6-di-acetyl-pyridine bis-(benzoyl-hydrazone), C23H21N5O2], are isotypic. The central lanthanide ions are nine-coordinate, being ligated by three N and two O atoms from the penta-dentate DAPBH2 ligand, and four O atoms from the coordinated methanol mol-ecule and three coordinated water mol-ecules. The coordination geometry of the lanthanide ion is a distorted capped square anti-prism. In the crystals, the various components are linked by O-H⋯Cl, N-H⋯Cl and O-H⋯O hydrogen bonds, forming three-dimensional supra-molecular frameworks. Within the frameworks, there are C-H⋯Cl and C-H⋯O hydrogen bonds and offset π-π inter-actions (inter-centroid distance ca 3.81 Å).

A hemiaminal-ether structure stabilized by lanthanide complexes with an imidazole-based Schiff base ligand.

Imidazole-based lanthanide complexes bearing a hemiaminal-ether pendant, [Ln(L'-OR)(NO3)2](NO3)·solvent (Ln = Tb and Gd; L'-OR = hemiaminal-ether ligand, 1,9-bis(1H-imidazole-4-yl)-5-[(R-1H-imidazole-4-yl)alkyl]-2,5,8-triazanon-1,8-dien; R = Me and Et; solvent = MeOH, EtOH, and H2O; abbreviated as LnL'-OR), were unexpectedly obtained from the reaction of diethylenetriamine with 4-imidazolecarbaldehyde in the presence of lanthanide nitrate in alcohol. These LnL'-OR structures were also obtained by treating dipodal imidazole-based lanthanide complexes [Ln(HL)(NO3)2(MeOH)](NO3)·MeOH (HL = bis{[2-{(imidazole-4-yl)methylidene}amino]ethyl}amine; abbreviated as LnHL) independently with (i) 4-imidazolecarbaldehyde, (ii) HNO3, or (iii) NO in alcohol. The results indicate that the hemiaminal-ether structure is stabilized by the lanthanide complex. The LnL'-OR structures were confirmed by X-ray analysis and cold electrospray ionization mass spectrometry in solution. The photophysical properties indicate that the L'-OR ligands can sensitize the Tb3+ luminescence. A significant spectral change was observed by the ligand component exchange. The exchange of the ligand component of the non-luminescent imidazole-based Tb complex, [Tb(MeL)(NO3)2](NO3) (MeL = bis{[2-{(1-methylimidazole-2-yl)methylidene}amino]ethyl}amine; abbreviated as TbMeL), results in a significant spectral change.

C-HCl- hydrogen bonds in solution and in the solid-state: HgCl2 complexes with cyclen-based cryptands.

Structural evidence is reported for C-HCl- hydrogen bonds in solution and in the solid state of HgCl2 complexes with cyclen-based cryptands. These cyclen-based cryptands (1) and (2) are bridged by di- and triethylene glycol units, respectively, between two aromatic rings. The X-ray structure indicates that the 2/HgCl2 complex contains an acetonitrile molecule in the cavity.

Homochiral 1D helical chain based on an achiral Cu(II) complex.

Self-assembly of an achiral [Cu(L)] complex produced a homochiral helical chain [Cu(L)](3)·2H(2)O (1) (L = 2-dimethylaminoethyl(oxamato)). Interestingly, complex 1 obtained in our laboratory exhibits only a left-handed helical chain without any chiral source. Single-crystal X-ray analysis revealed the absolute structure and homochirality of its helical chain structure in the space group of P3(2). Solid-state circular dichroism (CD) spectra confirmed the high enantio excess of the crystals obtained in different synthesis batches. Magnetic susceptibility measurements reveal a relatively strong intrachain antiferromagnetic interaction between Cu(II) centers via an oxamato bridge (J = -74.4 cm(-1)).

Luminescence tuning of imidazole-based lanthanide(III) complexes [Ln = Sm, Eu, Gd, Tb, Dy].

To tune the lanthanide luminescence in related molecular structures, we synthesized and characterized a series of lanthanide complexes with imidazole-based ligands: two tripodal ligands, tris{[2-{(1-methylimidazol-2-yl)methylidene}amino]ethyl}amine (Me(3)L), and tris{[2-{(imidazol-4-yl)methylidene}amino]ethyl}amine (H(3)L), and the dipodal ligand bis{[2-{(imidazol-4-yl)methylidene}amino]ethyl}amine (H(2)L). The general formulas are [Ln(Me(3)L)(H(2)O)(2)](NO(3))(3)·3H(2)O (Ln = 3+ lanthanide ion: Sm (1), Eu (2), Gd (3), Tb (4), and Dy (5)), [Ln(H(3)L)(NO(3))](NO(3))(2)·MeOH (Ln(3+) = Sm (6), Eu (7), Gd (8), Tb (9), and Dy (10)), and [Ln(H(2)L)(NO(3))(2)(MeOH)](NO(3))·MeOH (Ln(3+) = Sm (11), Eu (12), Gd (13), Tb (14), and Dy (15)). Each lanthanide ion is 9-coordinate in the complexes with the Me(3)L and H(3)L ligands and 10-coordinate in the complexes with the H(2)L ligand, in which counter anion and solvent molecules are also coordinated. The complexes show a screw arrangement of ligands around the lanthanide ions, and their enantiomorphs form racemate crystals. Luminescence studies have been carried out on the solid and solution-state samples. The triplet energy levels of Me(3)L, H(3)L, and H(2)L are 21 000, 22 700, and 23 000 cm(-1), respectively, which were determined from the phosphorescence spectra of their Gd(3+) complexes. The Me(3)L ligand is an effective sensitizer for Sm(3+) and Eu(3+) ions. Efficient luminescence of Sm(3+), Eu(3+), Tb(3+), and Dy(3+) ions was observed in complexes with the H(3)L and H(2)L ligands. Ligand modification by changing imidazole groups alters their triplet energy, and results in different sensitizing ability towards lanthanide ions.

Structural and magnetic study of O2 molecules arranged along a channel in a flexible single-crystal host family.

We report the magnetic behaviors of O(2) molecules, which aligned in the channels of four types of single-crystal adsorbents, [M(2)(bza)(4)(pyz)](n) (bza = benzoate; pyz = pyrazine; M = Rh(II) (1a) and Cu(II) (1b)) and [M(2)(bza)(4)(2-mpyz)](n) (2-mpyz = 2-methylpyrazine; M = Rh(II) (2a) and Cu(II) (2b)). The X-ray single-crystal structures at various temperatures from 10 to 298 K were determined for O(2)-included crystals of 1a and 2a. All adsorbed O(2) molecules exhibited abnormal magnetic phases above 4 T in the temperature range around 55-105 K. The magnetic behaviors of adsorbed O(2) molecules between four inclusions were discussed. The existence of the metastable state, which was also suggested by hysteresis on the M-H curves, was revealed by analysis of the time course of the magnetization. Considering that the abnormal magnetic behavior occurred at relatively high temperatures and a low magnetic field, it was suggested that these behaviors were induced because of the changes of magnetic interaction of included O(2) aggregates involving transformation which is supported by the surrounding of the channels of the single-crystal hosts under the applied magnetic field.

Single-molecule magnet behavior in heterometallic M(II)-Mn(III)(2)-M(II) tetramers (M(II) = Cu, Ni) containing Mn(III) salen-type dinuclear core.

The linear-type heterometallic tetramers, [Mn(III)(2)(5-MeOsaltmen)(2)M(II)(2)(L)(2)](CF(3)SO(3))(2) x 2H(2)O (MII = Cu, 1a; Ni, 2a), where 5-MeOsaltmen(2-) = N,N'-(1,1,2,2-tetramethylethylene) bis(5-methoxysalicylideneiminate), and H(2)L = 3-{2-[(2-hydroxy-benzylidene)-amino]-2-methyl-propylimino}-butan-2-one oxime, have been synthesized and characterized from structural and magnetic points of view. These two compounds are isostructural and crystallize in the same monoclinic P2(1)/n space group. The structure has a [M(II)-NO-Mn(III)-(O)(2)-Mn(III)-ON-M(II)] skeleton, where -NO- is a linking oximato group derived from the non-symmetrical Schiff-base complex [M(II)(L)] and -(O)(2)- is a biphenolato bridge in the out-of-plane [Mn(2)(5-MeOsaltmen)(2)](2+) dimer. The solvent-free compounds, 1b and 2b, have also been prepared by drying of the parent compounds, 1a and 2a, respectively, at 100 degrees C under dried nitrogen. After this treatment, the crystallinity is preserved, and 1b and 2b crystallize in a monoclinic P2(1)/c space group without significant changes in their structures in comparison to 1a and 2a. Magnetic measurements on 1a and 1b revealed antiferromagnetic Mn(III)---Cu(II) interactions via the oximato group and weak ferromagnetic Mn(III)---Mn(III) interactions via the biphenolato bridge leading to an S(T) = 3 ground state. On the other hand, the diamagnetic nature of the square planar Ni(II) center generates an S(T) = 4 ground state for 2a and 2b. At low temperature, these solvated (a) and desolvated (b) compounds display single-molecule magnet behavior modulated by their spin ground state.

Alcohol-vapor inclusion in single-crystal adsorbents [M(II)2(bza)4(pyz)]n (M = Rh, Cu): structural study and application to separation membranes.

The vapor absorbency of the series of alcohols methanol, ethanol, 1-propanol, 1-butanol, and 1-pentanol was characterized on the single-crystal adsorbents [M(II)2(bza)4(pyz)]n (bza = benzoate, pyz = pyrazine, M = Rh (1), Cu (2)). The crystal structures of all the alcohol inclusions were determined by single-crystal X-ray crystallography at 90 K. The crystal-phase transition induced by guest adsorption occurred in the inclusion crystals except for 1-propanol. A hydrogen-bonded dimer of adsorbed alcohol was found in the methanol- and ethanol-inclusion crystals, which is similar to a previous observation in 2 x 2EtOH (S. Takamizawa, T. Saito, T. Akatsuka, E. Nakata, Inorg. Chem. 2005, 44, 1421-1424). In contrast, an isolated monomer was present in the channel for 1-propanol, 1-butanol, and 1-pentanol inclusions. All adsorbed alcohols were stabilized by hydrophilic and/or hydrophobic interactions between host and guest. From the combined results of microscopic determination (crystal structure) and macroscopic observation (gas-adsorption property), the observed transition induced by gas adsorption is explained by stepwise inclusion into the individual cavities, which is called the "step-loading effect." Alcohol/water separation was attempted by a pervaporation technique with microcrystals of 2 dispersed in a poly(dimethylsiloxane) membrane. In the alcohol/water separation, the membrane showed effective separation ability and gave separation factors (alcohol/water) of 5.6 and 4.7 for methanol and ethanol at room temperature, respectively.

Structural and magnetic study of N2, NO, NO2, and SO2 adsorbed within a flexible single-crystal adsorbent of [Rh2(bza)4(pyz)]n.

The crystalline one-dimensional compound, [Rh(II)2(bza)4(pyz)]n (1) (bza = benzoate, pyz = pyrazine) demonstrates gas adsorbency for N2, NO, NO2, and SO2. These gas-inclusion crystal structures were characterized by single-crystal X-ray crystallography as 1 x 1.5 N2 (298 K), 1 x 2.5 N2 (90 K), and 1 x 1.95 NO (90 K) under forcible adsorption conditions and 1 x 2 NO2 (90 K) and 1 x 3 SO2 (90 K) under ambient pressure. Crystal-phase transition to the P1 space group that correlates with gas adsorption was observed under N2, NO, and SO2 conditions. The C2/c space group was observed under NO2 conditions without phase transition. All adsorbed gases were stabilized by the host lattice. In the N2, NO, and SO2 inclusion crystals at 90 K, short interatomic distances within van der Waals contacts were found among the neighboring guest molecules along the channel. The adsorbed NO molecules generated the trans-NO...NO associated dimer with short intermolecular contacts but without the conventional chemical bond. The magnetic susceptibility of the NO inclusion crystal indicated antiferromagnetic interaction between the NO molecules and paramagnetism arising from the NO monomer. The NO2 inclusion crystal structure revealed that the gas molecules were adsorbed in the crystal in dimeric form, N2O4.

From an S(T) = 3 single-molecule magnet to diamagnetic ground state depending on the molecular packing of Mn(III)salen-type dimers decorated by N,N'-dicyano-1,4-naphthoquinonediiminate radicals.

Two manganese(III) tetradentate Schiff-base dimers to which N,N'-dicyano-1,4-naphthoquinonediiminate (DCNNQI) radicals are attached have been selectively synthesized by varying the solvents used in the reactions: [Mn2(5-MeOsaltmen)2(DCNNQI)2].MeOH (1) and [Mn2(5-MeOsaltmen)(2)(DCNNQI)(2)] x 2CH2Cl2.2CH3CN (2) [5-MeOsaltmen2- = N,N'-(1,1,2,2-tetramethylethylene)bis(5-methoxysalicylideneiminate)]. These two complexes share the same molecular core, [(DCNNQI.-)-Mn(III)-(O)2-Mn(III)-(DCNNQI.-)], where -(O)2- is a biphenolate bridge in the out-of-plane dimerized [Mn(2)(5-MeOsaltmen)2]2+ moiety. However, their packing arrangements are completely different. Whereas complex 1 is found to be relatively isolated, strong intermolecular dimerization of the DCNNQI moieties (with the nearest contact being approximately 3.0 A) is observed in 2, forming a one-dimensional chain of [-Mn(III)-(O)2-Mn(III)-(DCNNQI.-)2-](infinity). The magnetic susceptibility of 1 can be modeled with an [S = 1/2, 2, 2, 1/2] four-spin system including strong antiferromagnetic Mn(III)/DCNNQI radical coupling (J(Mn/rad)/kB = -23 K) and ferromagnetic Mn(III)/Mn(III) coupling through the biphenolate bridge (J(Mn/Mn)/kB = +2.0 K). These interactions lead to an ST = 3 ground state that possesses significant uniaxial anisotropy (D(S=3)/kB = -2.1 K). Low-temperature ac and dc magnetic data of 1 reveal its single-molecule magnet behavior with quantum tunneling of the magnetization. By contrast, 2 possesses the diamagnetic ground state induced by dominating Mn(III)-Mn(III) antiferromagnetic interactions mediated by the diamagnetic DCNNQI dimers and/or pi-pi contact along the b axis.

Tuning of spin density wave strengths in quasi-one-dimensional mixed-halogen-bridged nickel(III) complexes with strong electron correlation, [Ni(III)(chxn)(2)Cl(1-x)Br(x)](NO(3))(2).

This communication describes the syntheses of the quasi-one-dimensional mixed-halogen-bridged Ni(III) complexes with strong electron correlation [Ni(chxn)(2)Cl(1-x)Br(x)](NO(3))(2) and the tuning of the spin density wave strengths of these compounds. If the Cl 3p and Br 4p make one band in the compounds, we should observe a single peak in the electronic spectra. As a result, we should observe the single peak from 1.45 to 2.00 eV depending on the mixing ratios of Cl and Br ions. Therefore, the Cl 3p and Br 4p make one band. Then, we have succeeded in tuning the spin density wave strengths of the Ni(III) complexes with the strong electron correlation by mixing the bridging halogen ions successively.