Two-fold addition reaction of silylene to C60: structural and electronic properties of a bis-adduct.

The addition reaction of C60 with silylene 1, a silicon analog of carbene, yielded the corresponding bis-adduct 3. The structure of 3 was determined by single-crystal X-ray structure analysis, representing the first example of a crystal structure of a silirane (silacyclopropane) derivative of fullerenes. Electrochemical measurements confirmed that the redox potentials of 3 are shifted cathodically compared to those of the parent mono-adduct 2. Density functional theory (DFT) calculations provided the basis for the electronic properties of compound 3.

Preparation, Structural Determination, and Characterization of Electronic Properties of [5,6]- and [6,6]-Carbosilylated Sc3 N@Ih -C80.

Photochemical carbosilylation of Sc3 N@Ih -C80 with silirane 1 afforded two corresponding [5,6]-adducts, 2 and 3, and a [6,6]-adduct, 4. The structural and electronic properties of these products were characterized by means of spectroscopic, electrochemical, and theoretical methods. The structure of 2 was disclosed by means of single-crystal X-ray crystallographic analysis. Thermal isomerization of 3 to 2 was observed, whereas that of 2 to 3 proceeded less efficiently at 100 °C. Upon heating under the same conditions, adduct 4 underwent facile decomposition to afford Sc3 N@Ih -C80 , or isomerized into small amounts of 2 and 3. The relative stabilities of 2, 3, and 4 were rationalized through the results of theoretical calculations. In contrast, adducts 2, 3, and 4 were stable under the photolytic conditions employed for carbosilylation. The photochemical functionalization of Sc3 N@Ih -C80 represents a convenient synthetic method to obtain thermally labile fullerene-based products.

Role of Heavy Meromyosin in Heat-Induced Gelation in Low Ionic Strength Solution Containing L-Histidine.

The gelation of myosin has a very important role in meat products. We have already shown that myosin in low ionic strength solution containing L-histidine forms a transparent gel after heating. To clarify the mechanism of this unique gelation, we investigated the changes in the nature of myosin subfragments during heating in solutions with low and high ionic strengths with and without L-histidine. The hydrophobicity of myosin and heavy meromyosin (HMM) in low ionic strength solution containing L-histidine was lower than in high ionic strength solution. The SH contents of myosin and HMM in low ionic strength solution containing l-histidine did not change during the heating process, whereas in high ionic strength solution they decreased slightly. The heat-induced globular masses of HMM in low ionic strength solution containing L-histidine were smaller than those in high ionic strength solution. These findings suggested that the polymerization of HMM molecules by heating was suppressed in low ionic strength solution containing L-histidine, resulting in formation of the unique gel.

Spontaneous asparaginyl deamidation of canine milk lysozyme under mild conditions.

Asparaginyl deamidation is a common form of nonenzymatic degradation of proteins and peptides. As it introduces a negative charge spontaneously and irreversibly, charge heterogeneity can be accumulated in protein solution during purification, preservation, and experiments. In this study, canine milk lysozyme (CML), a useful model for the study of the molten globule state, exhibited charge heterogeneity after sample purification. Four Asn residues in CML deamidated rapidly under mild conditions: pH 8.0 and 30 degrees C. Other than these residues, one Asn residue, which was stable in the native state, was labile to deamidation in the unfolded state. This suggests that the structural formation around Asn can suppress deamidation. Substitutions of these labile Asn residues to Gln residues prevented deamidation effectively. Because the substitutions did not disrupt the structural formation of the native and molten globule states, they will enable more precise analyses for physical and structural studies.

Differential scanning calorimetry of a metalloprotein under controlled metal-ion activity.

In order to investigate the thermodynamics of the unfolding of metalloproteins, the thermal denaturation of bovine alpha-lactalbumin (BLA), a typical calcium-binding protein, was investigated under a wide variety of calcium ion activities by means of differential scanning calorimetry. The excess heat capacity obtained as above is composed of those of the following three reactions: (i) the release of a calcium ion from holo-BLA; (ii) the capture of the released calcium ion by the chelating reagent; and (iii) the denaturation of native apo-BLA. The results indicated that the presence of the chelating reagent had a remarkable effect on the apparent enthalpy change for the denaturation of holo-BLA. On the other hand, the influence of the chelator on the heat capacity change was shown to be negligible. Because the denaturation reaction of holo-BLA includes Reactions (i) and (iii), it had to be handled as a three-state reaction. Such an investigation of the unfolding has been scarcely found that the activity of the metal ion is controlled precisely in wide range.

Crystallization and preliminary X-ray crystallographic analysis of Ca2+-independent and Ca2+-dependent species of the type II antifreeze protein.

Ca2+-independent and Ca2+-dependent species of the type II antifreeze protein (AFP) were both crystallized using the hanging-drop vapour-diffusion method. It appeared that the crystal of the Ca2+-independent species from Brachyosis rostratus belongs to space group P2(1)2(1)2(1), with unit-cell parameters a = 43.3, b = 48.4, c = 59.7 A, and diffraction data were collected to 1.34 A resolution. For the Ca2+-dependent type II AFP species from Hypomesus nipponensis, crystallization was carried out for its Ca2+-free and Ca2+-bound states. 1.25 A resolution data were collected from the crystal in the Ca(2+)-free state, which exhibited P3(1)21 (or P3(2)21) symmetry, with unit-cell parameters a = b = 66.0, c = 50.3 A. Data collection could be extended to 1.06 A resolution for the crystal in the Ca2+ -bound state, which appeared to be isomorphous to the crystal in the Ca2+-free state (unit-cell parameters a = b = 66.0, c = 49.8 A). These data will allow us to determine the high-resolution structures of the two species of type II AFP.

Self-assembled meso-helicates of linear trinuclear nickel(II)-radical complexes with triple pyrazolate bridges.

Complexation of 3-nitronyl-nitroxide-substituted pyrazolate (pzNN) and 3-imino-nitroxide-substituted pyrazolate (pzIN) with nickel(II) gave [Ni3(pzNN)6] and [Ni3(pzIN)6], respectively. They were practically isomorphous and characterized as a linear trinuclear structure with neighboring nickel ions triply bridged by pyrazolate moieties. The space groups were P2(1)/n but the molecules have a pseudo three-fold axis. We found polymorphs in the crystals of [Ni3(pzIN)6] depending on the solvate molecules; another space group was a cubic Pa3[combining macron]. The opposite chirality around the inversion center at the central nickel(II) ion leads to a meso-helical symmetry in the whole molecule. The radical oxygen atoms participate in the 6-membered chelation at the terminal nickel(II) ions. Antiferromagnetic couplings were observed in both complexes, which are ascribable to interactions between the nickel and radical spins and between the nickel spins across the pyrazolate bridges.

Mechanism of the first-order phase transition of an acylurea derivative: observation of intermediate stages of transformation with a detailed temperature-resolved single-crystal diffraction method.

The process of the first-order solid-to-solid phase transition of 1-ethyl-3-(4-methylpentanoyl)urea (1) was observed by means of a detailed temperature-resolved single-crystal diffraction method, which resembles watching a series of stop-motion photographs. The transition consists of two elementary processes, one supramolecular and the other molecular. Crystal structures from before and after the phase transition are isostructural. The straight-ribbon-like one-dimensional hydrogen-bonding structure is formed and stacked to form a molecular layer. The geometry of the layer is retained during the phase transition. The relative position of the layer with its neighbours, on the other hand, changes gradually with increasing temperature. The change is accelerated at the temperature representing the start of the endotherm seen in the DSC curves of (1). The structural variation yields void space between the neighbouring layers. When the void space grows enough that the crystal is unstable, the 3-methylbutyl group on the last of the molecules turns into a disordered structure with drastic conformational changes to fill up the void space. The phase transition process is well supported with simple force-field calculations. A crystal of 1-(4-methylpentanoyl)-3-propylurea (2), which shows no solid-to-solid phase transitions, was also analysed by the same method for comparison.

Synthesis and Reactivity of New 1,4-Bis(alkylthio)-3,6-diarylthieno[3,4-c]thiophene Derivatives.

1,4-Bis(tert-butylthio)-3,6-diphenyl- and 3,6-di(2-thienyl)thieno[3,4-c]thiophenes (2a,b)were synthesized from 2-(tert-butylthio)-3-phenyl- and 3-(2-thienyl)cyclopropenethiones (1a,b) and triphenylphosphine in dry benzene at 50 degrees C, although similar treatment of 2,4,6-triisopropylphenyl, N,N-diethylamino, pyrrolidino, and diphenyl-substituted cyclopropenethiones (1c-h) did not result in the production of the corresponding thieno[3,4-c]thiophene derivatives. The possible reaction pathway for the formation of 2a,b is described. The protonation of 2a with trifluoroacetic acid (TFA) gave 4-(tert-butylthio)-3,6-diphenylthieno[3,4-c]thiophene-1(3H)-thione (13a), the treatment of which with sodium hydride and then isopropyl iodide led to 4-(tert-butylthio)-3,6-diphenyl-1-(isopropylthio)thieno[3,4-c]thiophene (16) by the regeneration of the thieno[3,4-c]thiophene ring system, this making possible the synthesis of other alkylthio-substituted thieno[3,4-c]thiophene derivatives. The reactions of 2a,b with N-phenylmaleimide (NPM) gave predominantly the endo-cycloadducts (17a,b) at the 1- and 3-positions, and that of 2a with dimethyl acetylenedicarboxylate (DMAD) led to the benzo[c]thiophene derivative (19) by desulfurization.