Symmetry or asymmetry in cheletropic additions forming cyclopropanes

Cheletropic additions forming cyclopropane rings were studied theoretically. Ten addition paths were traced by means of density-functional-theory calculations. Two 1,4-dienes, 1,4-pentadiene, and tricyclo[5.3.1.0^4,9]undeca-2,5-diene were adopted as substrates. CO, SO₂, C₂H₅PCl₂, CCl₂ and SiCl₂ were employed as cheletropic reagents (Xs). An orbital correlation diagram of the Woodward–Hoffmann (W–H) rule and frontier molecular orbital (FMO) interactions between them were investigated in detail. The FMO interactions, HOMO (1,4-diene)lumo (X) and homo (X)LUMO (diene), work reasonably for the progress of the reactions. Those cause the formation of two C–X bonds and a cyclopropane ring, and alternation of double bonds to single bonds. All the additions are concerted. The easiness of the ring formation depends upon the energy gap between HOMO and lumo and that between homo and LUMO, and the spatial directions of HOMO and LUMO extensions. Symmetry conservation of the W–H rule does not hold necessarily for those addition paths. The symmetry-breaking was discussed in terms of FMO interactions.Acknowledgement This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture, Japan and by Nishida Memorial Foundation for Fundamental Chemical Research.     

Theoretical prediction of optical absorption maxima for photosensory receptor mutants

We found a linear correlation between the theoretically predicted shifts and experimentally observed absorption spectra for various mutants of photoactive yellow protein, a photosensory receptor. Excitation energies of mutants were evaluated by the combination of the high level ab initio calculation for the chromophore inside and the low level ab initio calculation for the surrounding protein environment. Importantly, the electronic states of these two regions were treated both as variables and they are solved consistently to each other. The protein–chromophore interaction has been accurately reproduced by this method.     

Silylation of water glass. I. Solvent extraction and trimethylsilylation of silicic acid

Homogeneous trimethylsilylation of silicic acid in nonaqueous media was studied. The silylation process consisted of three steps: acidification of sodium silicate, solvent extraction. and trimethylation of silicic acid. Sodium silicate was first acidified with dilute sulfuric acid to form silicic acid: extraction followed with hydrophilic organic solvents. The Silicic acid obtained was finally allowed to react with trimethylchlorosilane to give trimethylsilylates with molecular weights of 1000–12,000. Optimum conditions for each step have been intensively sought to achieve trimethylsilylates in high yield. The trimethylsilylates exhibited excellent thermal stability and surface properties, such as water repellency and antifoaming properties, comparable to those of conventional polydimethylsiloxanes.     

Enantioselective synthesis of the optically active alpha-methylene-beta-hydroxy esters, equivalent compounds to Morita-Baylis-Hillman adducts, using successive asymmetric aldol reaction and oxidative deselenization

[Chemical reaction: See text] The asymmetric aldol reaction of a tetra-substituted ketene silyl acetal including an alkylseleno group with aldehydes has been developed by the promotion of Sn(OTf)2 coordinated with a chiral diamine to afford the corresponding aldols having chiral quaternary centers at the alpha-positions. The facile oxidative deselenization of these aldol compounds produces optically active alpha-methylene-beta-hydroxy esters which correspond to adducts prepared by the asymmetric Morita-Baylis-Hillman reaction.     

The biphasic virulence activities of gingipains: activation and inactivation of host proteins

Gingipains are trypsin-like cysteine proteinases produced by Porphyromonas gingivalis, a major causative bacterium of adult periodontitis. Rgps (HRgpA and RgpB) and Kgp are specific for -Arg-Xaa- and -Lys-Xaa- peptide bonds, respectively. HRgpA and Kgp are non-covalent complexes containing separate catalytic and adhesion/hemagglutinin domains, while RgpB has only a catalytic domain with a primary structure essentially identical to that of the cata-lytic subunit of HRgpA. The multiple virulence activities of gingipains are reviewed in view of the biphasic mechanisms: activation and inactivation of host proteins. Rgps enhanced vascular permeability through prekallikrein activation or direct bradykinin release in combination with Kgp. This Rgp action is potentially associated with gingival edema and crevicular fluid production. Rgps activate the blood coagulation system, leading to progression of inflammation and consequent alveolar bone loss in the periodontitis site. Rgps also activate protease-activated receptors and induce platelet aggregation, which, together with the coagulation-inducing activity, may explain an emerging link between periodontitis and cardiovascular disease. Kgp is the most potent fibrinogen/fibrin degrading enzyme of the three gingipains in human plasma, being involved in the bleeding tendency at the diseased gingiva. Gingipains stimulate expression of matrix metalloproteinases (MMPs) in fibroblasts and activate secreted latent MMPs that can destroy periodontal tissues. Gingipains degrade cytokines, components of the complement system and several receptors, including macrophage CD14, T cell CD4 and CD8, thus perturbing the host-defense systems and thereby facilitating sustained colonization of P. gingivalis. Gingipains are potent virulence factors of P. gingivalis, and in many regards their pathogenic activities constitute new mechanisms of bacterial virulence.     

Preparation of polysiloxanes from silicic acid. III. Preparation and properties of polysilicic acid butyl esters

Polysilicic acid butyl esters were prepared from 1-butanol and silicic acid extracted from sodium metasilicate solution and water glass with tetrahydrofuran and hydrochloric acid. The properties of the esters were dependent on the degree of esterification (DE). The esters with a DE of less than ca. 60% could be isolated by precipitation with hexane and were highly liable to self-condensation, whereas those with more than 60% DE were soluble in solvents and could not be precipitated easily. They underwent further condensation to form highly polymerized esters. Subsequent silylation allowed the isolation and characterization of these esters. The solvent-soluble silylated esters had moderate thermal stability with decomposition points between 210 and 260°C and number-average molecular weights of 10,000–30,000. From the evaluation of units structures of silylate it was suggested that polymer backbone was a pseudoladder structure.     

Effects of a nonplanar porphyrin rings on the spin-spin interactions in low-spin ferric porphyrin radical cationst

Low-spin ferric porphyrin radical cations formed by the oxidation of chloro(meso-tetraalkylporphyrinato)iron(III) followed by the addition of bulky 2-methylimidazole show antiferromagnetic coupling, which is interpreted in terms of the interaction between porphyrin a2u and iron d(xy), orbitals caused by the S4 ruffling of the porphyrin core.     

Difference in spin crossover pathways among saddle-shaped six-coordinated iron(III) porphyrin complexes

The electronic states of a series of saddle-shaped porphyrin complexes [Fe(OMTPP)L(2)](+) and [Fe(TBTXP)L(2)](+) have been examined in solution by (1)H NMR, (13)C NMR, and EPR spectroscopy and by magnetic measurements. While [Fe(OMTPP)(DMAP)(2)](+) and [Fe(TBTXP)(DMAP)(2)](+) maintain the low-spin (S = (1)/(2)) state, [Fe(OMTPP)(THF)(2)](+) and [Fe(TBTXP)(THF)(2)](+) exhibit an essentially pure intermediate-spin (S = (3)/(2)) state over a wide range of temperatures. In contrast, the Py and 4-CNPy complexes of OMTPP and TBTXP exhibit a spin transition from S = (3)/(2) to S = (1)/(2) as the temperature was decreased from 300 to 200 K. Thus, the magnetic behavior of these complexes is similar to that of [Fe(OETPP)Py(2)](+) reported in our previous paper (Ikeue, T.; Ohgo, Y.; Yamaguchi, T.; Takahashi, M.; Takeda, M.; Nakamura, M. Angew. Chem., Int. Ed. 2001, 40, 2617-2620) in the context that all these complexes exhibit a novel spin crossover phenomenon in solution. Close examination of the NMR and EPR data of [Fe(OMTPP)L(2)](+) and [Fe(TBTXP)L(2)](+) (L = Py, 4-CNPy) revealed, however, that these complexes adopt the less common (d(xz), d(yz))(4)(d(xy))(1) electron configuration at low temperature in contrast to [Fe(OETPP)Py(2)](+) which shows the common (d(xy))(2)(d(xz), d(yz))(3) electron configuration. These observations have been attributed to the flexible nature of the OMTPP and TBTXP cores as compared with that of OETPP; the relatively flexible OMTPP and TBTXP cores can ruffle the porphyrin ring and adopt the (d(xz), d(yz))(4)(d(xy))(1) electron configuration at low temperature. Therefore, this study reveals that the rigidity of porphyrin cores is an important factor in determining the spin crossover pathways.