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181.
Mitani M Wakamatsu Y Katsurada T Yoshioka Y 《The journal of physical chemistry. A》2006,110(51):13895-13914
We report the geometrical features and electronic structures of di-mu-oxo-bridged Mn-Mn binuclear complexes with H2O ligands [Mn2O2(H2O)8]q+ in the iso- and mixed-valence oxidation states. All of the combinations among Mn(II), Mn(III), and Mn(IV) ions are considered the oxidation states of the Mn-Mn center, and the changes in molecular structure induced by the different electron configurations of Mn-based orbitals are investigated in relation to the oxygen-evolving complex (OEC) of photosystem II. The stable geometries of complexes are determined by using the hybrid-type density functional theory for both of the highest- and lowest-spin couplings between Mn sites, and the lowest-spin-coupled states are energetically more favorable than the highest-spin-coupled states except in the case of the complexes with the Mn(II) ion. The coordination positions of H2O ligands at the Mn(II) site tend to shift from the octahedral positions in contrast to those at the Mn(III) and Mn(IV) sites. The shape of the Mn2O2 core and the distances between the Mn ions and the H2O ligands vary depending on the electron occupations of the octahedral eg orbitals on the Mn site with an antibonding nature for the Mn-ligand interactions, indicating the trend as Mn(II)-O > Mn(III)-O and Mn(IV)-O, O-Mn(II)-O > O-Mn(III)-O > O-Mn(IV)-O among the iso-valence Mn2O2 cores, and O-Mn(lower)-O < O-Mn(higher)-O within the mixed-valence Mn2O2 core, and as Mn(II)-OH2 and Mn(III)-OH2 > Mn(IV)-OH2 for the axial H2O ligand. The optimized geometries of model complexes are compared with the X-ray structure of the OEC, and it is suggested that the cubane-like Mn cluster of the active site may not contain a Mn(II) ion. The effective exchange integrals are estimated by applying the approximate spin projection to clarify the magnetic coupling between Mn sites, and the superexchange pathways through the di-mu-oxo bridge are examined on the basis of the singly occupied magnetic orbitals derived from the singlet-coupled natural orbitals in the broken-symmetry state. The comparisons of the calculated results between [Mn2O2(H2O)8]q+ in this study and [Mn2O2(NH3)8]q+ reported by McGrady et al. suggest that the symmetric pathways are dominant to the exchange coupling constant, and the crossed pathway would be less important for the former than it would for the latter in the Mn(III)-Mn(III), Mn(IV)-Mn(IV), and Mn(III)-Mn(IV) oxidation states. 相似文献
182.
Sueishi Y Yoshioka D Yoshioka C Yamamoto S Kotake Y 《Organic & biomolecular chemistry》2006,4(5):896-901
Using a competitive spin trapping method, relative spin trapping rates were quantified for various short-lived radicals (methyl, ethyl, and phenyl radicals). High static pressure was applied to the competitive spin-trapping system by employing high-pressure electron spin resonance (ESR) equipment. Under high pressure (490 bar), spin trapping rate constants for alkyl and phenyl radicals increased by 10 to 40%, and the increase was dependent on the structure of nitrone spin traps. A maximum increase was obtained when tert-butyl(4-pyridinylmethylene)amine N-oxide (4-POBN) was used as a spin trap. Activation volumes (DeltaDeltaV(double dagger)) for the two spin trapping reactions were calculated to be -17-(-9) cm(3) mol(-1) for the 4-POBN system. 相似文献
183.
Masao Kawai Masao Muto Yoshiaki Murase Shuki Araki Komei Kafuku Hiroshi Yoshioka Kazumi Nakatsu Yasuo Butsugan 《Journal of chemical crystallography》1999,29(11):1193-1196
In order to confirm the structures of the autoxidation products of 2-tert-butyl-4-methoxyphenol (BHA), X-ray crystallographic analyses have been undertaken. One of the products was converted to a dibenzoate, which was subjected to the analysis to establish the structure as the O
6,O
7-dibenzoyl derivative of (1R*,2R*,6S*,7S*,9R*)-4,9-di-tert-butyl-6,7-dihydroxytricyclo[5.2.2.02, 6]undec-4-ene-3,8,10-trione. Crystallographic analysis of the major isomer of the two isomeric products gave its structure as (E)-4-tert-butyl-2-(3-tert-butyl-4-oxocyclopent-2-en-1-ylidene)cyclopent-4-ene-1,3-dione, which also established the structure of the minor isomer as the corresponding (Z)-isomer. 相似文献
184.
185.
A novel stable organic radical, 2-(naphth[2,3-d]imidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxyl-3-oxide (4), has been designed, synthesized, and structurally characterized to examine the effects of ring extension on 2-(benzimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxyl-3-oxide (2). 4 forms four-centered intramolecular and intermolecular hydrogen bonds, and the hydrogen bonds are repeated along the c-axis to form a one-dimensional chain structure. This hydrogen-bonding motif contrasts that of 2, which forms three-centered intramolecular and intermolecular hydrogen bonds. The magnetic susceptibility measurement of 4 reveals that an antiferromagnetic interaction is dominant between spins, and the magnetic behavior is reproduced by the Bonner-Fisher model with J = -14 cm-1. Because each hydrogen-bonded chain is well isolated, a magnetic interaction pathway was thought to exist along the chain direction. Two interaction pathways have been assumed: (i) through-space interaction between the O atoms of the nitroxide and (ii) through the NH...ON intermolecular hydrogen bond. We have concluded that pathway (i) is predominant, by considering the identical magnetic data between the NH nondeuterated and deuterated samples. The hydrogen bond mainly has a role in crystal scaffolding. 相似文献
186.
Fujita M Kim WH Sakanishi Y Fujiwara K Hirayama S Okuyama T Ohki Y Tatsumi K Yoshioka Y 《Journal of the American Chemical Society》2004,126(24):7548-7558
The reaction of 4-substituted cyclohex-1-enyl(phenyl)iodonium tetrafluoroborate with tetrabutylammonium acetate gives both the ipso and cine acetate-substitution products in aprotic solvents. The isomeric 5-substituted iodonium salt also gives the same mixture of the isomeric acetate products. The reaction is best explained by an elimination-addition mechanism with 4-substituted cyclohexyne as a common intermediate. The cyclohexyne formation was confirmed by deuterium labeling and trapping to lead to [4 + 2] cycloadducts and a platinum-cyclohexyne complex. Cyclohexyne can also be generated in the presence of some other mild bases such as fluoride ion, alkoxides, and amines, though amines are less effective bases for the elimination. Kinetic deuterium isotope effects show that the anionic bases induce the E2 elimination (k(H)/k(D) > 2), while the amines allow formation of a cyclohexenyl cation in chloroform to lead to E1 as well as S(N)1 reactions (k(H)/k(D) approximately 1). Bases are much less effective in methanol, and methoxide was the only base to efficiently afford the cyclohexyne intermediate. Nucleophiles react with the cyclohexyne to give regioisomeric products in the ratio dependent on the ring substituent. The observed regioselectivity of nucleophilic addition to substituted cyclohexynes is rationalized from calculated LUMO populations, which are governed by the bond angles at the acetylenic carbons: The less deformed carbon has a higher LUMO population and is preferentially attacked by the nucleophile. 相似文献
187.
Tomohisa Nagamatsu Keiichi Kinoshita Kenji Sasaki Taiji Nakayama Takashi Hirota 《Journal of heterocyclic chemistry》1991,28(2):513-515
A simple and highly efficient methodology for the synthesis of 1,2,4,5-tetrahydro[1]benzothiepino[4,5-e]-imidazo[1,2-c]pyrimidine ( XI ) having a novel ring system via 4-substituted 5,6-dihydro[1]benzothiepino-[5,4-d]pyrimidines VII-X is described. The anti-platelet aggregation activity for it and its related compounds against collagen-induced aggregation of rabbit blood platelets in vitro was found. 相似文献
188.
High-energy protons and neutrons produce various radionuclides in the air, mainly through the nuclear spallation of atmospheric
elements. Air samples were collected from the EP-2 tunnel of the KEK 12 GeV proton synchrotron facility with a filter pack,
which consisted of a membrane filter for aerosols, a Na2CO3-impregnated filter for acidic gases, and an activated carbon fiber filter for non-acidic gases. Sulfur-38 was found on the
membrane and Na2CO3-impregnated filters, 38Cl and 39Cl were on the membrane, Na2CO3-impregnated, and ACF filters, and 82Br was only on the ACF filter. The results on the relative abundances of aerosol and gaseous acidic components have indicated
that 38Cl produced by thermal neutron capture, which is the main reaction for 38Cl production in the EP-2 tunnel air, are likely not to be present as aerosol or acidic gas. This was similar to the case
of 82Br produced by thermal neutron capture. 相似文献
189.
Hasegawa T Haraguchi S Numata M Li C Bae AH Fujisawa T Kaneko K Sakurai K Shinkai S 《Organic & biomolecular chemistry》2005,3(24):4321-4328
Schizophyllan interacts with various 1,4-diphenylbutadiyne derivatives to induce their chirally-twisted packing. A series of referential experiments using other polysaccharides (amylose, pullulan, dextran, etc.) and a carbohydrate-appended detergent (dodecyl-beta-d-glucopyranoside) indicates that these 1,4-diphenylbutadiyne derivatives are accommodated within a tubular cavity constructed by a helical superstructure of schizophyllan. In these 1,4-diphenylbutadiyne derivatives, 1,4-bis(p-propionamidophenyl)butadiyne can be easily polymerized through UV-irradiation, in which schizophyllan acts as a one-dimensional mold to produce the corresponding poly(diacetylene)s with fibrous morphologies. Detailed investigations on this unique approach to prepare the nanofibers revealed that it includes two individual processes, that is, 1) UV-mediated polymerization of encapsulated 1,4-bis(p-propionamidophenyl)butadiyne to produce immature nanofibers and 2) their reorganization through hydrophobic interfiber interactions into ordered nanofibers. The other 1,4-diphenylbutadiyne derivatives could not be polymerized through UV-irradiation, indicating that the p-propionamido-functionalities play substantial roles for a suitable packing of the monomer for the polymerization. The other 1,4-diphenylbutadiyne derivatives, however, can be also polymerized through gamma-ray irradiation in the presence of schizophyllan to give the corresponding poly(diacetylene)-nanofibers, emphasizing the wide applicability of the schizophyllan-based strategy for polymerization of various 1,4-diphenylbutadiyne derivatives. 相似文献
190.
Yoshioka S Tosha T Takahashi S Ishimori K Hori H Morishima I 《Journal of the American Chemical Society》2002,124(49):14571-14579
Structural and functional roles of the hydrogen bonding network that surrounds the heme-thiolate coordination of P450(cam) from Pseudomonas putida were investigated. A hydrogen bond between the side chain amide of Gln360 and the carbonyl oxygen of the axial Cys357 was removed in Q360L. The side chain hydrogen bond and the electrostatic interaction between the polypeptide amide proton of Gln360 and the sulfur atom of Cys357 were simultaneously removed in Q360P. The increased electron donation of the axial thiolate in Q360L and Q360P was evidenced by negative shifts of their reduction potentials by 45 and 70 mV, respectively. Together with the results on L358P in which the amide proton at position 358 was removed (Yoshioka, S., Takahashi, S., Ishimori, K., Morishima, I. J. Inorg. Biochem. 2000, 81, 141-151), we propose that the side chain hydrogen bond and the electrostatic interaction of the amide proton with the thiolate ligand cause approximately 45 and approximately 35 mV of positive shifts, respectively, of the redox potential of the heme in P450(cam). The resonance Raman spectra of the ferrous-CO form of the Q360 mutants showed a downshifted Fe-CO stretching mode at 482 approximately 483 cm(-)(1) compared with that of wild-type P450(cam) at 484 cm(-)(1). The Q360 mutants also showed the upshift by 4 approximately 5 cm(-)(1) of the Fe-NO stretching mode in the ferrous-NO form. These Raman results indicate the increase in the sigma-electron donation of the thiolate ligand in the reduced state of the Q360 mutants and were in contrast to the increased pi-back-donation of the thiolate in L358P having an upshifted Fe-CO stretching mode at 489 cm(-)(1). The catalytic activities of the Q360 mutants for the unnatural substrates were similar to those of the wild-type enzyme, indicating that the increased sigma-electron donation does not promote the O-O bond heterolysis in the Q360 mutants, although the increased pi-electron donation in L358P promoted the heterolysis of the O-O bond. We conclude that the functions of the proximal hydrogen bonding network in P450(cam) are to stabilize the heme-thiolate coordination, and to regulate the redox potential of the heme iron. Furthermore, we propose that the pi-electron donation, not the sigma-electron donation, of the thiolate ligand promotes the heterolysis of the O-O bond of dioxygen. 相似文献