SPECTROSCOPIC STUDIES ON THE STRUCTURES OF RETINOCHROME and METARETINOCHROME*

Abstract– Spectroscopic measurements of retinochrome and the related photoproduct, metaretino-chrome, were carried out to determine the conformation of the retinal and the protein. Absorption spectra with fourth derivatives indicate that the tryptophan residues are located in a hydrophobic core and that the environment around these residues does not change after light irradiation. Circular dichroism measurements indicate that retinochrome has a high helical content which is not altered by the conversion of retinochrome to metaretinochrome. Fourier transform infrared difference spectra demonstrate that retinochrome has the all-trans retinal and metaretinochrome has the 11-CM retinal. Retinochrome has an absorption due to amino acid residue(s) which changes in metaretinochrome. This work demonstrates that conformational changes are not induced during the conversion but the electrical charge(s) of the protein are changed by irradiation.     

BACTERIORHODOPSIN RECONSTITUTED FROM TWO INDIVIDUAL HELICES AND THE COMPLEMENTARY FIVE-HELIX FRAGMENT IS PHOTOACTIVE

Bacteriorhodopsin (bR), a light-driven proton pump, consists of a bundle of seven membrane-spanning alpha-helices connected to each other by short extramembranous loops. Previously it has been shown that bR can be reconstituted from three fragments corresponding to the first helix, the second helix, and the remaining five helices, and that this reconstituted material reforms the native structure of bR. In this study, it is shown that the native function is also recovered. Low-temperature spectroscopy was used to examine the photochemical properties of bR reconstituted from three fragments. At room temperature at pH 6, the reconstituted material shows essentially the same absorption spectrum as native bR, while upon raising the pH at room temperature or cooling the sample in glycerol, a second, blue-shifted peak appears. The pH and temperature dependence of the absorption spectrum indicates that the reconstituted bR is in an equilibrium between two pigments, which we call P560 and P480. Both pigments convert to their own K intermediates, which differ in absorption maxima, upon illumination with green light at -180 degrees C. Each K intermediate can be reverted to its initial state by light. Similarly, both pigments convert to their own M intermediates upon irradiation with yellow light at -77 degrees C. The M intermediate of both species can be reverted only to P560 by light. Both pigments are therefore photoactive. These unique photochemical properties of bR reconstituted from three fragments may be attributable to the lack of a covalent linkage in the loop connecting the A and B helices, and thus possibly to a change in the orientation of the B helix.     

Mechanism of antifeedant activity of plumbagin, a compound concerning the chemical defense in carnivorous plant

Dionaea muscipula Ellis accumulates a large amount of plumbagin (1), which operates as an antifeedant against predators. Content of 1 reached 0.5% weight of the fresh trap lobes. It was found that other carnivorous plants also accumulated similar naphthoquinone-type strong antifeedant. Thus, naphthoquinone analogs are widely used as an antifeedant among the carnivorous plants. By using several analogs of 1, we clarified that both the high volatility and high redox potential of 1 are important for its strong antifeedant activity. It was known that plumbagin stimulates the mitochondrial electron transport system as a result of intercepting electrons. These results suggested that the Droseraceae family possesses a universal defensive mechanism against predators, that is, accumulation of volatile naphthoquinone with high redox potential as defensive substance. Thus, it is estimated that highly volatile naphthoquinone of moderately high redox potential would be used as an antifeedant of weak toxicity.     

Modified guanidines as potential chiral superbases. 3. Preparation Of 1,4,6-triazabicyclooctene systems and 1,4-disubstituted 2-iminoimidazolidines by the 2-chloro-1,3-dimethylimidazolinium chloride-induced cyclization of guanidines with a hydroxyethyl substituent

Simple preparation methods of modified guanidines have been explored as potential chiral superbases. Thus, 3,7,8-trisubstituted and 3,6,7, 8-tetrasubstituted 1,4,6-triazabicyclooctene systems were prepared from (1S,2S)-1,2-diphenylethylenediamine through stepwise 2-chloro-1, 3-dimethylimidazolinium chloride (DMC)-induced cyclizations of protected thioureas to the corresponding 2-iminoimidazolidines and then of 2-(2-hydroxyethylimino)imidazolidines to the bicyclic systems. Linear guanidines with a 2-hydroxyethyl functional group were prepared by the reaction of carbodiimides with 2-amino alcohols. Reaction of linear-type guanidines with DMC followed by base treatment afforded 1,4-disubstitued 2-iminoimidazolidines. Furthermore, another type of 1,4,6-triazabicyclooctene was also prepared through double DMC-induced cyclization of guanidines with two 2-hydroxyethyl substituents.     

Ruthenium complex-catalyzed anti-Markovnikov hydration of terminal alkynes

[reaction: see text]. Highly regioselective, efficient, and substituent-tolerant anti-Markovnikov hydration of terminal alkynes occurs to give n-aldehyde by use of a catalytic amount of easily available cyclopentadienylruthenium complexes bearing appropriate bidentate or monodentate phosphine ligands. Typically, RuCpCl(dppm) (1 mol %) catalyzes the addition of water to 1-hexyne at 100 degrees C to give hexanal in 95% yield: 2-hexanone is not detected at all.     

Reduction of carbon dioxide with methane over Ni-catalyst

The reaction CO₂+CH₄=2CO+2H₂+60 kcal/mol was carried out over some transition metal catalysts (Ni, Fe and Co) and Ni/Al₂O₃ catalyst was then found to be the best for the reaction. The reaction rate expression and the apparent activation energy were also obtained over the Ni catalyst.

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CO₂+CH₄=2CO+2H₂+60 / (Ni, Fe Co). Ni/Al₂O₃ . Ni-.

     

A new entry of nucleophiles in rhodium-catalyzed asymmetric 1,4-addition reactions: addition of organozinc reagents for the synthesis of 2-aryl-4-piperidones

A rhodium-catalyzed asymmetric 1,4-addition reaction has been applied to the synthesis of 2-aryl-4-piperidones. While other conventional nucleophiles fail, organozinc reagents have been successfully utilized for the construction of these useful compounds in very good yield and enantiomeric excess.     

Ruthenium-Catalyzed Intermolecular Hydroamination of Terminal Alkynes with Anilines: A Practical Synthesis of Aromatic Ketimines

Without solvent and in the open air, the hydroamination of phenylacetylene with aniline in the presence of [Ru(3)(CO)(12)]/NH(4)PF(6) proceeds with high regioselectivity [Eq. (1)]. Simple distillation gives a product with greater than 99 % purity in an excellent yield. The application of this method to a new two-component reaction gives the important class of substances: the quinolines.     

Cross-coupling reaction of thermally stable titanium(II)-alkyne complexes with aryl halides catalysed by a nickel complex

The first cross-coupling reaction of thermally stable titanium(II)-alkyne complexes with aryl iodides in the presence of a catalytic amount of Ni(cod)2 is presented.     

Photochemical dechlorination of DDT catalyzed by a hydrophobic vitamin B12 and a photosensitizer under irradiation with visible light

Dechlorination of 1,1-bis(4-chlorophenyl)-2,2,2-trichloroethane (DDT) was catalyzed by a hydrophobic vitamin B(12), heptamethyl cobyrinate perchlorate, with a visible light irradiation system containing a [Ru(ii)(bpy)(3)]Cl(2) photosensitizer, and the hydrophobic vitamin B(12) showed high catalytic efficiency and stability during the reaction.