Ab initio study on the electronic structures of styrene in the Franck-Condon region

The electronic structures of styrene in the Franck‐Condon region have been theoretically examined by means of ab initio complete active space self‐consistent field (CASSCF) and the second order multireference Møller‐Plesset calculations. The optimized structure of styrene in S₀ is planar but the torsional motion of the phenyl group is very floppy. The S₁ state is assigned to the local π–π* excitation within the benzene ring. On the other hand, S₂, above S₁ by 0.561 eV, is assigned to a state that resembles the so‐called V‐state of ethylene. The transition intensity of S₀–S₁ is weak, while that of S₀–S₂ is strong. This is in good agreement with the experimental absorption spectrum where the S₀–S₁ and S₀–S₂ transitions are in the energy range of 290–220 nm. The optimized geometry of S₁, characterized by an enlarged benzene ring and its vibrational analyses, further justifies the assignment of the S₁ state. © 2002 Wiley Periodicals, Inc. J Comput Chem 9: 928–937, 2002     

Hexameric and pentameric slipped-cofacial dimers: toward an artificial light-harvesting complex

[structure: see text] We prepared a zinc complex of bis(1-methylimidazolyl)-m-gable porphyrin. The automatically assembled coordinate species showed a complex mixture of wide molecular weight distributions accompanied with assemblies of specific assembly numbers. When this zinc complex was once dissociated by the addition of methanol and reorganized again by elimination of the methanol under high-dilution conditions in chloroform to facilitate intramolecular coordinate structure formation, two convergent assemblies were obtained through analysis by gel permeation chromatography. These assemblies gave round-shaped particles on solid substrates by probe microscopies (atomic force and scanning tunneling microscopies). These two components were separated by GPC and evidence from small-angle X-ray scattering measurements in solution with synchrotron radiation was consistent with hexameric and pentameric macrorings of the gable porphyrin. The porphyrin macrorings did not show any fluorescence quenching by assembly formation, and we anticipate that the macrorings of gable porphyrins represent a good model for an artificial light-harvesting complex.     

CF3Br?H2 reaction in shock waves

CF₃Br? H₂ mixtures highly diluted with Ar were studied by using a time-resolved IR-emission of HBr and a gas-chromatography for reaction products. The temperature range covered was 1000–1600 K and the total pressure behind the reflected shock waves used was 1.2–2.6 atm. CF₃H, C₂F₆, and C₂F₄ were produced and the yields of these products were determined as a function of temperature. The main product under our experimental conditions was CF₃H. The mechanism and the rate constants of CF₃Br? H₂ reaction at high temperatures were discussed. The experimental data was satisfactorily modeled using a 14-reaction mechanism. Reaction (5) played an important role in the formation of CF₃H together with reaction (4). The rate constant expression k₅ = 2.2 × 10¹³ exp(?12 kcal/RT) cm³ mol^?1 s^?1 gave the best agreement between the calculated and observed results. © 1993 John Wiley & Sons, Inc.     

Facile synthesis of membrane-embedded peptides utilizing lipid bilayer-assisted chemical ligation

Lipid bilayer-assisted chemical ligation between thiolester and N-terminal cysteine peptides has been developed with successful application to the synthesis of membrane protein segments possessing both two transmembrane and one extracellular regions.     

Stereoselective conjugate addition of lactams to nitroalkenes and formal total synthesis of indolizidine 167B

Optically active 5-substituted pyrrolidin-2-ones underwent conjugate addition to nitroalkenes to give the corresponding adducts in a diastereoselective manner. The presence of 18-crown-6 was crucial to achieve good stereoselective addition. Addition of 6-substituted piperidin-2-ones also gave the corresponding adduct in a stereoselective manner. The adduct was readily converted into a bicyclic lactam through intramolecular nitroaldol reaction, and the formal synthesis of indolizidine 167B was achieved.     

Functional monomers and polymers. CXVII. Photochemical reactions on oligo- and polyethyleneimines containing pendant thymine bases: Effect of the spacer

The photodimerization reaction of thymine bases incorporated in the modified oligo- and polyethyleneimines, in which α-alanine or ß-alanine unit was inserted between the main chain and the pendant thymine bases, was studied both in aqueous and in dimethyl sulfoxide solutions. It was found that the quantum yields for the photodimerization for the oligomers were closely related to the intramolecular interaction in the ground state, while those of the copolymers were not related to the ground state interaction. It was suggested that the singlet energy migration was present in the case of the polyethyleneimine derivatives. This result is discussed in terms of the effects of the nearest-neighboring thymine units, the structure of the side chain, and the singlet energy migration, particularly for the copolymers.     

Constraints of opsin structure on the ligand-binding site: studies with ring-fused retinals

Ring-fused retinal analogs were designed to examine the hula-twist mode of the photoisomerization of the 9-cis retinylidene chromophore. Two 9-cis retinal analogs, the C11-C13 five-membered ring-fused and the C12-C14 five-membered ring-fused retinal derivatives, formed the pigments with opsin. The C11-C13 ring-fused analog was isomerized to a relaxed all-trans chromophore (lambda(max) > 400 nm) at even -269 degrees C and the Schiff base was kept protonated at 0 degrees C. The C12-C14 ring-fused analog was converted photochemically to a bathorhodopsin-like chromophore (lambda(max) = 583 nm) at -196 degrees C, which was further converted to the deprotonated Schiff base at 0 degrees C. The model-building study suggested that the analogs do not form pigments in the retinal-binding site of rhodopsin but form pigments with opsin structures, which have larger binding space generated by the movement of transmembrane helices. The molecular dynamics simulation of the isomerization of the analog chromophores provided a twisted C11-C12 double bond for the C12-C14 ring-fused analog and all relaxed double bonds with a highly twisted C10-C11 bond for the C11-C13 ring-fused analog. The structural model of the C11-C13 ring-fused analog chromophore showed a characteristic flip of the cyclohexenyl moiety toward transmembrane segments 3 and 4. The structural models suggested that hula twist is a primary process for the photoisomerization of the analog chromophores.     

Molecular inclusion in calixarenes. XVIII. Crystal and molecular structure of thep-tert-butylcalix[7]arene 1 : 3 pyridine complex/clathrate

The structure of thep-tert-butylcalix[7]arene 1:3 pyridine complex/clathrate (C₇₇H₉₈O₇ · 3 C₅H₅N) has been determined by X-ray crystallography. The crystal data are as follows: monoclinic, space groupP2₁/c,a = 37.790(4),b = 9.423(2),c = 29.075(4)Å; = 112.74(2)°;V = 8285(3)ų;Z = 4;D _calc = 1.10 g·cm^–3. The hostp-tert-butylcalix[7]arene links one pyridine molecule through a hydrogen bond between one of the seven hydroxy groups and the nitrogen of the pyridine. Two other guest molecules have been found in the crystal lattice, so the title compound may be defined as a complex/clathrate hybrid. The unambiguous conformation of the macrocycle is described by an alternative method, which avoids the use of dihedral angles (and the ambiguities in their values).Part XVII. Ref. [2].     

Simple, rapid and sensitive determination of plasma taurine by high-performance liquid chromatography using pre-column derivative formation with fluorescamine.

A simple, rapid and sensitive method for the determination of plasma taurine by high-performance liquid chromatography in the isocratic mode has been developed. The deproteinized plasma was treated with fluorescamine. These derivatives were separated on a LiChrospher 100 RP-8 column within 15 min. The detection limit for taurine was 0.2 microM. The plasma taurine contents of yellowtail fish, Seriola quinqueradiata, beef cattle, dairy cows and chickens were determined to be 125 +/- 54, 5.6 +/- 1.4, 2.2 +/- 0.7 and 20.0 +/- 9.6 micrograms/ml, respectively.     

Functional monomers and polymers. CIV. Synthesis and properties of oligomer models of polyethyleneimine derivatives with spacer-separated nucleic acid bases

A series of polyethyleneimine derivatives and their oligomer models with pendant thymine or adenine bases separated by β-alanyl groups as spacers were prepared by an activated ester method. To elucidate the nature of intramolecular interaction of thymine and adenine bases ultraviolet (UV) spectrophotometric studies were undertaken for the series of compounds and the results were compared with those obtained for the corresponding oligomers without spacers. In the thymine derivatives, the nature of the intramolecular interaction resembled that of the corresponding compounds without β-alanyl units, although larger hypochromicities were found in the series with β-alanyl units. In the adenine derivatives, contrary to the corresponding thymine systems, UV hypochromicities of the compounds decreased by incorporation of the spacer group into side chains. It became evident that the intramolecular interaction of adenine bases was due to stacking neutral species, hydrogen bonding, and residual hypochromicity for the protonated species.