THE ATTACHMENTS OF PHYCOBILINS TO CYSTEINYL RESIDUES

The attachments of phycobilins to cysteinyl residues have been worked out through the reactions of phycoerytbrobilin dimethyl ester and phycocyanobilin dimethyl ester with cysteine methyl ester and reduced glutathione dimethyl ester respectively. A series of model compounds which carry the same conjugative skeletons as the chromophores in phycobillproteins have been purified anti identified. These compounds are characterized with strong fluorescence emission and circular dichroism effects which appeared weaker for the common bilinoid chromophores. Analyses of the circular dichroism effects of these compounds indicated that the major reaction products of phycobilins with cysteine methyl ester and reduced glutathione dimethyl ester carried the same stereochemical configuration as the chromophores in phvcobilinoroteins     

Molecular and quantum mechanical studies on the monomer recognition of a highly-regular β-helical antifreeze protein

The possible interaction models for an antifreeze protein from Tenebrio molitar (TmAFP) have been systematically studied using the methods of molecular mechanics, molecular dynamics and quantum chemistry. It is hoped that these approaches would provide insights into the nature of interaction between protein monomers through sampling a number of interaction possibilities and evaluating their interaction energies between two monomers in the course of recognition. The results derived from the molecular mechanics indicate that monomer's β-sheets would be involved in interaction area and the side chains on two β-faces can match each other at the two-dimensional level. The results from molecular mechanics and ONIOM methods show that the strongest interaction energy could be gained through the formation of H-bonds when the two β-sheets are involved in the interaction model. Furthermore, the calculation of DFT and analysis of van der Waals bond charge density confirm further that recognition between the two     

Kinetics of wetting of liquid on a solid surface

To consider a sessile drop on an ideal solid surface in equilibrium with a vapor phase, the classic Young equation was given. The derivation of the Young equation was based on both the mechanics and the energy knowledge. According to the constant volume of the liquid in the wetting process of the liquid on a smooth and homogeneous solid surface and the low energy law, Young equation was ob-tained through the mathematic method in this paper. The previous work indicated that the contact angle θ was a function...     

Synthesis and Characterization of Linked Clusters Capped by Alkylidyne

1 INTRODUCTION Constructing higher nuclearity clusters with well-defined dimensions and structures provide a rather active field of chemistry with potential applications in areas including nanotechnology, molecular recognition and catalysis[1~4]. A continuing effort has been directed toward developing a better methodology for systematic synthesis of supracluster compounds through molecular design [5,6]. On the basis of extensive investigation on the metal exchange reaction in cluster com…     

Highly Enantioselective Diels-Alder Reactions Promoted by Non C2-Symmetric Chiral Ligands

Ligand design and synthesis occupy a central theme in asymmetric catalysis. [1] In developing chiral ligands, their design should provide an opportunity for tuning through easy modification of substituents. In a large number of cases, this means starting from a new chiral source. We have been interested in design/synthesis of novel chiral ligands that allow for diversity to be introduced with ease. We have designed a new ligand system containing a core dihydropyrazole moiety with chiraliuy residing remotely to the fluxional group as shown in Scheme 1. There are three points of diversity and these are indicated on the structure.     

Studies on the Total Synthesis of Hainanolide (VII) - Analysis of the Relative Stereochemistry of key Intermediate 2

The natural product, hainanolide 1, also under the name harringtonolide2 demonstrated antitumor and antiviral activities in preliminary test. Its structure was determined by X-ray diffraction. The total synthesis of 1 was reported recently by Mander3. A different scheme of its synthesis has been studied in our laboratory4. Here the determination of the stereochemical structure of the key intermediate 2 in the synthesis was reported. HMBC and HMQC spectra identified the skeleton and H,…     

Supramolecular Self-assembly and Functionalization of Porphyrin-based Systems

Porphyrins are abundant in nature. They have been frequently employed as building blocks in the construction of nanoarchitectures and functional supramolecular systems. Recently, a series of novel porphyrin molecules including small molecules and polymers have been originally designed and synthesized with the aim of producing nanostructures with controllable-growth and materials with high-performance. Literature coverage is through 2004–2012. This review gives a full summary of related studies in our group.     

Structure of cis-[Pt(NH_3)(2-picoline)]~(2+) and DNA adduct and its bonding characteristics

Several methods including molecular mechanics, molecular dynamics, ONIOM that combines quantum chemistry with molecular mechanics and standard quantum chemistry are used to study the configuration and electron structures of an adduct of the DMA segment d(ATACATG*G*TACATA)-d(TATGTACCATGTAT) with cis-[Pt(NH3)(2-Picoline)]2+. The investigation shows that the configuration optimized by ONIOM is similar to that determined by NMR. Strong chemical bonds between Pt of the complex and two N7s of neighboring guanines in the DNA duplex and hydrogen bond between the NH3of the complex and O6 of a nearby guanine have a large impact on the configuration of the adduct. Chemical bonds, the aforementioned hydrogen bond, and the interaction between a methyl of the complex and a methyl of the base in close proximity are critical for the complex to specifically recognize DNA.     

Synthesis of New Chiral Phosphine Ligands and Their Application in Asymmetric Hydrogenation of Ketones

The design of new chiral ligands is the key in the development of transition metal catalyzed asymmetric synthesis. Many chiral diphosphine ligands have been prepared and applied in asymmetric catalytic reactions with excellent enantioselectivities. Among the chiral diphosphine ligands that have been reported, the atropisomeric C2-symmetric phosphines with a biaryl scaffold initiated by Noyori and co-workers with BINAP were found to have the widest application in the transition metal catalyze…     

Effects of matrix molecular weight on structure and reinforcement of high density polyethylene/mica composites

Three types of high-density polyethylene(HDPE)with different molecular weights(high,medium and low)were adopted to evaluate the influence of matrix molecular weight on the structure-property relation of injection-molded HDPE/mica composites through a combination of SEM,2d-WAXS,DSC,DMA and tensile testing.Various structural factors including orientation,filler dispersion,interfacial interaction between HDPE and mica,etc.,which can impact the macroscopic mechanics,were compared in detail among the three HDPE/mica composites.The transcrystallization of HDPE on the mica surface was observed and it exhibited strong matrix molecular weight dependence.Obvious transcrystalline structure was found in the composite with low molecular weight HDPE,whereas it was hard to be detected in the composites with increased HDPE molecular weight.The best reinforcement effect in the composite with low molecular weight HDPE can be understood as mainly due to substantially improved interfacial adhesion between matrix and mica filler,which arises from the transcrystallization mechanism.     

Design and synthesis of 2,4-difluorophenylpyruvic acid and of its azlactone precursor for macrophage migration inhibitory factor (MIF) tautomerase activity

The macrophage migration inhibitory factor (MIF) is an important cytokine implicated in several diseases and currently a target for drug development. This study aimed to synthesize phenylpyruvic acid like structures that fit the molecular requirements of MIF with respect to keto/enol tautomerism and E/Z isomerism of the enol forms. The synthesis of 2,4-difluorophenylpyruvic acid and its azlactone precursor as potential ligands to interact with the active site of MIF is reported here. Both the E and Z isomers of the azlactone of 2,4-difluorobenzaldehyde were synthesized using different synthetic methods. Hydrolysis of these isomeric azlactones yielded similar enol/keto tautomeric mixtures of 2,4-difluorophenylpyruvic acid, with the enol form predominating. NMR spectroscopy was used to confirm the structures of these compounds, while an X-ray crystallographic study also confirmed the configuration of the E isomer of the azlactone.     

Calculated conformer energies for organic molecules with multiple polar functionalities are method dependent: Taxol (case study)

Background  

Molecular mechanics (MM) and quantum chemical (QM) calculations are widely applied and powerful tools for the stereochemical and conformational investigations of molecules. The same methods have been extensively used to probe the conformational profile of Taxol (Figure 1) both in solution and at the β-tubulin protein binding site.     

Keto/enol tautomerism in phenylpyruvic acids: structure of the o-nitrophenylpyruvic acid

The synthesis of a tautomeric keto/enol mixture of o-nitrophenylpyruvic acid followed the acid hydrolysis of the azlactone of o-nitrobenzaldehyde was carried out. The structures of the two tautomeric forms were assigned by NMR spectroscopy. X-ray diffraction of a single crystal revealed that the crystalline form corresponds to the keto tautomer. Quantum mechanics calculations in the gas phase confirmed the experimental findings in solution.     

Ultraviolet-visible spectroscopic study of 3,5-dithio-2,7-dimethyl-[1,2,4]-triazepine: qualitative analysis of tautomeric behaviour

Room temperature UV-vis absorption spectra of the tautomeric system 3,5-dithio-2,7-dimethyl-[1,2,4]-triazepine in solution, in acetonitrile (CH3CN), are measured at different water volume percentages. The comparison of the obtained measures to the calculated allowed absorption transitions, using ZINDO/S package, reveals the coexistence of all the tautomeric forms in aprotic polar solution with a high dominance of the dithione form. The solute-water hydrogen bond, which seems to be of 1:n-type, favours the dithiol and monothiol conformers to the detriment of the dithione one. Further experimental investigations lead to the result that standing at ambient Laboratory conditions in the dark favours the dithione tautomeric isomer, while standing at indirect sunlight shifts the tautomeric equilibrium away from this species.     

Syntheses and Properties of Two Heterocyclically Substituted Hypericin Derivatives: 10,11-Dibenzothiazolyl-10,11-didemethylhypericin and 10,11-Dibenzoxazolyl-10,11-didemethylhypericin

Summary. The syntheses of the two heterocyclically substituted title hypericin derivatives were achieved starting either from 6-benzothiazolyl-tri-O-methyl-6-demethylemodin or 6-benzoxazolyl-tri-O-methyl-6-demethylemodin. The use of microwave assisted synthesis for the preparation of these anthraquinone synthons and the chemical as well as photochemical properties of the corresponding unique hypericin derivatives, which might constitute new photodynamic therapy agents, are reported. The tautomeric and stereochemical aspects of these hypericin derivatives were investigated by means of semiempirical calculations (AM1).     

Syntheses and Properties of Two Heterocyclically Substituted Hypericin Derivatives: 10,11-Dibenzothiazolyl-10,11-didemethylhypericin and 10,11-Dibenzoxazolyl-10,11-didemethylhypericin

The syntheses of the two heterocyclically substituted title hypericin derivatives were achieved starting either from 6-benzothiazolyl-tri-O-methyl-6-demethylemodin or 6-benzoxazolyl-tri-O-methyl-6-demethylemodin. The use of microwave assisted synthesis for the preparation of these anthraquinone synthons and the chemical as well as photochemical properties of the corresponding unique hypericin derivatives, which might constitute new photodynamic therapy agents, are reported. The tautomeric and stereochemical aspects of these hypericin derivatives were investigated by means of semiempirical calculations (AM1).     

Absolute Asymmetric Synthesis: The Origin, Control, and Amplification of Chirality

Biomolecular homochirality, the origin of which is still a puzzle, has challenged scientists to design chemical systems that provide chiral molecules through absolute asymmetric synthesis and to amplify a small stereochemical bias in such systems. The photoresolution of the enantiomers of helical-shaped, sterically overcrowded alkene 1 with circularly polarized light and the transduction of the stereochemical information by triggering the helical arrangement of a large collection of achiral molecules in a twisted nematic liquid crystalline phase (2) are examples of control and amplification of chirality.     

Spatial and electronic structure of monomeric and dimeric complexes of carnosine with zinc

A molecular mechanics method in the MM⁺ semi-empirical field potentials and a quantum chemical method in the PM3 approximation are used to calculate the geometric and energy parameters of a biocarnosine molecule in two tautomeric forms of the imidazole ring. The electronic structures of monomeric and dimeric complexes of both carnosine forms with zinc are investigated     

Intramolecular hydrogen bonds in crystals of thiophosphorylbenzopyrane derivatives – X-ray and FT-IR studies

The crystal structures of two new benzopyrane derivatives are analyzed and compared with previous X-ray investigations on related compounds. A particular attention is focused on intramolecular interactions. For the chromone derivatives (1 and 3) only one kind of interaction is possible, i.e., N–HO, whereas for the coumarine derivatives (2 and 4) two types of intramolecular hydrogen bonding are observed – N–HO and O–HN. Two types of H-bond for coumarine derivatives are the result of the existence of two tautomeric forms – enamine and iminoenol. Combined spectroscopic, NMR and IR measurements confirm such tautomeric equilibrium in solution. The influence of the additional intermolecular hydrogen bonds on the stabilization of tautomeric forms in crystals is also discussed here.