Theoretical evaluation of atomic charges to be integrated into conformational analyses of neutral lipids

Conformational analyses of large molecules as fatty acids and triglycerides are usually amenable by molecular mechanics. A correct evaluation of the electrostatic energy term is thus crucial in determining reliable results. In this contribution, we have considered the most abundant fatty acids in biomembranes, i.e., lauric, stearic, oleic, and elaidic acid, and the corresponding triglycerides, i.e., trilaurin, tristearin, triolein, and trielaidin, and estimated the Mulliken and potential-derived charges both at the semiempirical AM 1 and ab initio HF MO STO -3G level. Atomic charges obtained by the Mulliken population analysis do not take into account the full geometry of the molecule. On the contrary, the change of conformation, due to different chains length or the presence of a trans or cis double bond, greatly influences the repartition of the potential-derived charges. A systematic comparative analysis shows that charges calculated by AM 1 are not suitable because as they do not reproduce potential-derived charges obtained by ab initio. © 1993 John Wiley & Sons, Inc.     

Skipping in a hybrid polyketide synthase. Evidence for ACP-to-ACP chain transfer

A tetraketide synthase containing a loading module (LM), the extension modules erythromycin module 1, rapamycin module 2, and erythromycin module 2 (LM-Ery1-Rap2-Ery2-TE), when expressed in Saccharopolyspora erythraea strain JC2, produced as previously reported a mixture of tetraketide lactones (minor products) and triketide lactones (major products). Several alternative plausible mechanisms by which this "skipping" phenomenon might occur may be proposed. Site-directed mutagenesis of the ketosynthase (KS) and acylcarrier protein (ACP) domains in the interpolated module has shown that skipping within the hybrid PKS involves passage of the growing polyketide through the interpolated module, by direct ACP-to-ACP transfer of the polyketide chain.     

Dendrimer-functionalized self-assembled monolayers as a surface plasmon resonance sensor surface

We report here a multistep route for the immobilization of DNA and proteins on chemically modified gold substrates using fourth-generation NH(2)-terminated poly(amidoamine) dendrimers supported by an underlying amino undecanethiol (AUT) self-assembled monolayer (SAM). Bioactive ultrathin organic films were prepared via layer-by-layer self-assembly methods and characterized by fluorescence microscopy, variable angle spectroscopic ellipsometry, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and attenuated total internal reflection Fourier transform infrared spectroscopy (ATR-FTIR). The thickness of the AUT SAM base layer on the gold substrates was determined to be 1.3 nm from ellipsometry. Fluorescence microscopy and AFM measurements, in combination with analyses of the XPS/ATR-FTIR spectra, confirmed the presence of the dendrimer/biopolymer molecules on the multilayer sensor surfaces. Model proteins, including streptavidin and rabbit immunoglobulin proteins, were covalently attached to the dendrimer layer using linear cross-linking reagents. Through surface plasmon resonance measurements, we found that sensor surfaces containing a dendrimer layer displayed an increased protein immobilization capacity, compared to AUT SAM sensor surfaces without dendrimer molecules. Other SPR studies also revealed that the dendrimer-based surfaces are useful for the sensitive and specific detection of DNA-DNA interactions. Significantly, the multicomponent films displayed a high level of stability during repeated regeneration and hybridization cycles, and the kinetics of the DNA-DNA hybridization process did not appear to be influenced by surface mass transport limiting effects.     

A ring-fission/C–C bond cleavage reaction with an N-alkyl-N-methyl-N-[(5-phenyl-1,2,4-oxadiazol-3-yl)methyl]amine

The reaction of N-(3,4-dichlorophenethyl)-N-methylamine (1) with 3-chloromethyl-5-phenyl-1,2,4-oxadiazole (2) was investigated. Employment of an equimolar amount of 1 and 2 in the presence of potassium carbonate led to the expected tertiary amine 3 (N-[(3,4-dichlorophenyl)ethyl]-N-methyl-N-[(5-phenyl-1,2,4-oxadiazol-3-yl)methyl]amine), whereas an excess of 1 and prolonged reaction time resulted in ring fission of the oxadiazole system in 3 and finally in the formation of N′-benzoyl-N-[(3,4-dichlorophenyl)ethyl]-N-methylguanidine (4) and N,N′-bis[(3,4-dichlorophenyl)ethyl]-N,N′-dimethylmethanediamine (5). The structures of products 3–5 were determined by means of ¹H and ¹³C NMR-spectroscopy, mass spectrometry and IR-spectroscopy, for 3 (as picrate) and 4 also X-ray structure analysis was employed. A possible mechanism of the reaction pathway leading to compounds 4 and 5 is proposed.     

New compounds obtained by enzymatic oxidation of phloridzin

Oxidation of phloridzin was studied in a model system in the presence of apple polyphenol oxidase. In addition to 3-hydroxy phloridzin, two major oxidation products were purified by reversed phase HPLC at the semi-preparative scale. Their structures were elucidated by UV, ESI-MSⁿ and NMR spectroscopies. The first compound was a colourless product, whose novel structure strongly differs from its precursor showing a biphenyl moiety and a propionic acid chain. The second product was an oxidised form of the first one and corresponded to a stable yellow pigment with two isomeric forms. A mechanism of formation of these products, which implied successive oxidation and nucleophilic addition steps was proposed.     

Preparation of 3-methyl-4-vinyl-1,2,5-oxadiazole and 3-methyl-4-vinyl-1,2,5-thiadiazole

Short and high yielding preparations of 3-Methyl-4-vinyl-1,2,5-oxadiazole ( 6a ) and 3-Methyl-4-vinyl-1,2,5-thiadiazole ( 6b ) are described.     

Beryllium-tetrakis-[trimethylsilyl]-diamid-eine Verbindung mit zweibindigem Beryllium

Monomeres [(CH₃)₃Si]₂NBeN[Si(CH₃)₃]₂ (I) entsteht in 68% Ausb. bei der Reaktion von BeCl₂-Ätherat mit NaN[Si(CH₃)₃]₂. Eigenschaften, IR-, Raman- und¹H-NMR-Spektren sprechen für einen NBeN-Winkel von 180°. Die SiN- und BeN-Valenzkraftkonstanten werden zu 3,40 bzw. 2,96 mdyn/Å errechnet.Monomeric I has been prepared (yield 68%) by the reaction of BeCl₂ with NaN[Si(CH₃)₃]₂ in ether. Properties, infrared, raman and¹H-NMR-spectra support a NBeN angle of 180°. SiN and BeN force constants were calculated to be 3,40 and 2,96 mdyn/Å, respectively.

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9. Mitt. über silylsubstituierte Metallamide; 8. Mitt.:H. Bürger, W. Sawodny undU. Wannagat, J. Organometall. Chem.2, 113, (1965).     

X-ray crystal structures of three nonbenzodiazepinic ligands for the benzodiazepine receptor sites: SR95926, CMW1842, and L16317:nab initio MO study of the electronic properties

The crystal structures ofp-methoxyphenyl-3-triazolo [4,3-a] isoquinoline (SR95926),p-methoxyphenyl-3-triazolophtalazine (CMW1842), andp-methoxyphenyl-3-N-dimethoxyethylamino-6-triazolophtalazine (L16317) have been solved by direct methods from single-crystal X-ray diffraction data, and refined by full-matrix least squares. SR95926: monoclinic,P2₁/n,a=20.950(3),b=6.769(1),c=9.465(2) Å,=100.90(1)°. CMW1842: triclinic,P¯1,a=8.784(1),b=9.160(4),c=8.555(1) Å,=99.10(2),=93.90(1), =106.77(1)°. L16317: monoclinic,P2₁/_n,a=20.124(3),b=9.586(1),c=10.788(1) Å,=91.91(1)°. FinalR factors are 0.034, 0.037, and 0.053, respectively. Experimental geometries were used to perform STO-3Gab initio molecular-orbital calculations. A relationship between the electronic pattern within the molecules and the affinity of the benzodiazepine receptor sites is pointed out.     

Swallowtail porphyrins: synthesis, characterization and incorporation into porphyrin dyads

The incorporation of symmetrically branched tridecyl ("swallowtail") substituents at the meso positions of porphyrins results in highly soluble building blocks. Synthetic routes have been investigated to obtain porphyrin building blocks bearing 1-4 swallowtail groups. Porphyrin dyads have been synthesized in which the zinc or free base (Fb) porphyrins are joined by a 4,4'-diphenylethyne linker and bear swallowtail (or n-pentyl) groups at the nonlinking meso positions. The swallowtail-substituted Zn(2)- and ZnFb-dyads are readily soluble in common organic solvents. Static absorption and fluorescence spectra and electrochemical data show that the presence of the swallowtail groups slightly raises the energy level of the filled a(2u)(pi) HOMO. EPR studies of the pi-cation radicals of the swallowtail porphyrins indicate that the torsional angle between the proton on the alkyl carbon and p-orbital on the meso carbon of the porphyrin is different from that of a porphyrin bearing linear pentyl groups. Regardless, the swallowtail substituents do not significantly affect the photophysical properties of the porphyrins or the electronic interactions between the porphyrins in the dyads. In particular, time-resolved spectroscopic studies indicate that facile excited-state energy transfer occurs in the ZnFb dyad, and EPR studies of the monocation radical of the Zn(2)-dyad show that interporphyrin ground-state hole transfer is rapid.     

Solvothermal synthesis and structure of a new selenium-rich selenophosphate K(3)PSe(4).2Se(6)

The compound K(3)PSe(4).2Se(6) was synthesized at 110 degrees C via solventothermal techniques from binary starting materials and Se in acetonitrile. The compound crystallizes in the space group Fd macro 3 of the cubic system with eight formula units in a cell with a dimension of a = 16.415(2) A at T = 193 K. The structure contains an unusual intermixing of ionic and uncharged species. The selenophosphate tetrahedral trianions PSe(4)(3-) are surrounded by potassium cations; other potassium cations in the structure are coordinated to 12 selenium atoms from four Se(6) rings in a tetrahedral arrangement. There are no short contacts between adjacent selenium rings. Heating the same reaction mixture to 160 degrees C results in the formation of only needles of trigonal selenium.