Search results for "Effects"
showing 10 items of 2634 documents
Binding abilities of dehydropeptides towards Cu(II) and Ni(II) ions. Impact of Z–E isomerization on metal ion binding
2002
The study on the binding ability of dehydro-tri- and tetrapeptides has shown that the alpha,beta-double bond has a critical effect on the peptide coordination to metal ions. It may affect the binding of the vicinal amide nitrogens by the electronic effect and stabilize the complex due to steric effects. The (Z) isomer is the most effective in stabilizing of the complexes formed. The presence of large side chain in the dehydroamino acid residue may also be critical for the coordination mode in the metallopeptide systems.
Aminomethyl-Substituted Ferrocenes and Derivatives: Straightforward Synthetic Routes, Structural Characterization, and Electrochemical Analysis
2013
A variety of aminomethyl-substituted ferrocenes and the parent compounds (iminomethyl)ferrocenes, azaferrocenophanes, and diferrocenylamines can be selectively synthesized from reductive amination of 1,1′-diformylferrocene or formylferrocene. The optimized one- or two-step reactions have delivered 13 new compounds, isolated in 65–97% yields, which include tertiary (ferrocenylmethyl)amines and azaferrocenophanes by using NaBH(OAc)3 as a mild reducing agent and (iminomethyl)ferrocenes and secondary (ferrocenylmethyl)amines by using LiAlH4. X-ray structures of representative members of these ferrocene derivative families have evidenced the preferred conformation adopted by ferrocene backbones,…
Noncovalent assembly of functional groups on calix[4]arene molecular boxes
1997
Calix[4]arenes diametrically substituted at the upper rim with two melamine units spontaneously form well-defined box-like assemblies in the presence of two equivalents of 5,5-diethylbarbituric acid. These assemblies, consisting of nine different components, are held together by 36 hydrogen bonds and are stable in apolar solvents at concentrations of up to 10-4M. This paper reports the first X-ray crystal structure, and the MALDI TOF mass spectra together with the complete 1H NMR spectroscopic characterization of these hydrogen-bonded assemblies. The crystal structure clearly shows that the assemblies are stereogenic, as a result of the antiparallel orientation of the two rosette motifs. Fu…
EPR spectroscopic characterization of persistent germyl-substituted Pb(III)- and Sn(III)-radicals.
2010
In this report we present the synthesis and the detailed electron paramagnetic resonance (EPR) spectroscopic characterization of novel trivalent lead- and tin-based radicals comprising sterically demanding germyl substituents. The investigated radicals are derived from the recently reported trihypersilyl-substituted tetryl radicals *PbHyp3 and *SnHyp3. The tetryl radicals *Pb(Ge(SiMe3)3)3 (8), *Pb(Ge(SiMe3)3)2Si(SiMe3)3 (9), *PbGe(SiMe3)3(Si(SiMe3)3)2 (10), and *Sn(Ge(SiMe3)3)3 (11) show substitution patterns derived from stepwise (9, 10) or complete (8, 11) substitution of hypersilyl groups (Hyp = Si(SiMe3)3) in *PbHyp3 and *SnHyp3 by homologous hypergermyl groups (Hge = Ge(SiMe3)3). They …
Synthesis of oligodeoxynucleotides containing diastereomeric dihydrodiol epoxide-N6-deoxyadenosine adducts of polycyclic aromatic hydrocarbons
1993
Abstract A generally applicable route is reported for the synthesis of oligodeoxynucleotides which contain structurally defined N 6 -deoxyadenosine adducts, derived from sterically highly hindered dihydrodiol epoxides of polycyclic aromatic hydrocarbons (PAH).
A study of the diels-alder reaction of 3-oxabicyclo[3.3.0]oct-1(5)-ene-2,4-dione with cyclopentadiene and some related dienes
1984
Abstract The reaction of 3-oxabicyclo[3.3.0]oct-l(5)ene-2,4-dione, 2, with cyclopentadiene, 3, 5,5-diethoxycyclopentadiene, 4, and 6,6-dimethylfulvene, 5, is studied. Reaction of 2 and 3 yields the expected endo-adduct, while reaction of 2 and 5 gives a mixture of the endo- and exo-adducts whose ratio is slightly dependent on the reaction conditions. Attractive electrostatic interactions in the transition state leading to the exo-adduct can explain the absence of endo stereoselectivity in the last reaction. Steric factors seem to be responsible for the absence of reaction between 2 and 4. The analysis of the 13C NMR spectra of these adducts and some derived compounds confirms the tentative …
An ab initio study of the enolboration of 3-pentanone mediated by boron monochlorides L2BCl
2001
Abstract Using ab initio methods at the HF/6-31G∗∗ level, we have studied the mechanism of the enolboration of 3-pentanone mediated by boron monochlorides L2BCl (L=H, methyl and isopropyl) and trimethyl amine. The size of the L group has been found to have a decisive influence on the configuration (E or Z) of the formed boron enolate. Thus, whereas our calculations predict that dimethylboron chloride yields the Z enolate with high stereoselectivity, diisopropylboron chloride is found to yield predominantly the E enolate. The difference in behavior is due mainly to steric features related to the conformational bias of the respective ketone–boron chloride complexes formed reversibly in the fi…
Donor-π-Acceptor Species Derived from Functionalised 1,3-Dithiol-2-ylidene Anthracene Donor Units Exhibiting Photoinduced Electron Transfer Propertie…
1998
Steric interactions between the anthraquinoid core and the 1,3-dithiole and dicyanomethylene groups play a key role in determining the physical properties of system 1. The intramolecular charge transfer properties of this donor–π-acceptor species have been explored and cyclic voltammetric data, X-ray crystal structures and ab initio calculations are also reported.
Protonation and oxidation chemistry of a pentaethylcyclopentadienyl-containing molybdenum(IV) trihydride complex
2000
International audience; Compound CpEtMoCl4 (CpEt=η5-C5Et5) (1) can be transformed into CpEtMoH3(dppe) (2) and CpEtMoD3(dppe) (2-d3) [dppe=1,2-(diphenylphosphino)ethane] by reaction with LiAlX4 (X=H and D, respectively). The protonation and oxidation studies of these two compounds, in comparison with previously reported studies on (C5Me5) analogs, show important differences that may be attributed to a kinetic stabilization of the products, which is steric in nature. Protonation of 2 with HBF4 in acetonitrile affords [CpEtMoH4(dppe)]+ (3), which only slowly decomposes to [CpEtMoH2(MeCN)(dppe)]+ (4). Further protonation of the latter affords the monohydride species [Cp*MoH(dppe)(MeCN)2]2+ in t…
Investigation of Temperature-Induced Phase Transitions in DOPC and DPPC Phospholipid Bilayers Using Temperature-Controlled Scanning Force Microscopy
2004
Under physiological conditions, multicomponent biological membranes undergo structural changes which help define how the membrane functions. An understanding of biomembrane structure-function relations can be based on knowledge of the physical and chemical properties of pure phospholipid bilayers. Here, we have investigated phase transitions in dipalmitoylphosphatidylcholine (DPPC) and dioleoylphosphatidylcholine (DOPC) bilayers. We demonstrated the existence of several phase transitions in DPPC and DOPC mica-supported bilayers by both atomic force microscopy imaging and force measurements. Supported DPPC bilayers show a broad L(beta)-L(alpha) transition. In addition to the main transition …