Search results for "Isomerism"
showing 10 items of 747 documents
Differential interaction of the two cholesterol-dependent, membrane-damaging toxins, streptolysin O and Vibrio cholerae cytolysin, with enantiomeric …
2003
AbstractMembrane cholesterol is essential to the activity of at least two structurally unrelated families of bacterial pore-forming toxins, represented by streptolysin O (SLO) and Vibrio cholerae cytolysin (VCC), respectively. Here, we report that SLO and VCC differ sharply in their interaction with liposome membranes containing enantiomeric cholesterol (ent-cholesterol). VCC had very low activity with ent-cholesterol, which is in line with a stereospecific mode of interaction of this toxin with cholesterol. In contrast, SLO was only slightly less active with ent-cholesterol than with cholesterol, suggesting a rather limited degree of structural specificity in the toxin–cholesterol interact…
Effect of L-Histidine on the Survival of a T-Strain of Mycoplasma
1975
The addition of L-histidine to the growth medium prolongs the stationary phase and the survival of a T-strain of mycoplasma. Results of an experiment performed with 14 C-labeled urea demonstrate that the action of L-histidine is based on the retardation of the rise of pH.
Enzymatic effects on reactant and transition states. The case of chalcone isomerase.
2007
Chalcone isomerase catalyzes the transformation of chalcone to naringerin as a part of flavonoid biosynthetic pathways. The global reaction takes place through a conformational change of the substrate followed by chemical reaction, being thus an excellent example to analyze current theories about enzyme catalysis. We here present a detailed theoretical study of the enzymatic action on the conformational pre-equilibria and on the chemical steps for two different substrates of this enzyme. Free-energy profiles are obtained in terms of potentials of mean force using hybrid quantum mechanics/molecular mechanics potentials. The role of the enzyme becomes clear when compared to the counterpart eq…
Stereoselective syntheses of piperidinones and their modification by organometallic coupling reactions
2007
Dehydropiperidinones stereoselectively obtained from N-arabinosyl imines were iodinated at the enaminone structure. Knochel iodine-magnesium exchange afforded Grignard compounds of these piperidinone derivatives which reacted, either directly or after transmetalation to zinc or copper intermediates, with alkyl-, aryl- or acylhalides to give correspondingly substituted piperidinones. Stereoselective conjugate allyl cuprate addition to a thus obtained 5-allyl dehydropiperidinone and ring-closing metathesis of the product gave a hydroquinolinone containing three stereogenic centers.
Dielectric friction effects on rotational reorientation of three cyanine dyes in n-alcohol solutions
1997
We have estimated the effect of dielectric friction on the rotational correlation times of three cationic cyanine dyes. Dielectric corrections were evaluated by using the Stokes–Einstein-Debye hydrodynamic continuum model including the dielectric friction for DiIC2, DiIC6, and DiIC14 in different n-alcohol solutions at room temperature. The dielectric corrections were done to cis and trans conformations of the cyanine dyes. For the trans conformations, which were found more stable than cis conformations, the dielectric model seemed to be more properly suited. The ground and excited state dipole moments for the calculations were evaluated from ab initio molecular orbital calculations and for…
Geometric Structure and Torsional Potential of Biisothianaphthene. A Comparative DFT and ab Initio Study
1997
We present a study of the torsional potential of biisothianaphthene and compare it to that of bithiophene. The calculations are performed at the ab initio and semiempirical Hartree−Fock (HF), ab initio post-Hartree−Fock, and density functional theory (DFT) levels. Our study has two major aims: (i) on the physico-chemical side, to asses the optimal conformation of biisothianaphthene and evaluate the rotational barriers toward coplanar structures and (ii) on the methodological side, to asses the usefulness of DFT approaches. In contrast to previous estimates, the torsional potential of biisothianaphthene is found to differ markedly from that of bithiophene. For biisothianaphthene, strongly r…
Theoretical study of electronically excited cis- and trans-glyoxal
1997
Abstract The equation-of-motion coupled cluster method for excitation energies in the singles and doubles approximation (EOMEE-CCSD) is applied to an investigation of the structure and harmonic frequencies of planar conformers of glyoxal in their first excited singlet states. For the trans-isomer, agreement between calculated harmonic frequencies and observed fundamentals is generally satisfactory, although the theoretical values are slightly more than 10% too high for the carbonyl stretching modes. Parallel calculations of the corresponding ground state properties allow for an empirical prediction of the excited state frequencies in which calculated differences in normal-mode frequencies a…
1H, 13C NMR spectral and single crystal structural studies of toxaphene congeners. Quantum chemical calculations for preferred conformers of 2,5-endo…
2005
Abstract The 1H and 13C NMR chemical shifts for six toxaphene congeners: 2-exo,3-endo,6-exo,8,9,10-hexachloro- (1), 2-exo,3-endo,5-exo,9,9,10,10-heptachloro- (2), 2-exo,3-endo,6-exo,8,9,10,10-heptachloro- (3), 2-exo,3-endo,5-exo,6-endo,8,9,10-heptachloro- (4), 2-exo,3-endo,5-exo,6-endo,8,9,9,10-octachlorobornane (5) and 2,5-endo,6-exo,8,9,9,10,10-octachloro-2-bornene (6) are reported. Their chemical shift assignments have been obtained by means of Pulsed Field Gradient (PFG) Double Quantum Filtered (DQF) 1H,1H correlation spectroscopy (COSY), PFG 1H,13C Heteronuclear Multiple Quantum Coherence (HMQC) and PFG 1H,13C Heteronuclear Multiple Bond Correlation (HMBC) experiments. A single crystal…
Chlorodicyclopentadienyloxoniobium(V) complexes revisited: The origin of the asymmetry in the 1H- and 13C-NMR spectra, X-ray crystal structures and a…
2000
Abstract 1H- and 13C-NMR spectra of chlorodicyclopentadienyloxoniobium(V) complex I and its four 1,1′-dialkyl substituted derivatives II–V have been recorded and assigned based on DQF 1H,1H-COSY and PFG 1H,13C-HMQC and HMBC experiments. Non-equivalences of all cyclopentadienyl protons and carbons in II–V (as reflected by their different 1H- and 13C-NMR chemical shifts) are explained by synchronous and out-of-phase rotations of the substituted cyclopentadienyl rings. A non-equivalence of the methyls in III (1,1′-di-isopropyl) is explained by a detailed inspection of the rotamers of the isopropyl groups. The X-ray structural data show that III and IV (1-methyl-1′-tert-butyl) crystallize in th…
Electro-optical and laser spectrofluorimetry study of coumarins 7 and 30: evidence for the existence of the close-lying electronic states and conform…
1997
Our experiments and calculations reveal a set of anomalous and interesting properties of coumarin 7 (CU7) and coumarin 30 (CU30) in solution. The spectral dependence of electro-optical coefficients and, consequently, dipole moments indicates that the absorption band is a superposition of several (at least two) electronic transitions. Probably, there are two close-lying π,π* singlet electronic excited states which contribute to the first absorption band. The calculations using the MM2 program show that both dyes in the ground state have two stable plane anti conformers. From time-resolved spectrofluorimetric measurements we obtained evidence for the existence of conformers in paraffin oil so…