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51.
Vacca A Nativi C Cacciarini M Pergoli R Roelens S 《Journal of the American Chemical Society》2004,126(50):16456-16465
A new tripodal receptor for the recognition of monosaccharides is described. The prototypical host 1 features a 1,3,5-substituted 2,4,6-triethylbenzene scaffold bearing three convergent H-bonding units. The binding ability of the t-octyl derivative 1a toward a set of octylglycosides of biologically relevant monosaccharides, including Glc, Gal, Man, and GlcNAc, was investigated by 1H NMR in CDCl3. A protocol for the correct evaluation of binding affinities was established, which can be generally applied for the recognition of monosaccharides by 1H NMR spectroscopy. A three-constant equilibrium model, including 1:1 and 2:1 host-guest association and dimerization of the receptor, was ascertained for the interaction of 1a with all the investigated glycosides. An affinity index, which we defined median binding concentration BC50 in analogy to the IC50 parameter, intended to address the general issue of comparing dimensionally heterogeneous binding data, and a limiting BC0(50)quantity describing intrinsic binding affinities were developed for evaluating the results. BC0(50) values for 1a range from 1 to 6 mM, indicating an intrinsic binding affinity in the millimolar range and a selectivity factor of 5 toward the investigated glycosides. The treatment has been extended to include any generic host-guest system involved in single or multiple binding equilibria. 相似文献
52.
Giorgio Favini Demetrio Pitea Manuela Sottocornola Roberto Todeschini 《Journal of Molecular Structure》1982,87(1):53-64
The conformational geometries and possible interconversion paths for some oxa derivatives of bicyclo[5.1.0] octane have been studied by the molecular mechanics method. The theoretical results are compared with the experimental data for the molecular geometry of bicyclo[5.1.0] octane and 3,5,8-trioxabicyclo[5.1.0] octane, the free energy of activation for cycloheptene epoxide and 3,5-dioxabicyclo[5.1.0] octane, the dipole moments and molar Kerr constants in solution for cycloheptene epoxide, 3,5-dioxa- and 3,5,8-trioxabicyclo[5.1.0] octane. 相似文献
53.
This is a thermodynamic analysis of the Province of Ravenna (Italy) and its districts (Ravenna, Faenza and Lugo) and an evaluation of entropy waste production based on a balance sheet of greenhouse gases. The method used is energy analysis. The results show that the Province and Ravenna are characterized by a heavy exploitation of local non renewable resources, Faenza strikes a good balance between economic development and environment conservation and Lugo is less sustainable than the others. The greenhouse gas balance shows that the Province emits 10.5 times the quantity of greenhouse gases that it adsorbs and that the emissions reduction is required in the energy sector, which is responsible for 92% of the total. 相似文献
54.
Steady-state mass transfer experiments were done to determine whether certain strains of bacteria in homogeneous suspension will accumulate at the gas:liquid interface and thereby significantly increase the oxygen transfer rate to the suspension. In particular, with suspensions ofB. licheniformis, the measured transfer rates are as much as three times the rate expected for a uniform suspension. In contrast, suspensions ofM. luteus show no increase in oxygen flux above expected values. The fact thatB. licheniformis is motile, whereasM. luteus is not suggests that cell motility may play an important role in the accumulation process.
相似文献55.
56.
An experimental design procedure was applied to optimize the operating conditions of an axially-viewed inductively coupled plasma emission spectrometer instruments equipped with echelle optics with cross dispersion and charge transfer device. The multivariate effect of carrier gas flow rate and r.f. power on several analytical figures was investigated and discussed. Both ultrasonic and pneumatic nebulization were used. For the final choice of the optimum, different criteria were taken into account, mainly plasma robustness, instrumental precision, analyte and background net emission, detection limits and signal-to-background ratios. It was found that the use of moderate power (1100W) and mean carrier gas flow rate (0.75 L/min) allows to obtain sufficient plasma robustness, satisfactory precision, and excellent signal-to-background ratios and limits of detection, favorable for ultratrace element determinations in environmental matrices. 相似文献
57.
de Oliveira PR Rittner R 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2005,62(1-3):30-37
The conformational equilibria of 3-X-cyclohexanol [X=F (1), Cl (2), Br (3), I (4), Me (5), NMe(2) (6) and MeO (7)] and of 3-X-methoxycyclohexane [X=F (8), Cl (9), Br (10), I (11), Me (12), NMe(2) (13) and MeO (14)] cis isomers were determined from low temperature NMR spectra and PCMODEL calculated coupling constants. The energy differences between aa and ee conformers were obtained from these data (DeltaG(J)(av) and DeltaG(PC)(av), respectively) and also by the additivity principle from data for the monosubstituted cyclohexanes (DeltaG(Ad)). H-1 and H-3 hydrogen vicinal coupling constants and DeltaG(J)(av) values showed that the diequatorial conformer is predominant in the conformational equilibrium of the compounds studied at low temperature. However, DeltaG(PC)(av) data show that compounds 6 and 7 constitute an exception, since they are almost equally populated by ee and aa at room temperature, due to stabilization of their aa conformer by an intramolecular hydrogen bond. DeltaG(Ad) values, obtained according to the additivity principle, show a better agreement for compounds 2 and 3, since the 1,3-diaxial steric effect is counterbalanced by the formation of an intramolecular hydrogen bond (IAHB). For the remaining compounds, DeltaG(Ad) values underestimate the energy differences, since the 1,3-diaxial steric effect, between X and OH or OCH(3), is absent in the monosubstituted compounds used as references. Moreover, the DeltaG(PC)(av), calculated from the coupling constants, obtained through the PCMODEL program, are rather smaller than the DeltaG(J)(av) values, since the program does not have parameters for the effect, observed in this report, of a substituent at gamma position on coupling constants values for the hydrogen under consideration. 相似文献
58.
The thermal behaviour of [Ba(C2H6O2)4][Sn(C2H4O2)3] used as a BaSnO3 precursor, and its phase evolution during thermal decomposition are described. The initially formed transient barium tin oxycarbonate phase disintegrates into BaCO3 and SnO2, reacting subsequently to BaSnO3. The existence of the intermediate oxycarbonate phase is evidenced by Fourier transformed infrared (FT-IR), X-ray powder diffraction (XRD) and electron energy loss spectroscopy (EELS (ELNES)) investigations. 相似文献
59.
Improta R Mele F Crescenzi O Benzi C Barone V 《Journal of the American Chemical Society》2002,124(26):7857-7865
The importance of vicinal and long-range interresidue effects in determining the stability of the collagen triple helix has been investigated by quantum mechanical (QM) and molecular mechanical (MM) computations on suitable model polypeptides, taking into account solvent effects by the polarizable continuum model (PCM). At the QM level, the PII conformation corresponds to an energy minimum for pentapeptide analogues incorporating the sequence Gly-Pro-Pro-Gly, irrespective of the down or up puckering of the pyrrolidine ring. However, our computations indicate that the alternation of down and up prolines characterizing collagen and collagen-like peptides is not due to an intrinsic preference of the Pro-Pro-Gly sequence. This result is confirmed by MM computations of longer polypeptides. Next, MM computations on model triple helices show that a better packing is obtained for specific values of backbone dihedrals, which, in turn, favor the alternation of down and up prolines along each chain. 相似文献
60.
Michelle Bitencourt Matheus P. Freitas Roberto Rittner 《Journal of Molecular Structure》2007,840(1-3):133-136
The effect of attaching an additional fluorine atom at C-2 in 1-fluoropropane (FP), giving 1,2-difluoropropane (DFP), on its conformational equilibrium, is theoretically evaluated. This substitution causes critical implications on the conformer stabilities of DFP (TG, GT and GG conformations) and the steric and electrostatic interactions should favor the conformer with fluorine atoms trans. However, the gauche effect plays a major role in describing the energies balance in DFP, shifting the equilibrium towards the conformation in which the two fluorine atoms are gauche. The origin of this effect is discussed through an NBO analysis, which allows the evaluation of both classical and non-classical (hyperconjugation and bent bonds) interactions as the prevailing factors governing the conformational equilibrium of molecules containing the 1,2-difluoroethane fragment. 相似文献