Comparison of the phytoestrogen trans-resveratrol (3,4',5-trihydroxystilbene) structures from x-ray diffraction and solution NMR

The NMR-derived solution structure of trans-3,4',5-trihydroxystilbene (resveratrol) was compared with two recent literature crystal x-ray structures, resveratrol in complex with human transthyretin (TTR-RES) from 1DVS.pdb and resveratrol bound to chalcone synthase (CHS-RES) from 1CGZ.pdb. 1H and 13C NMR spectra of resveratrol were acquired in DMSO-d6. Assignments were obtained from an analysis of DQF-COSY, TOCSY, DEPT, HMQC/HSQC, HMBC and INADEQUATE NMR spectra. Past 1H and 13C NMR literature assignments are corrected. The dihedral angle 2-1-1'-2' provides an indication of the relative spatial orientation of the two phenolic rings. Values of 1.62, - 54.10 and 12.6 +/- 1.1 degrees were found for the 1DVS.pdb, 1CGZ.pdb and NMR resveratrol structures, respectively. The 1DVS.pdb resveratrol structure is 'flat' with the two phenolic rings along the same plane. The 1CGZ.pdb structure has these two rings almost orthogonal to each other, and the NMR structure has these two rings much closer to being along the same plane. The angles 1-alpha--alpha' and 1'-alpha'--alpha are along the same trace and of similar magnitude for the 1CGZ.pdb and NMR resveratrol structures. For the 1DVS.pdb resveratrol structure, these angles are about 7-10 degrees greater, with alpha and alpha' being 180 degrees out-of-phase from the other two structures. The alpha rings did not overlap, with the NMR result representing a 'median model' of the two x-ray structures.     

Closely stacked oligo(phenylene ethynylene)s: effect of π-stacking on the electronic properties of conjugated chromophores

In this work, a bicyclo[4.4.1]undecane scaffold is used to hold oligo(phenylene ethynylene) units in a cofacially stacked arrangement along the entire length of the conjugated units. We study the impact that the resulting strong interchain interactions have on the photophysical properties. The length of the individual oligomer branches was varied from three to five rings to investigate the effect of conjugation on the electronic properties of the stacked segments. Absorption and fluorescence spectra were recorded and compared to those of the corresponding unstacked analogues. Time-dependent density functional theory calculations were carried out and helped to rationalize the low-energy features present in the fluorescence spectra of the stacked systems. The calculations indicate that the low-energy emissions are due to the presence of excimer-like states. The stronger intensity of the low-energy fluorescence band observed in the five-ring stacked system compared to the three-ring analogue is attributed to the smaller activation barrier that separates the local intrachain state and the excimer-like state in the former compound.     

Increased activity of in situ catalysts for alkyne metathesis

[reaction: see text] Reaction of an enyne (1,1-diphenyl-pent-1-ene-3-yne) with a preheated mixture of Mo(CO)6/4-chlorophenol/3-hexyne at 130 degrees C furnished 1,1,6,6-tetraphenylhex-1,5-diene-3-yne in an 80% yield. If the starting material was heated with a mixture of Mo(CO)6 and 4-chlorophenol under identical conditions, no reaction was observed.     

Energy dependence of multiplicity in proton-nucleus collisions and models of multiparticle production

This is a continuation of our earlier investigation (Gurtuet al 1974Phys. Lett. 50 B 391) on multiparticle production in proton-nucleus collisions based on an exposure of emulsion stack to 200 GeV/c beam at the NAL. It is found that the ratioR _em = 〈n _s〉/〈n _ch〉, where 〈n _ch〉 is the charged particle multiplicity in pp-collisions, increases slowly from about 1 at 10 GeV/c to 1·6 at 68 GeV/c and attains a constant value of 1·71 ± 0·04 in the region 200 to 8000 GeV/c. Furthermore,R _em = 1·71 implies an effectiveA-dependence ofR _A =A ^0.18,i.e., a very weak dependence. Predictions ofR _em on various models are discussed and compared with the emulsion data. Data seem to favour models of hadron-nucleon collisions in which production of particles takes place through adouble step mechanism,e.g., diffractive excitation, hydrodynamical and energy flux cascade as opposed to models which envisage instantaneous production.     

Computational Investigation of the Substituent Effect on the Intramolecular Proton Transfer Reaction of 3-Hydroxytropolone

The effects of substituent type and position on the proton transfer reaction of 3-hydroxytropolone(3-OHTRN) have been investigated theoretically by using density functional theory at the level of B3LYP/ 6-31+G** method. The influence of solvent on the proton transfer reactions of substituted 3-OHTRN has been examined using the self-consistent isodensity polarized continuum model(SCI-PCM) in water. As a result, while the proton transfer reaction is kinetically the easiest by substitution on position 3 of-NH2 group in the gas phase, it is kinetically the easiest by substitution on position 5 of the same group in water. In addition, these reactions are either kinetically or thermodynamically easier in the gas phase than that in water, except the reaction of structure with-NH2 group at position 6.     

Competition Between π–π and CH/π Interactions: A Comparison of the Structural and Electronic Properties of Alkoxy‐Substituted 1,8‐Bis((propyloxyphenyl)ethynyl)naphthalenes

The structural and electronic consequences of π–π and C?H/π interactions in two alkoxy‐substituted 1,8‐bis‐ ((propyloxyphenyl)ethynyl)naphthalenes are explored by using X‐ray crystallography and electronic structure computations. The crystal structure of analogue 4 , bearing an alkoxy side chain in the 4‐position of each of the phenyl rings, adopts a π‐stacked geometry, whereas analogue 8 , bearing alkoxy groups at both the 2‐ and the 5‐positions of each ring, has a geometry in which the rings are splayed away from a π‐stacked arrangement. Symmetry‐adapted perturbation theory analysis was performed on the two analogues to evaluate the interactions between the phenylethynyl arms in each molecule in terms of electrostatic, steric, polarization, and London dispersion components. The computations support the expectation that the π‐stacked geometry of the alkoxyphenyl units in 4 is simply a consequence of maximizing π–π interactions. However, the splayed geometry of 8 results from a more subtle competition between different noncovalent interactions: this geometry provides a favorable anti‐alignment of C?O bond dipoles, and two C?H/π interactions in which hydrogen atoms of the alkyl side chains interact favorably with the π electrons of the other phenyl ring. These favorable interactions overcome competing π–π interactions to give rise to a geometry in which the phenylethynyl substituents are in an offset, unstacked arrangement.     

Conformational and electronic engineering of twisted diphenylacetylenes

[structure: see text] Three tethered diphenylacetylene derivatives were prepared by alkyne metathesis. In these cycles, the twist angle between the two benzene rings is variable and determined by the nature of the linker. The engineering of the twist angle leads to a change of the UV-vis spectra of the cycles. The larger the twist angle in the macrocycles, the more blue shifted their lambda(max) (UV-vis), the lower their fluorescence quantum yield, and the lower field shifted their (13)C NMR signals of the alkyne carbons are.