Conformational isomerism

About: Conformational isomerism is a research topic. Over the lifetime, 11563 publications have been published within this topic receiving 199312 citations.

Energy Increment Method Based on Quantum Chemical Results: A General Recipe for Approximative Prediction of Isomerization and Tautomerization Energies of Pyrimidine and Purine Nucleic Acid Bases and Related Compounds

Abstract: On the basis of HF/6-31G(d,p) and MP2/6-31G(d,p) quantum chemical computations of fully optimized structures of all isomers of cytosine, isocytosine, uracil, thymine, adenine, guanine, xanthine, and many model compounds, a system of additive energy increments has been derived, from which energies of conversion of geometric isomers (conformers) and tautomers of these nucleic acid bases may be reconstructed within approximately 0.5 kcal/mol. The increments are associated with specific structural fragments perceptible in the conventional structural formula. Physically, they correspond to repulsions between H atoms of −OH, −NH2, and NH substituents and H atoms bound to C or N ring atoms and to attractions between H atoms of such substituents and sp2 lone electron pairs localized at N atoms of the ring systems or of imino substituents. Tautomerization energies associated with displacements of H atoms among ring N atoms were included in the estimation process of the increments. Keto−enol and amino−imino tautome...

Plicatin B conformational landscape and affinity to copper (I and II) metal cations. A DFT study.

Abstract: The conformational landscape of the prenylchalcone plicatin B and some of its tautomers has been investigated at the B3LYP/6-31G* level in analogy to prior studies of ours on two structurally related prenylated pterocarpans. Since the antioxidant activity of these natural compounds is supposed to be related to their copper chelation ability, several complexes with Cu(I) and Cu(II) metal cations, Cu+ and Cu2+, have been taken into account with the metal ions described by effective core potentials in the LANL2DZ valence basis set. The preferred binding sites on low-energy conformers of E and Z plicatin B have been determined and their metal ion affinity (MIA) values have been compared. Both cations give stable complexes with plicatin B, but the stability order of the metallated species at the various coordination sites strongly depends on the cation nature. In particular, for the E configuration the most stable Cu+-plicatin B ground-state structure features the metal cation bridged between the hydroxy O lone pairs and the prenyl π density, while in the most stable Cu2+ complex the cation is coordinated with the inner lone pairs of the oxygens in the methyl ester moiety bearing an anti methyl group. For the Z configuration, in contrast, the most stable Cu2+ complexes are found with the metal ions dentated between the Z ester side chain and the prenyl π density, while Cu+ in addition is close to the aromatic ring density as well. A comparison of the Cu+ and Cu2+ affinity values demonstrates however that the affinity to Cu2+ is decidedly much higher (by a factor of 3–4, depending on the arrangement type) than that to Cu+, even including the possible B3LYP overestimate of the Cu2+ binding energy with respect to BHLYP. A tentative evaluation of MIA in aqueous solution using the polarizable continuum model of the solvent shows a remarkable decrease for Cu(II).

Laser spectroscopic study on the conformations and the hydrated structures of benzo-18-crown-6-ether and dibenzo-18-crown-6-ether in supersonic jets

Abstract: The laser-induced fluorescence spectra of jet-cooled benzo-18-crown-6 (B18C6) and dibenzo-18-crown-6 (DB18C6) exhibit a number of vibronic bands in the 35 000–37 000 cm−1 region. We attribute these bands to monomers and hydrated clusters by fluorescence-detected IR-UV and UV-UV double resonance spectroscopy. We found four and two conformers for bare B18C6 and DB18C6, and the hydration of one water molecule reduces the number of isomers to three and one for B18C6-(H2O)1 and DB18C6-(H2O)1, respectively. The IR-UV spectra of B18C6-(H2O)1 and DB18C6-(H2O)1 suggest that all isomers of the monohydrated clusters have a double proton-donor type (bidentate) hydration. That is, the water molecule is bonded to B18C6 or DB18C6 via two O–H⋯O hydrogen bonds. The blue shift of the electronic origin of the monohydrated clusters and the quantum chemical calculation suggest that the water molecule in B18C6-(H2O)1 and DB18C6-(H2O)1 prefers to be bonded to the ether oxygen atoms near the benzene ring.

Stimuli-responsive phenothiazine-based donor-acceptor isomers: AIE, mechanochromism and polymorphism

Abstract: The development of solid-state stimuli responsive materials has escalated following their intriguing colour switching properties and versatile material and optoelectronic applications. In this article, we have designed and synthesized donor–acceptor (D–A) isomers, namely p-PTZ, m-PTZ and o-PTZ, by functionalizing the para, meta and ortho positions of the benzothiazole (BT) ring with the phenothiazine (PTZ) moiety. The Suzuki cross-coupling reaction of the mono boronate ester of phenothiazine PTZ with the corresponding bromo derivatives of phenyl BT generated the isomers p-PTZ, m-PTZ and o-PTZ. The attachment of donor PTZ at different positions of the acceptor phenyl BT could exert influence on the donor–acceptor character and the molecular packing modes in the isomers, resulting in discrete photophysical and electronic properties. The isomers manifest contrasting emission properties in different solvents as a consequence of the twisted intramolecular charge transfer (TICT) state. The non-planar twisted structures of the isomers bring about aggregation induced emission (AIE) characteristics and reversible mechanofluorochromism (MFC) behaviour. The isomers display self-reversible colour switching by virtue of the different accessible conformers of the PTZ moiety. The p-PTZ isomer occurs as polymorphs owing to the conformational flexibility of the PTZ moiety. The structural and morphological differences in the polymorphs of p-PTZ were studied in detail using single crystal X-ray analysis, scanning electron microscopy (SEM) and powder X-ray diffraction (PXRD) studies. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations were used to explore the electronic properties of the isomers in the ground and excited state. The isomers were capable of sensing trifluoroacetic acid in solution. A detailed comparison of the photophysical, electronic and structural properties of p-PTZ, m-PTZ and o-PTZ has been done with a view to studying the outcome of the positional change. The adopted design opens up different possibilities for study of novel stimuli responsive materials.

Conformational analysis and structure-activity relationships of selective dopamine D-1 receptor agonists and antagonists of the benzazepine series.

Abstract: Comprehensive conformational analysis using molecular mechanics calculations (MM2(85)) has been carried out for the potent and selective dopamine D-1 receptor agonist 7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine (1; SKF SCH 23390), and several analogues, including conformationally constrained ones Calculated conformational energies have been related to pharmacological and biochemical data in an attempt to identify the biologically active conformations of 1 and 8 It is concluded that the most probable receptor-bound conformation in both cases is a chair conformation with an equatorial phenyl ring and for 8 an equatorial N-methyl group It is suggested that the orientation of the phenyl ring in the receptor-bound molecule does not deviate in terms of dihedral angles by more than about 30 degrees from the preferred phenyl group rotamer in which the planes of two aromatic rings are essentially orthogonal