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Conformational isomerism

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


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Journal ArticleDOI
TL;DR: A conformational analysis of the valine side chains of ribonuclease T1 (RNase T1) was performed using NMR spectroscopy, in particular homonuclear and heteronuclear vicinal spin-spin coupling constants as obtained from E.COSY-type NMR experiments.
Abstract: A conformational analysis of the valine side chains of ribonuclease T1 (RNase T1) was performed using NMR spectroscopy, in particular homonuclear (1H, 1H and 13C, 13C) and heteronuclear (1H, 15N and 1H, 13C) vicinal spin-spin coupling constants as obtained from E.COSY-type NMR experiments. The coupling constants related to the chi 1 dihedral angle in valine, 3JH alpha H beta, 3JNH beta, 3JC'H beta, 3JH alpha C gamma 1, 3JH alpha C gamma 2, 3JC'C gamma 1, and 3JC'C gamma 2, were evaluated in a quantitative manner. The analysis of 3J data allowed for the stereospecific assignment of the valine methyl resonances. On the basis of various models for motional averaging of coupling constants, a fit of the torsion angles chi 1 to a set of the experimental 3J coupling constants (3JH alpha H beta, 3JNH beta, 3JC'H beta) was carried out. The resulting side-chain conformations were examined with respect to NOE distance informations. Single rotameric states emerged for Val16, Val67, Val79, and Val101, while conformational equilibria between staggered rotamers were found for Val33 and Val78. Using a different model approach, Val52 and Val89 are also likely to exhibit unimodal chi 1 angle distributions. The analysis was found to depend critically on the set of Karplus parameters used. Except for Val52 and Val78, the predominant rotamers obtained from 3J coupling informations agree with the conformation in the crystal structure of ribonuclease T1 (Martinez-Oyanedel et al., 1991).

46 citations

Journal ArticleDOI
TL;DR: The photophysical properties of trans-1- and trans-2-styrylnaphthalene (1-N and 2-StN) have been investigated by measuring the solvent and temperature effects and by m.o. calculations on the nature of the lowest excited states as mentioned in this paper.
Abstract: The photophysical properties of trans-1- and trans-2-styrylnaphthalene (1-StN and 2-StN) have been studied. The conformational isomers of 2-StN have been reinvestigated by measuring the solvent and temperature effects and by m.o. calculations (CNDO/S) on the nature of the lowest excited states. Fluorescence quantum yields and lifetimes and kinetic parameters of decay have been obtained for the two conformers in n-hexane and ethanol. The energy difference between the two conformers and the activation energy for their trans→cis photoisomerization have been obtained from the temperature effect in both solvents. Evidence has been obtained for two photoisomerization mechanisms, a triplet mechanism predominant below room temperature and a singlet mechanism favoured in ethanol and/or at higher temperatures.

46 citations

Journal ArticleDOI
TL;DR: The study offers new insights into how the trans conformation is stabilized in TFE clusters of increasing size, and eventually becomes a dominant conformation in the liquid phase.
Abstract: Rotational spectra of the three most stable conformers (I, II, III) of the ternary 2,2,2-trifluoroethanol (TFE) cluster were measured using Fourier transform microwave spectrometers, and unambiguously assigned with the aid of ab initio calculations. The most stable conformer, I, contains one trans-TFE subunit which is unstable in its isolated gas phase form. The study offers new insights into how the trans conformation is stabilized in TFE clusters of increasing size, and eventually becomes a dominant conformation in the liquid phase. A detailed analysis shows that while O-H⋅⋅⋅O-H and O-H⋅⋅⋅F-C hydrogen bonds are the most significant attractive interactions which stabilize all three conformers, the main driving force for the stability of I over III, which has all gauche-TFE subunits, is the attractive interaction of C-F⋅⋅⋅F-C contact pairs. A new type of three-point F⋅⋅⋅F⋅⋅⋅F attractive contact interaction is also identified.

46 citations

Journal ArticleDOI
TL;DR: The results are correlated with steady-state and dynamic fluorescence measurements at various temperatures in order to investigate further degrees to which ground and excited singlet state conformations affect the different photoreactivity channels available to the aryl esters.
Abstract: An aryl alkanoate, 2,4,6-trimethylphenyl (S)-(+)-2-methylbutyrate, whose ester group has a chiral center alpha to the carbonyl carbon and in which photo-Fries rearrangements are blocked by methyl substituents, undergoes facile photodecarboxylation under a variety of conditions and with complete retention of configuration. In fact, the decarboxylation process has many of the attributes of a symmetry-allowed suprafacial [1,3]sigmatropic rearrangement. The process requires concerted extrusion of carbon dioxide in a spiro-lactonic transition state, which has been investigated using high level DFT and CIS calculations: thermally less stable s-cis conformers in the ground and excited singlet states play an important role in determining the competitive efficiency of the process. Conformational control has also been imposed by substrate structure, solvent interactions, temperature, and applying external pressure, as well as using constraining media such as cyclodextrins and polyethylene films. The results are co...

46 citations

Journal ArticleDOI
TL;DR: The geometries of various tautomers and rotamers of N-hydroxyurea (aminoformohydroxamic acid), its anions and protonated forms were optimized at the Becke3LYP DFT level using the 6-311+G(d,p) basis set as mentioned in this paper.
Abstract: The geometries of various tautomers and rotamers of N-hydroxyurea (aminoformohydroxamic acid), its anions and protonated forms were optimized at the Becke3LYP DFT level using the 6-311+G(d,p) basis set. The calculations showed that the molecule of neutral acid should exist in several forms very close in energy. The hydroxamic tautomer is about 42 kJ mol-1 more stable than the hydroximic structure. The most stable conformer of N-hydroxyurea is non-planar with the OH hydrogen atom out of the plane of heavy atoms. In the structure of the anion the intramolecular hydrogen bond stabilizes the structure and makes the most stable conformations more planar. The HON- anion is more stable than the O-anion, hence N-hydroxyurea in the gas phase is an N-acid. N-hydroxyurea is a weak acid with calculated acidity of about 1470 kJ mol-1. The hydroxyamide tautomer of N-hydroxyurea is an oxygen base in the gas phase. However, the effective polarization of the cations of extended planar hydroximic tautomers causes the unusual increase of the stability of N-cations. Hence the hydroximic tautomer should be protonated with almost the same probability. The proton affinity of N-hydroxyurea was computed to be -856.0 kJ mol-1.

45 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
2023303
2022618
2021217
2020219
2019228
2018268