And conformational equilibria

Im W, Chen J, Brooks CL III (2006) Peptide and protein folding and conformational equilibria Theoretical treatment of electrostatics and hydrogen bonding with implicit solvent models. Adv Protein Chem 72 173-198. 

Model analogs of the green type chromophore HBI have been chemically synthe-tized in different forms carrying blocking groups in place of the protein polypeptide chain [21, 24, 68, 69]. However, the covalent structure of HBI does not uniquely define its optical properties, because the molecule undergoes several protonation and conformational equilibria that directly affect its electronic structure. 

Preferred Confobmers and Conformational Equilibria of 1,3-Dioxan-2-ones in CDCL AT Room Temperature [83ACSA(B)289]... 

No. l,3-Dioxan-2-one Anancomeric conformations and conformational equilibria (kcal mol" )... 

Raman spectroscopy, ab initio model calculations and conformational equilibria in ionic liquids... 

NMR spectroscopy has become an indispensable tool for synthetic chemists, and an additional and very useful technique for examining tautomeric and conformational equilibria. 

Possible correlations between optical activity and conformational equilibria in poly-a-olefins will be discussed later on. 

Pioneering work in the field of conformational analysis was contributed by O. Hassel (Norway) and D. R. H. Barton (Britain), for which they shared the Nobel Prize in chemistry in 1969. Hassel s work involved the physical determination of preferred conformations of small molecules, whereas Barton was the first to show the general importance of conformation to chemical reactivity. Study of conformations and conformational equilibria has direct application to explaining the extraordinary specificity exhibited by com-... 

D Cis-Trans Isomerism and Conformational Equilibria for Cyclohexane Derivatives... 

The SM2/AM1 model was used to examine anomeric and reverse anomeric effects and allowed to state that aqueous solvation tends to reduce anomeric stabilization [58]. Moreover, SM2/AM1 and SM3/PM3 models were accounted for in calculations of the aqueous solvation effects on the anomeric and conformational equilibria of D-glucopy-ranose. The solvation models put the relative ordering of the hydroxymethyl conformers in line with the experimentally determined ordering of populations. The calculations indicated that the anomeric equilibrium is controlled primarily by effects that the gauche/trans 0-C6-C5-0 hydroxymethyl conformational equilibrium is dominated by favorable solute-solvent hydrogen bonding interactions, and that the rotameric equilibria were controlled mainly by dielectric polarization of the solvent [59]. On the other hand, Monte Carlo results for the effects of solvation on the anomeric equilibrium for 2-methoxy-tetrahydropyran indicated that the AM1/SM2 method tends to underestimate the hydration effects for this compound [60]. 

D. Ronis, Phys. Rev. E, 49,5438 (1994). Statistical Mechanics of Ionic Colloids Interparticle Correlations and Conformational Equilibria in Suspensions of Polymer Coated Colloids. 

In a detailed H NMR study (employing also Pr(N03)3 and Eu(fod)3 as shift reagents), Palaima et al. (97MI74) estimated the stereoconfigurations and conformational equilibria of the cis/trans isomers of 3- and 4-aminocyclohexane carboxylic acid the cis/trans ratios of the isomeric mixtures were determined. 

Freeman et al. (00JMS(T)145, 98JCC1064) studied by ab initio quantum chemical calculations the geometry and conformational equilibria of all isomeric dihydrodioxines (185) and dihydrodithiines (186) (cf. Scheme 66). 

Abraham, R.J. Bretschneider, E. Medium Effects on Rotational and Conformational Equilibria, in Internal Rotation in Molecules Orville-Thomas, W.J., Ed. Wi ley-Interscience New York, 1974, pp. 481-584. 

Not only tautomeric equilibria are subject to considerable solvent effects. Other equilibria, such as rotational and conformational equilibria [81-83], cisitrans (or E/Z) isomerization, valence isomerization [84], ionization, dissociation, and association [85] (some of which are considered in Section 2.6), complex equilibria [86, 163, 262, 263], acid/base equilibria [264, 265] etc., are also strongly affected by the medium. Only a small number of representative examples will be considered in this Section in order to give an idea of how solvents can affect these different kinds of equilibria. 

The multipole expansion model has seen use in the examination of solvation effects on both reaction coordinates and conformational equilibria, including the isomerization of push-pull ethylenes o (e,g., nitroenamines), the ketene-imine [2-f 2]-cycloaddition to form p-lactam,24i and the Diels-Alder reaction.242,243 Again, only the ENP terms are considered in general. 

I. Benjamin, Theoretical study of the water/1,2-dichloroethane interface Stmcture, dynamics and conformational equilibria at the liquid-liquid interface, J. Chem. Phys. 97 (1992) 1432. 

Record, M.T. Jr., Anderson, C.F., Lohman, T.M. Thermodynamic analysis of ion effects on the binding and conformational equilibria of proteins and nucleic acids The roles of ion association or release, screening, and ion effects on water activity. Q. Rev. Biophys. 1978,11,103-78. 

Rotational barriers and conformational equilibria of 1,3-diphenylallyl lithium and a series of 2-substituted 1,3-diphenylallyl lithium compounds have been determined by Boche and coworkers 18). Table 3 summarizes the amounts of endo,endo, endo,exo-and exo.exo-conformers 27 and the free energies of activation of their mutual transformations. 

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