Asymmetry of Hydrogen Bonds in Solutions of Monoanions of Dicarboxylic Acids

Abstract

Is a hydrogen bond symmetric (single-well potential) or asymmetric (double-well potential)? The NMR method of isotopic perturbation of equilibrium was used to answer this question for the monoanions of a wide variety of 18O-labeled dicarboxylic acids. The observed 18O-induced isotope shifts, especially at the ipso carbons, demonstrate that these exist as a pair of equilibrating tautomers in both aqueous and organic solvents. This conclusion for organic solvents is opposite a previous one based on similar data. As a further test, primary isotope shifts, which had been diagnostic for a single-well potential, were reinvestigated. The monoanions of 1,2-cyclopentenedicarboxylic, 3,4-furandicarboxylic, and 3,4,5,6-tetrahydrophthalic acids have negative primary isotope shifts and are confirmed as having asymmetric hydrogen bonds. Although hydrogen phthalate has a positive primary isotope shift, it too is judged to have an asymmetric hydrogen bond, according to 18O-induced isotope shifts, which are considered more reliable.

α-helix to β-hairpin transition of human amylin monomer

“…A minimum free energy path between the α and β conformers can be found through a steepest descent algorithm. 76 A steepest-descent search starting from the saddle points in the vicinity of three meta-stable states (α-helix, β-hairpin and unstructured coil) result in primarily two pathways of transition from the α-helix to the β-hairpin state. The paths are represented in Figure 2 with solid black, solid red and dotted red lines.…”

Isotope effect on chemical shifts in hydrogen‐bonded systems

“…This is true for b-diketones, 54 malondialdehyde, 55,56 b-thioxoketones, 57 for monoprotonated dicarboxylic acids and for a number of enzyme systems. 58,59 For these systems it has so far not been possible to cool down enough for the single tautomers to be studied. The barrier to interconversion can be calculated in vacuum and the calculations predict very low barriers.…”

“…The change in the bond length can be estimated from a scan of the potential surface as demonstrated for o-hydroxyacetophenones. This approach has since been used for o-hydroxy Schiff bases 59 and in DNA and RNA base pairs 70 as well as for proteins. 36 For tautomeric systems calculation of nuclear shieldings is especially important because the chemical shifts for the two tautomers are normally not known and these are necessary to estimate the equilibrium contribution (see Equation (7)).…”

“…A rather good agreement with experimental values is obtained for Schiff bases. 59 A more advanced approach has been taken by Mavri and Co-workers 31 predicting isotope effects of picolinic acid N-oxide. In this approach the change was calculated on two-dimensional chemical shift surfaces.…”

Kombinierte Flüssigkeits‐NMR‐ und UV/Vis‐Spektroskopie: Untersuchung der Struktur starker OHO‐Wasserstoffbrücken von Phenolen mit Carbonsäuren

“…Darüber hinaus kann man mit der Kenntnis der Eigenschaften der beteiligten Spezies aus den NMR-Spektroskopiedaten die elektronische Bande von 7 genauer untersuchen, um Informationen über eine intrinsische Barriere oder eine Lösungsmittelbarriere für den Protonentransfer zu erhalten. [21,22] Der hier beschriebene Aufbau kann nützlich sein, wenn die Zusammensetzung eines Systems sehr empfindlich auf die Konzentrationen der wechselwirkenden Komponenten und auf die Temperatur reagiert, besonders in Systemen, in denen langsame chemische Reaktionen oder Photoreaktionen ablaufen. [23] Die UVNMR könnte auch zum Verständnis des Zusammenhangs zwischen der Struktur von Wasserstoffbrü-cken und den optischen Eigenschaften des Chromophors in aktiven Zentren von Signalproteinen wie PYP beitragen.…”