Deuterium isotope effects on ¹³C nuclear shielding of amino and acetamido compounds. Tautomerism and intramolecular hydrogen bonding

Abstract

Deuterium isotope effects on 13C nuclear shielding were investigated for a series of o-amino and acetamido azo compounds and for amino-and acetamido-9,lO-anthra-and leucoanthraquinones. A variety of these compounds form intramolecular hydrogen bonds and are possibly tautomeric in nature. Deuterium isotope effects on "C chemical shifts were used to probe both the tautomerism and the nature of the hydrogen bond. Deuterium isotope effects over two bonds reflect the strength of the hydrogen bond. In general, isotope effects do not depend on the solvent and, in this study, are independent of temperature. In the acetamido compounds isotope effects on the internal aromatic carbons, 'AC,,(NQ), vary much more than the 'ACO(ND) values of the external carbonyl carbon. Plots of ' A vs. 6(H) accordingly show different slopes. Investigations of these types of compounds allow a correlation between the hydrogen-bond enthalpy and 'ACO(ND) of amides. 'ACO(ND) values therefore form a basis for extending the use of deuterium isotope effects to, for example, biomolecules. Diamino-9,lOleucoanthraquinone represents, to our knowledge, the first case of a compound in which isotope effects from both a hydrogen-bonded and a free ND deuterium can be observed. Bis(pheny1amino)leucoanthraquinone exhibits interesting equilibrium isotope effects over many bonds. Many of these isotope effects are non-additive. Hydrogen bonding is discussed in general, based on these observations. KEY WORDS Deuterium isotope effects "C chemical shifts Intramolecular hydrogen bonding o-Amino azo compounds Tautomerism Hydrogen-bond strength Long-range isotope effects Non-additivity Multi-site deuteriation ~

The Interactions with Solvent, Heat Stability, and 13C‐Labelling of Alamethicin, an Ion‐Channel‐Forming Peptide

“…There are several published reports on the usefulness of the "C carbonyl resonance for providing information about peptide structure, dynamics, and interactions with solvent: (see Urry et a]., 1974; Jameson and Mason, 1987;Tuchsen and Hansen, 1988;Ando et al, 1988: Hansen et al, 1992Wishart et al, 1991 ;Wishart and Sykes, 1994). For example, helix-to-coil transitions in small alanine-based peptides were conveniently monitored i*iu the carbonyl chemical shift (Shalongo et al, 1994a,b).…”

Tautomeric behaviour and isotopic multiplets in the 13C NMR spectra of partially deuterated 5‐arylazo‐pyrimidine (1H,3H,5H)‐2,4,6‐triones and 5‐arylazo‐2‐thioxo‐pyrimidine (1H,3H,5H)‐4,6‐diones—evidence for elucidation of tautomeric forms

“…Carbon C5 must show a singlet in the form I as it is β to H/D at the oxygen atom in the forms II and III in pyrimidine ring moiety. In fact, deuterium isotope splitting multiplets are due to the low-frequency deuterium isotope effects on the carbon-13 chemical shifts: 0.07-0.116 ppm for 2 C α (ND), 0.01-0.05 ppm for 3 C α (ND) in amides [29,30] and 0.09-0.12 ppm for directly bonded hydroxyls 2 C α (OD) and 0.07 ppm or less for hydroxyl on vicinal carbons (γ -isotope effect). [16,31 -35] The two-bond isotope shift, 2 C α (ND) (β-isotope effect), depends on dihedral angle, ϕ.…”

“…The three-bond amide deuterium isotope shift, 3 C α (ND), depends primarily on the backbone angle, ψ. [29,30] As an example, Fig. 5(a) presents the isotopic multiplets in the 13 C NMR spectrum of 5c in (CD 3 ) 2 SO/CD 3 OD (≈80:12 (v/v)) with partially deuterated two amide and hydrazone protons.…”

Isotope effect on chemical shifts in hydrogen‐bonded systems

“…Electron-withdrawing groups will lead to more positive 4 DC ¼ OðODÞ. Very few compounds have been investigated 16,17 so far but the examples given in The values can again be explained by the distances around the six-membered hydrogen bond ring. An early negative example was that of 1,4-diaminophenyl-9,10-anthraquinone 16 (see Figure 9).…”

“…Very few compounds have been investigated 16,17 so far but the examples given in The values can again be explained by the distances around the six-membered hydrogen bond ring. An early negative example was that of 1,4-diaminophenyl-9,10-anthraquinone 16 (see Figure 9). The finding that the 4 DC ¼ OðNDÞ is primarily negative can be ascribed to a finding that the NH form is primarily on the charged form (see Figure 10).…”