Limiting values of the one-bond C-H spin-spin coupling constants of the imidazole ring of histidine at high-pH

作者：Vila Jorge A^*; Scheraga Harold A

来源：Journal of Molecular Structure, 2017, 1134: 576-581.

DOI：10.1016/j.molstruc.2017.01.022

• One-bond spin-spin coupling constant
• High-pH limiting values
• Histidine tautomers
• Protonated form of His
• Solvent effects

Assessment of the relative amounts of the forms of the imidazole ring of Histidine (His), namely the protonated (H+) and the tautomeric N-epsilon 2-H and N delta 1-H forms, respectively, is a challenging task in NMR spectroscopy. Indeed, their determination by direct observation of the N-15 and C-13 chemical shifts or the one-bond C-H,(1)J(CH), Spin-Spin Coupling Constants (SSCC) requires knowledge of the "canonical" limiting values of these forms in which each one is present to the extent of 100%. In particular, at high-pH, an accurate determination of these "canonical" limiting values, at which the tautomeric forms of His coexist, is an elusive problem in NMR spectroscopy. Among different NMR-based approaches to treat this problem, we focus here on the computation, at the DFT level of theory, of the high-pH limiting value for the (1)JCH SSCC of the imidazole ring of His. Solvent effects were considered by using the polarizable continuum model approach. The results of this computation suggest, first, that the value of 1J(C epsilon 1H) = 205 +/- 1.0 Hz should be adopted as the canonical high-pH limiting value for this SSCC; second, the variation of (l)J(CeH) SSCC during tautomeric changes is minor, i.e., within +/- 1Hz; and, finally, the value of 1J(C delta 2H) SSCC upon tautomeric changes is large (15 Hz) indicating that, at high-pH or for non-protonated His at any pH, the tautomeric fractions of the imidazole ring of His can be predicted accurately as a function of the observed value of (1)J(C delta 2H) SSCC.

• 出版日期2017-4-15