(15)N and (1)H Solid-State NMR Investigation of a Canonical Low-Barrier Hydrogen-Bond Compound: 1,8-bis(dimethylamino) Naphthalene.
(15)N and (1)H Solid-State NMR Investigation of a Canonical Low-Barrier Hydrogen-Bond Compound: 1,8-bis(dimethylamino) Naphthalene.
J Phys Chem B. 2015 Aug 5;
Authors: White PB, Hong M
Abstract
Strong or low-barrier hydrogen bonds have been often proposed in proteins to explain enzyme catalysis and proton transfer reactions. So far 1H chemical shifts and scalar couplings have been used as the main NMR spectroscopic signatures for strong H-bonds. In this work, we report simultaneous measurements of 15N and 1H chemical shifts and N-H bond lengths by solid-state NMR in 15N-labeled 1,8-bis(dimethylamino) naphthalene (DMAN), which contains a well known strong NHN H-bond. We complexed DMAN with three different counter anions to examine the effects of the chemical environment on the H-bond lengths and chemical shifts. All three DMAN compounds exhibit significantly elongated N-H distances compared to the covalent bond length, and the 1HN chemical shifts are larger than ~17 ppm, consistent with strong NHN H-bonds in the DMAN cation. However, the 15N and 1H chemical shifts and the precise N-H distances differ among the three compounds, and the 15N chemical shifts show opposite dependences on the proton localization from the general trend in organic compounds, indicating the significant effects of the counter anions on the electronic structure of the H-bond. These data provide useful NMR benchmarks for strong H-bonds, and caution against the sole reliance on chemical shifts for identifying strong H-bonds in proteins, since neighboring sidechains can exert similar influences on chemical shifts as the bulky organic anions in DMAN. Instead, N-H bond lengths should be measured, in conjunction with chemical shifts, as a more fundamental parameter of H-bond strength.
PMID: 26244754 [PubMed - as supplied by publisher]
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