Fast proton exchange in histidine: Measurement of rate constants through indirect detection by NMR spectroscopy

Akansha Ashvani Sehgal, Luminita Duma, Geoffrey Bodenhausen, Philippe Pelupessy

Research output: Contribution to journalArticlepeer-review

Abstract

Owing to its imidazole side chain, histidine participates in various processes such as enzyme catalysis, pH regulation, metal binding, and phosphorylation. The determination of exchange rates of labile protons for such a system is important for understanding its functions. However, these rates are too fast to be measured directly in an aqueous solution by using NMR spectroscopy. We have obtained the exchange rates of the NH3 + amino protons and the labile NHε2 and NH δ1 protons of the imidazole ring by indirect detection through nitrogen-15 as a function of temperature (272K<T<293K) and pH (1.3<pH<4.9) of uniformly nitrogen-15- and carbon-13-labeled L-histidine-HCl-H2O. Exchange rates up to 8.5×10 4s-1 could be determined (i.e., lifetimes as short as 12μs). The three chemical shifts δHi of the invisible exchanging protons Hi and the three one-bond scalar coupling constants 1J(N,Hi) could also be determined accurately. Seeking the invisible: The protons attached to the N atoms of histidine in an aqueous solution have hitherto escaped detection owing to the very fast exchange of these protons with the solvent. The exchange rate constants along with the one-bond scalar couplings and chemical shifts of these elusive protons have been determined for a pH range in which the three N nuclei are protonated (see figure).

Original languageEnglish (US)
Pages (from-to)6332-6338
Number of pages7
JournalChemistry - A European Journal
Volume20
Issue number21
DOIs
StatePublished - May 19 2014
Externally publishedYes

Keywords

  • NMR spectroscopy
  • amino acids
  • exchange rate constants
  • protonation

ASJC Scopus subject areas

  • Catalysis
  • Organic Chemistry

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