Prev. Article Next Article Table of Contents

Article

NMR Studies of Solvent-Assisted Proton Transfer in a Biologically Relevant Schiff Base: Toward a Distinction of Geometric and Equilibrium H-Bond Isotope Effects

Supporting Info

Shasad Sharif,^† Gleb S. Denisov,^‡ Michael D. Toney,^§ and Hans-Heinrich Limbach*^†

Contribution from the Institut fr Chemie, Takustrasse 3, Freie Universitt Berlin, D-14195 Berlin, Germany, the Institute of Physics, St. Petersburg State University, 198504 St. Petersburg, Russian Federation, and the Department of Chemistry, University of California-Davis, Davis, California 95616

J. Am. Chem. Soc., 2006, 128 (10), pp 3375–3387

DOI: 10.1021/ja056251v

Publication Date (Web): February 18, 2006

†

Freie Universität Berlin.

‡

St. Petersburg State University.

University of California-Davis.

In papers with more than one author, the asterisk indicates the name of the author to whom inquiries about the paper should be addressed.

, limbach@chemie.fu-berlin.de

Abstract

The tautomeric equilibrium in a Schiff base, N-(3,5-dibromosalicylidene)-methylamine 1, a model for the hydrogen bonded structure of the cofactor pyridoxal-5‘-phosphate PLP which is located in the active site of the enzyme, was measured by means of ¹H and ¹⁵N NMR and deuterium isotope effects on ¹⁵N chemical shifts at variable temperature and in different organic solvents. The position of the equilibrium was estimated using the one-bond ¹J(OHN) and vicinal ³J(H_αCNH) scalar coupling constants. Additionally, DFT calculations of a series of Schiff bases, N-(R₁-salicylidene)-alkyl(R₂)amines, were performed to obtain the hydrogen bond geometries. The latter made it possible to investigate a broad range of equilibrium positions. The increase of the polarity of the aprotic solvent shifts the proton in the intramolecular OHN hydrogen bond closer to the nitrogen. The addition of methanol and of hexafluoro-2-propanol to 1 in aprotic solvents models the PLP−water interaction in the enzymatic active site. The alcohols, which vary in acidity and change the polarity around the hydrogen bond, also stabilize the equilibrium, so that the proton is shifted to the nitrogen.