NMR Studies of Solvent-Assisted Proton Transfer in a Biologically Relevant Schiff Base: Toward a Distinction of Geometric and Equilibrium H-Bond Isotope Effects
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- 作者：Shasad Sharif ; Gleb S. Denisov ; Michael D. Toney ; and Hans-Heinrich Limbach
- 刊名：Journal of the American Chemical Society
- 出版年：2006
- 出版时间：March 15, 2006
- 年：2006
- 卷：128
- 期：10
- 页码：3375 - 3387
- 全文大小：259K
- 年卷期：v.128,no.10(March 15, 2006)
- ISSN：1520-5126
文摘
The tautomeric equilibrium in a Schiff base, N-(3,5-dibromosalicylidene)-methylamine 1, a modelfor the hydrogen bonded structure of the cofactor pyridoxal-5'-phosphate PLP which is located in the activesite of the enzyme, was measured by means of 1H and 15N NMR and deuterium isotope effects on 15Nchemical shifts at variable temperature and in different organic solvents. The position of the equilibriumwas estimated using the one-bond 1J(OHN) and vicinal 3J(H
CNH) scalar coupling constants. Additionally,DFT calculations of a series of Schiff bases, N-(R1-salicylidene)-alkyl(R2)amines, were performed to obtainthe hydrogen bond geometries. The latter made it possible to investigate a broad range of equilibriumpositions. The increase of the polarity of the aprotic solvent shifts the proton in the intramolecular OHNhydrogen bond closer to the nitrogen. The addition of methanol and of hexafluoro-2-propanol to 1 in aproticsolvents models the PLP-water interaction in the enzymatic active site. The alcohols, which vary in acidityand change the polarity around the hydrogen bond, also stabilize the equilibrium, so that the proton isshifted to the nitrogen.
CNH) scalar coupling constants. Additionally,DFT calculations of a series of Schiff bases, N-(R1-salicylidene)-alkyl(R2)amines, were performed to obtainthe hydrogen bond geometries. The latter made it possible to investigate a broad range of equilibriumpositions. The increase of the polarity of the aprotic solvent shifts the proton in the intramolecular OHNhydrogen bond closer to the nitrogen. The addition of methanol and of hexafluoro-2-propanol to 1 in aproticsolvents models the PLP-water interaction in the enzymatic active site. The alcohols, which vary in acidityand change the polarity around the hydrogen bond, also stabilize the equilibrium, so that the proton isshifted to the nitrogen.