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Coupling of Functional Hydrogen Bonds in Pyridoxal-5‘-phosphate−Enzyme Model Systems Observed by Solid-State NMR Spectroscopy
journal contribution
posted on 2007-04-11, 00:00 authored by Shasad Sharif, David Schagen, Michael D. Toney, Hans-Heinrich LimbachWe present a novel series of hydrogen-bonded, polycrystalline 1:1 complexes of Schiff base
models of the cofactor pyridoxal-5‘-phosphate (PLP) with carboxylic acids that mimic the cofactor in a
variety of enzyme active sites. These systems contain an intramolecular OHN hydrogen bond characterized
by a fast proton tautomerism as well as a strong intermolecular OHN hydrogen bond between the pyridine
ring of the cofactor and the carboxylic acid. In particular, the aldenamine and aldimine Schiff bases
N-(pyridoxylidene)tolylamine and N-(pyridoxylidene)methylamine, as well as their adducts, were synthesized
and studied using 15N CP and 1H NMR techniques under static and/or MAS conditions. The geometries of
the hydrogen bonds were obtained from X-ray structures, 1H and 15N chemical shift correlations, secondary
H/D isotope effects on the 15N chemical shifts, or directly by measuring the dipolar 2H−15N couplings of
static samples of the deuterated compounds. An interesting coupling of the two “functional” OHN hydrogen
bonds was observed. When the Schiff base nitrogen atoms of the adducts carry an aliphatic substituent
such as in the internal and external aldimines of PLP in the enzymatic environment, protonation of the ring
nitrogen shifts the proton in the intramolecular OHN hydrogen bond from the oxygen to the Schiff base
nitrogen. This effect, which increases the positive charge on the nitrogen atom, has been discussed as a
prerequisite for cofactor activity. This coupled proton transfer does not occur if the Schiff base nitrogen
atom carries an aromatic substituent.
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Schiff base modelsSchiff base nitrogen atomproton transferMASOHN hydrogen bondSchiff base nitrogencarboxylic acidcarboxylic acids15 N CP15 N chemical shift correlationsaldiminecofactor activitynovel seriesadductring nitrogen shiftsnitrogen atom1 H15 N chemical shiftsPLPpyridine ringSchiff base nitrogen atomsaliphatic substituent1 H NMR techniquesintramolecular OHN hydrogen bondhydrogen bondsproton tautomerismFunctional Hydrogen Bonds
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