Conformational analysis of the cyclised pyridoxal Schiff base of L-tryptophan. X-Ray crystal structure, nuclear magnetic resonance and molecular orbital studies of 3-carboxy-1-{3-hydroxy-2-methyl-5-[(phosphonooxy)methyl]-4-pyridyl}-1,2,3,4-tetrahydro-β-carboline

Abstract

In order to deduce the structural features of cyclised Schiff bases in their in vivo regulation of pyridoxal-requiring enzymes, the molecular conformation of 3-carboxy-1-{3-hydroxy-2-methyl-5-[(phosphonooxy)methyl]-4-pyridyl}-1,2,3,4-tetrahydro-β-carboline, a cyclised Schiff base between pyridoxal 5-phosphate and L-tryptophan, has been investigated by X-ray crystal analysis, ¹H NMR measurements and molecular orbital calculations. In the crystalline state the molecule, which is in a double zwitterionic state with the hydroxy and phosphate oxygen atoms deprotonated and the pyridine and β-carboline nitrogen atoms protonated, takes a rigid conformation stabilized by two intramolecular hydrogen bonds between the carboline NH and pyridoxal O^– and between the indole NH and phosphate O^– atoms. This solid conformation also appears to be the preferred conformation in DMSO solution, as judged from appreciable ROEs between protons and J values. Conformational analysis by CNDO/2 calculations showed the conformation observed in the crystal also to be energetically the most favourable. The importance of this molecular conformation for the biological function of the cyclised Schiff base is discussed.