X-ray structural investigations of selected vitamin B6 model systems
Abstract (Summary)Restricted Item. Print thesis available in the University of Auckland Library or available through Inter-Library Loan. This thesis describes the X-ray structural analyses of a selection of non-enzymatic Vitamin B model systems with a view to examining structures and possible electronic shifts in complexed intermediates. The compound (3-methoxysalicylidene-DL-valinato)aquacopper(II), in which the biological cofactor has been replaced by a substituted salicylaldehyde, exhibits square planar geometry, the coordination sphere being occupied by the tridentate Schiff base ligand with a water molecule in the fourth position. In the complex N,N'-ethylene-bis (pyridoxylideneiminato)copper(II) four-coordinate, square planar geometry has been achieved through binding to the two imine nitrogen and two phenolic oxygens. The dimethylene bridge has adopted a gauche conformation with a short carbon-carbon bond of 1.467(7) Å. No hydrogen bonding exists to the oxygen donor atoms in either instance. Comparisons with data for analogous chelated and unchelated compounds provide some evidence for electron redistributions existing in the ground state thus helping to explain the coordination geometries adopted. The structural determinations of a complex with pyridoxal and L-histidine, in which cyclisation of the Schiff base ligand has occurred prior to chelation of the copper ion, and the compound (pyridoxylidene-L-isoleucinato) aquacopper (II) have highlighted the great difficulty in obtaining crystals and of maintaining them during data collection. The incorporation of large numbers of solvent water molecules (260-280 and 100-110 per unit cell respectively) and the number of atoms (more than 150 in both instances) within the structures have precluded accurate analyses. The molecular structures are, however, of primary concern and have been unequivocally resolved. In the former the phenolic oxygen, secondary nitrogen and carboxylate oxygen of one ligand, with histidine nitrogen from another, occupy the four planar positions about each copper, water being in the fifth, apical site. The structure is trimeric with three copper atoms in an approximately equilateral triangle bridged by the ligands. In the complex with isoleucine a similar situation exists but this time six molecules are bound in a zig-zag polymeric arrangement through bonding by the pyridine nitrogen of a neighbouring molecule at the fourth in-plane site. Discussion of modes of binding and the positioning of substituents at the ?-carbon atom of the amino acid in relation to the proposed reactivity of the systems is given.
School Location:New Zealand
Source Type:Master's Thesis
Date of Publication:01/01/1982