Reduced Schiff Base Research Articles

Macrocyclic and macroacyclic compartmental Schiff bases: synthesis, characterization, X-ray structure and interaction with metal ions

Acyclic, potentially hexadentate (H 4L AH 4L D) or decadentate (H 6L E) compartmental Schiff bases have been obtained by reaction in methanol of 2,3-dihydroxybenzaldehyde and 1,2-diaminoethane, 1,2-diamino-2-methylpropane, 1,3-diamino-2,2-dimethylpropane, 1,2-diaminobenzene or tris(2-aminoethyl)amine in a 2:1 or 3:1 molecular ratios as appropriate. The reaction of 2,6-diformyl-4-Z-phenol (ZCl−, CH 3−) with 1,2-diaminobenzene in methanol produced the partially reduced macrocycles (H 2L F and H 2L G). Similarly symmetric tetraimine macrocyclic ligands have been synthesized by cyclo-condensation of the same 2,6-diformyl-4-substituted phenols and 1,5-diamino-3- azapentane (H 2L H) or 1,4-bis(3-aminopropyl)piperazine (H 2L I). A step by step procedure has been employed for the preparation of the asymmetric macrocycle (H 2L L). The ligands are yellow or yellow-orange solids and have been characterized by elemental analyses, IR, 1H NMR spectroscopy and electron impact (EI) or fast atom bombardment (FAB) mass spectrometry. The X-ray crystal structures of H 4L C and H 2L F have also been solved. Crystals of H 4L C, grown from CHCl 3, are monoclinic, space group P2 1/ c with a=6.767(4), b=14.106(5), c=18.163(5) Å, β=91.05(4)° for Z=4. The molecule is composed of two identical dihydroxybenzaldimine moieties connected by the carbon atom C(9). Three hydroxylic oxygens are protonated while the fourth is negatively charged. An intermolecular hydrogen system, involving the hydroxyl groups, gives rise to the formation of polymer chains parallel to c axis. Crystals of H 2L F, grown from CHCl 3, are monoclinic, space group P2 1/ a with a=22.481(7), b=8.989(5), c=11.911(5) Å, β93.28(5)° for Z=4. The molecule shows some unexpected bond lengths and angles. Two nitrogen atoms of each molecule are iminic, the other two aminic in character. A reduction of two CN bonds occurs during the synthetic procedures with the consequent formation of the partially reduced Schiff base. The coordination properties of the acyclic and cyclic ligands towards d and/or f metal ions have been tested, and the results are discussed in detail.

Structure of lysine adducts with 16α-hydroxyestrone and cortisol

Recent studies indicate that steroids containing a vicinal hydroxyketone moiety can react with proteins both in vitro and in vivo to form covalent addition products. This reaction is non-enzymatic and occurs via the Heyns rearrangement of an initial Schiff base adduct between the steroid carbonyl and the ϵ-amino group of lysine residues. The present study describes the synthesis, isolation, and structural analysis of model adducts prepared by the incubation of 16α-hydroxyestrone or cortisol with NaCNBH 3 and lysine derivatives blocked in the N α-position. The product formed from the reaction of 16α-hydroxyestrone and lysine was found to have the structure predicted for a reduced Schiff base between these molecules. A stable, cortisol-lysine adduct was similarly synthesized and isolated. This conjugate was found not to be the expected reduced Schiff base but rather a C-20 cyano amine. This compound most likely was formed by the nucleophilic addition of cyanide during the course of the incubation. The observation that the cortisol-lysine Schiff base is not reducible with NaCNBH 3 accounts for the observation that the incorporation rate of glucocorticoids into proteins is not increased by the presence of NaCNBH 3.

The radiolytic reduction of the Schiff base in bacteriorhodopsin.

Abstract —Nanosecond electron beam pulses cause radiolytic reduction of the retinal Schiff base in bacteriorhodopsin. The effects of different scavengers show that both H atoms and the formate radical anion can act as the reducing species. Model system studies on the radiolysis of retinal indicate transient formation of a retinal radical anion and suggest an analogous two‐step reaction in bR. Delocalization of the radical in the polyene system and reaction with surrounding groups would account for the relatively low yield of reduced Schiff base and the appearance of intra‐ and intermolecular crosslinks in irradiated bacteriorhodopsin.

Aging of human bone and articular cartilage collagen: changes in the reducible cross-links and their precursors.

The age-dependent changes in reducible collagen cross-links, Schiff bases and their precursors were followed for human bone and articular cartilage throughout the whole life span from 3 to 89 years of age. With aging, the reduced Schiff bases dihydroxylysinonorleucine remarkable decreased, wheras hexitollysine peaks increased significantly in both collagens. Dihydroxynorleucine, the precursor of the cross-link, was found to increase slightly with age. But the changes in hydroxylysinonorleucine and lysinonorleucine were comparatively small. These changes were discussed in relation to the increased insolubility observed in both collagens at higher age.