Carmen Ibarra

Determination of acid dissociation constants of anomers of amino sugars by 1H NMR spectroscopy

Abstract Acid dissociation constants of α- and β- d -glucos-, mannos-, and galactos-ammonium ions have been determined from 1H NMR chemical shifts of the individual anomers in D2O. Values of pKa(D) for the α- and β-ammonium ions are, respectively: glucosamine, 8.12 and 7.87, mannosamine, 7.78 and 8.50, galactosamine, 8.49 and 8.02. The differences are ascribed largely to differences in the hydration requirements of ammonium and amino groups in the axial and equatorial positions and hydration at upper and lower faces of the sugars. Acid dissociation constants of the 1-hydroxyl group of nonionic d -glucosamine and d -glucose are higher for the β than the α anomer.

Optical activity of compounds formed by aminocomplexes of Co(III) and carbohydrates—I

Abstract The preparation of aminocomplexes of Co(III) containing d -ribose, l -sorbose or d -glucosamine as second ligand is described. Their spectral features and optical activity are reported. Formation in solution of complex species with other chosen carbohydrates gives ground for discussion as to the origin and sign of the observed Cotton effects.

Complexes of cobalt(III) with D-glucosamine and amines

Abstract d -Glucosamine and ammonia, 1,2-ethylenediamine, or 1,10-phenanthroline form complexes with Co(III) * . The complexes are: [Co(NH 3 ) 3 ·H 2 O· d -glucosamine] 3+ , [Co(en) 2 · d -glucosamine] 2+ and [Co(phen) 2 · d -glucosamine] 3+ . Formation of the last two complexes is stereospecific with Co(III) with predominance of the Δ over the Λ diastereomer. The Δ and Λ diastereomers of the ethylenediamine and phenanthroline complexes with d -glucosamine were separated and their configurations about Co(III) assigned from the CD and ORD spectra. The ethylenediamine complex has p K a ∼ 3.4 and is more acidic than the other complexes, pointing to coordination of the alkoxide residue at position 1 of glucosamine.

Structures of complexes of cobalt(III) and glucosamine. Applications of molecular mechanics and NMR spectroscopy

Abstract The 1 H NMR signals of the hydrochlorides of 2-amino-2-deoxy-α- and β- d -glucose (α- and β- d -glucosamine) have been assigned and chemical shifts and coupling constants determined from their phase-sensitive COSY spectra in D 2 O. Based on the Karplus relation the observed coupling constants fit H-H dihedral angles calculated with MM2 parameters. The COSY spectrum of the Λ isomer of [Co(en) 2 · d -glucosamine] 2+ shows that it is formed from the α anomer. The coupling constants, except for J 2,3 and J 3,4 , agree qualitatively with dihedral angles predicted by MM2 parameters. The structure of triammine-Co(III)- l -malate simulated by using MM2 parameters agrees reasonably well with that from X-ray crystallography.

Complexation of amino sugars with cobalt(III)bis(phenanthroline)

Abstract Complexes of Co(III)bis(phenanthroline) with β - d -mannosamine and α - d -galactosamine can be isolated as the triiodide salts. The complex with d -mannosamine has the Δ -configuration at Co(III). Complexation with d -galactoamine gives the Δ - in excess over the Λ - complex, and the Δ -complex can be isolated chromatographically. Complexation involves cis -1-OH and 2-NH 2 groups, eq , ax respectively with β -mannosamine and ax , eq respectively, with α -galactosamine, as with α -glucosamine, based on 1 H NMR spectra. Galactosamine complexes in aqueous solution decompose to the free sugar, but the mannosamine complex is much more stable.

Origin of chirality in charge transfer bands of Cu(II) acidates

Abstract Complexes of Cu(II)_with L-aminoacids have positive charge transfer bands in their CD spectra at ca. 230 nm and negative at ca. 270 nm. Examination of complexes with L-malic acid or L-1,2-diamino propane shows that the positive band is due to charge transfer from nitrogen and the negative from oxygen to copper. This sign difference is consistent with a theoretical model. The effects of added nonchiral ligands on the CD spectra in the charge transfer region are analyzed.

Mixed complexes of cobalt(III) with phenanthroline and galactose or arabinose

Abstract Mixed complexes of bis-phenanthroline cobalt(III) and α- d -galactose or β- d - or l -arabinose are identified in aqueous solution from their 1H NMR and circular dichroism (CD) spectra. Galactose forms only the Δ-complex, but d -arabinose gives preferentially Δ-, and l -arabinose preferentially Λ-complexes, consistent with structural optimization with PM3 (tm) parameters. The Λ-complex is formed initially from β- d -arabinose, but the Δ-complex is preferred thermodynamically. Examination of the absorption and CD spectra gives information on configuration at cobalt(III) and allows assignments of electronic transitions in Co(III) and the phenanthroline residues.

Triammine cobalt(III)-L-malate. Malate ion as a tridentate ligand

Abstract The crystal structure of the title compound has been determined from single crystal X-ray diffraction data. There is little angle strain in the complex which has two carboxylate ions and one alkoxide ion bound to cobalt(III). These conclusions agree with IR, CD and NMR spectral data except that 1H coupling constants show that the conformation of the malate ligand differs slightly in solid and solution. The three ammonia ligands have different 1H chemical shifts and different rates of hydrogen exchange in D2O.

Complexation of sucrose with cobalt(III)bis(phenanthroline)

The delta-[Co(III)bis(phenanthroline)(sucrose)]3+ complex forms with little perturbation of the sucrose conformation, and complexation by HO-2(g) and HO-1(f).

Complexes of cobalt(III) with d-fructose and phenanthroline

Abstract d -Fructose and 1,10-phenanthroline form complexes with Co(III). Configurations about Co(III) are assigned from the CD and ORD spectra of the separated Λ and Δ diastereomers, and Λ strongly predominates in the mixture. Most of the 1 H NMR signals of the complexes are shifted strongly upfield, relative to those of d -fructose, due to shielding by aromatic residues, and the effect is especially strong for hydrogens at positions 1 and 6. Signals of the fructose residue are assigned for the Λ diastereomer and coupling constants estimated, but only some of the signals can be assigned for the Δ diastereomer. The marked changes in chemical shifts on formation of the complexes from fructose are rationalized in terms of predicted conformations based on molecular-mechanics calculations with MM2 parameters, which predict the higher stability of the Λ over the Δ diastereomer, and also complexation at positions 2 and 3.