Investigations of different carbohydrate anomers in copper(II) complexes with d-glucose, d-fructose, and d-galactose by Raman and EPR spectroscopy

With the aim of verifying different carbohydrate anomers coordinated to copper(II) ions, some copper(II) complexes with d-glucose (Glc), d-fructose (Fru), and d-galactose (Gal) were prepared and investigated by spectroscopic techniques. Their compositions were verified by elemental, ICP–AES and thermal analyses, in addition to conductivity measurements. The compounds isolated were consistent with the formula Na2[Cu2(carbohydrate)3]·8H2O and Na[Cu2(carbohydrate)3]·6H2O for the aldoses Glc and Gal, respectively, and Na2[Cu3(carbohydrate)4]·8H2O in the case of the ketose, Fru. EPR spectra of these solids showed a rhombic environment around the metal center and suggested the presence of different anomers of the carbohydrates in each case. By Raman spectroscopy, it was possible to verify the predominance of the β anomer of d-glucose in the corresponding copper complex, while in the free ligand the α anomer is predominant. In the case of the analogous complex with d-galactose, the spectrum of the complex shows bands of both anomers (α and β) in approximately the same relative intensities as those observed in the isolated free ligand spectrum. On the other hand, for the complex with d-fructose a mixture of both furanose (five-membered ring) and pyranose (six-membered ring) structures was detected with prevalence of the furanose structure. Based on variations in the relative intensities of characteristic Raman bands, the binding site for copper in the fructose ligand was identified as most likely the 1-CH2OH and the anomeric 1-OH, while in β-d-glucose it is presumably the anomeric 1-OH and the O-5 atom. These results indicated that EPR and Raman spectroscopy are suitable supporting techniques for the characterization of carbohydrate anomers coordinated to paramagnetic ions.

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