ISBN: 978-83-60043-10-3 - eurobic9

Eurobic9, 2-6 September, 2008, Wrocław, Poland
P197. Coordination Ability of Niacin Towards Nickel(II) Ions
J. Urbańska and H. Podsiadły
Faculty of Chemistry, University of Wrocław, F. Joliot-Curie 14, 50-383 Wrocław, Poland
Niacin is a water-soluble type of vitamin B including nicotinic acid and nicotinamide. Nicotinic acid is converted
in vivo to nicotinamide and although the two compounds are identical in their vitamin functions, their
pharmacologic and toxic effects are different.
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N
O
OH
Nicotinamide is an reactive moiety of two coenzymes with similar structures: nicotinamide adenine dinucleotide
(NAD) and nicotinamide adenine dinucleotide phosphate (NADP) which are necessary for enzymatic catalysis of
several vitally important redox processes.
Metal complexes of biologically important ligands are sometimes more effective than the free ligands.
In spite of the great biological importance of nicotinic acid and nicotinamide their interaction with metal ions is
rarely studied polarographically. This is probable due to the electrochemical activity of these ligands which
reduction waves overlap with the reduction wave of some metal ions or their complexes [1, 2, 3].
In this communication we present the polarographic, potentiometric and spectroscopic (UV-VIS) results on the
coordination ability of these ligands towards Ni(II) ions.
Ni(II) ions in non-complexing electrolyte undergo reduction process at mercury electrode irreversible at about -
1.0 V. Addition of very small amounts of nicotinamide or nicotinate ions causes the appearence of the new wave
(prewave), at less negative potentials (- 0.7 V). This prewave increase with concentration of ligands in the
solution, whereas Ni(II) wave decreases and finally disappears. The appearance of this prewave implies the
formation of at least one Ni(II)-niacin complex which is reduced at a lower potential than that required for the
reduction of aquaion. Detailed analysis polarographic curves on the concentration of reagents (ligands and Ni(II)
ions), pH values and mercury drop time allow us to establish the stoichiometry of the complexes reduced at the
electrode and those existing in the solution as well as their stability constants. Such a peculiar behaviour of
nickel(II) ions at the dropping mercury electrode was already observed in the presence of the other N and N, O
donor ligands [4, 5, 6, 7].
Potentiometric and spectroscopic studies display that nicotinic acid and nicotinamide are relatively week ligands
towards nickel(II) ions. The complexation occurs in the pH range 3 - 6; at higher pH the formation of poorly
soluble species (probably Ni(OH)2) has been observed. The stability constants of complexes determined from
potentiometric data are in good accordance with that obtained polarographically.
UV-VIS spectroscopy confirms monodentate coordination mode of both ligands to nickel(II) ions, by the
nitrogen of pyridine rings.
References:
[1] L. Campanella, P. Cignini and G. De Angelis, Rev.Roumaine Chim., 18, 1269 (1973).
[2] R. Rodriguez-Amaro, R. Perez, V. Lopez and J.J. Ruiz, J. Electroanal. Chem., 278, 307 (1990).
[2] E. Mathieu, R. Meunier-Prest and E. Laviron, Electrochim. Acta, 42, 331 (1997).
[4] D.R. Crow and M.E. Rose, Electrochimica Acta, 24, 41 (1979).
[5] P.K. Aggarwal and D.R. Crow, Electrochimica Acta, 25, 411 (1980).
[6] J. Urbańska, H. Kozłowski, A. Delannoy and J. Henion, Anal. Chim. Acta, 207, 85 (1988).
[7] J. Urbańska and H. Kozłowski, J.Coord.Chem., 42, 197 (1992).
N
O
NH 2