Pb(II)-binding capability of aminohydroxamic acids: Primary hydroxamic acid derivatives of α-amino acids as possible sequestering agents for Pb(II)

Abstract

Complexes of aminohydroxamic acids, d,l-α-alaninehydroxamic acid (α-Alaha), sarcosinehydroxamic acid (Sarha), d,l- N-methyl-α-alaninehydroxamic acid ( N-Me-α-Alaha), β-alaninehydroxamic (β-Alaha), l-aspartic acid-β-hydroxamic acid (Asp-β-ha), l-glutamic acid-γ-hydroxamic acid (Glu-γ-ha) and l-histidinehydroxamic acid (Hisha) with lead(II) in aqueous solution were studied by pH-potentiometric, 1H NMR and electrospray ionization mass spectrometric (ESI MS) methods. The results were compared to those of a simple monohydroxamic acid, acetohydroxamic acid and the effects of the amino group, hydroxamate-N, as well as, additional side chain donors on the co-ordination mode and on the stability of the complexes formed were evaluated. It was found that the amino nitrogen atom situating in β- or in γ-position (β-Alaha, Asp-β-ha, Glu-γ-ha) does not co-ordinate to Pb(II), only hydroxamate type chelates are formed before the hydrolytic processes. However, the amino-N in α-position (α-Alaha, Sarha, Hisha) seems to form a stable 5-membered ( N, N)-type chelate together with the deprotonated hydroxamate-N above pH 6. On the other hand, the hydroxamate ( O, O)-type chelate also exists. Since steric reasons do not allow the coordination of these two chelates of a molecule to the same Pb(II) ion, polynuclear complexes with mixed co-ordination modes are formed with the α-derivatives above pH 6. Simple hydroxamate type complexes are formed with N-Me-α-Alaha, where the hydroxamate-N is not able to co-ordinate. The co-ordination of the side chain imidazole of Hisha is not measurable, while a weak interaction of the side chain carboxylates of Asp-β-ha and especially of Glu-γ-ha can be suggested.