Potentiometric Investigation of Silver (I) Complexation by Arginine

Abstract

Arginine is an important amino acid involved in the production of NO, which is a vasodilatator. Its property as ligand towards cations was little studied. Also its protonation constants, even if subject of several studies, were not accurately determined. In this paper electromotive force measurements are performed by using a hydrogen electrode to accurately determine the three protonation constant of arginine. At 25°C and in 1.00 mol dm -3 NaClO 4 , as ionic medium, they are log k 1 =12.53 ± 0.05; log k 1 k 2 = 21.36 ± 0.03 log k 3 = 2.20 ±0.07, while in 1.00 mol dm -3 NaCl, the following values were obtained: log k 1 =12.35 ± 0.05; log k 1 k 2 = 21.18 ± 0.03; log k 3 = 2.08 ±0.07. Galvanic cells involving silver and glass electrodes were employed to study the behaviour of L-arginine as ligand towards silver (I). The experimental data, obtained at 25°C and in 1.00 mol dm -3 NaClO 4 , were explained by assuming the formation of the species Ag(HL) (log β 1,1,1 = 3.79 ± 0.04) and Ag(HL) 2 (log β 1,2,2 = 7.36 ± 0.03) The results of this paper will be used for future investigation.

An Investigation on the Equilibria between Arginine and Iron(II) and Iron(III)

“…the constants relative to eq 13 and eq 14 can be expressed, respectively: The value of log K w = 13.78 was obtained previously 11 in the same experimental conditions. The high values for the constants can explain the little percentage of hydrolyzed iron(III) found in this investigation.…”

“…In the same paper, silver and glass electrodes were employed to study the behavior of L-arginine as a ligand toward silver(I). 11 The knowledge of the protolytic equilibria of arginine and its complex formation with silver(I) at 25 °C and 1.00 mol kg −1 NaCl and 1.00 mol kg −1 NaClO 4 , as ionic media, was very useful to investigate the behavior of arginine toward calcium(II) and magnesium(II) in the same experimental conditions. 12 The behavior of arginine as a ligand toward iron(II) and (III) was investigated very little probably for difficulties connected with the properties of iron(II) and iron(III).…”

Complex formation between arginine and calcium (II) and magnesium (II)

“…On the basis of the stability constants values found for the complexes silver (I) -arginine, it was reasonable to suppose that two five-membered planar chelate complexes were formed between silver (I) and L-arginine (Antonilli et al, 2007). In contrast, it is hard to believe that in the case of calcium (II) and magnesium (II) chelate rings can be formed.…”

“…However, to explain the data in terms of complex formation, it is necessary to consider the basic behaviour of arginine and to know the protonation constants in both the studied ionic media. These were determined in a previous paper (Antonilli et al, 2007) and are listed in Table 2, both for 1.00 mol dm À 3 NaCl and 1.00 mol dm À 3 NaClO 4 .…”

“…measurements were performed by means of a hydrogen electrode to accurately determine all three-protonation constant of arginine. In the same paper, galvanic cells involving silver and glass electrodes (GEs) were employed to study the behaviour of L-arginine as ligand towards silver (I) (M. Antonilli et al, 2007). Using these results, i.e.…”