Metal ion chelating peptoids with potential as anti-oxidants: complexation studies with cupric ions

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Abstract

The cupric ion binding characteristics of the chelator EDTA bis (ethyl tyrosinate) are reported. Potentiometric studies in aqueous solutions over the pH range of 2.0–12.0 allowed identification and quantification of the species in solution. The principal species CuA predominates over the physiological pH range of 4.0–8.0 pH units. The logarithm of the stability constant (logβpqr) for this species is 16.43. The cupric ion binding characteristics were further assessed using electronic absorption spectroscopic investigations. These results support the use of this chelator as a metal binding anti-oxidant.

Introduction

Desferrioxamine which is commonly used to treat iron overload in haemachromatosis patients, is costly which has led to the development of a wide range of alternative chelators [1]. Orally active chelators which do not interfere with metalloproteins and do not exacerbate Fe-mediated radical damage are preferential for the treatment of iron overload conditions. Other chelator series including polyaminocarboxylic acids and their derivatives are under development for decorporation therapy and have been used for modelling metalloproteins. The ligand ethylenediaminetetraacetic acid bis (ethyl methioninate) demonstrated the ability of this class of chelators to form monomeric six-coordinate cupric complexes centred on the EDTA moiety [2].

We have adopted these systems to explore new therapeutic approaches to treat inflammation which is characterised by oxidative stress, an imbalance between oxidants and anti-oxidants [3], [4]. Excessive RONS can lead to cellular damage and release of cuprous and ferrous ions from storage proteins [5], [6]. Redox-active transition metal ions have been shown to partake in a wide number of reactions contributing to oxidative stress. These include the Fenton reaction, the Haber Weiss reaction, the Udenfriend’s system and amplification of aromatic nitration by peroxynitrite [7], [8]. Analysis of synovial fluid aspirated from patients suffering with rheumatoid arthritis showed increased levels of non-protein-bound Cu(II) and Fe(III) ions [9], [10]. These species can further exacerbate oxidative stress and anti-oxidant therapies can be designed to remove them.

We have developed a series of chelators for redox-active metal ions which demonstrate: (i) anti-oxidant activities against hydrogen peroxide and peroxynitrite [11], [12], (ii) superoxide dismutase and catalase activities [13] and (iii) the potential for development as an assay for oxidative stress [14]. Here, we report the cupric ion binding characteristics of these peptoids. Potentiometric studies in aqueous solutions allowed identification and quantification of the cupric species in solution and the determination of their stability constants (logβpqr). The cupric ion binding characteristics and the stability of the complex in the presence of albumin were determined using electronic absorption spectroscopy.

Section snippets

Species distribution studies

The species distribution of the cupric complexes of EDTA bis (ethyl tryrosinate) ET2 was calculated using the Sarkar–Kruck method. By processing the titration data using PLOT 11 program the presence of likely species (MpHqAr) were tested for. Processing using the program LEASK 11 to determine βpqr values gave the final species distribution curve [15].

Spectroscopic studies of cupric ion complexation

The complexation characteristics of the Cu(II)–ET2 complex were studied using spectrophotometric titrations. Solutions of 0.01 M ET2 and 0.05 M

Results and discussion

The ligands ET2 and EDTA bis (ethyl phenylalaninate) (EP2) (Fig. 1) are readily prepared in good yields using standard amine synthetic routes following ester protection of the amino acid. The methyl ester analogues were prepared by the same route using the methyl ester of tyrosine. Purification is effected by recrystallisation from hot alcohol. During the course of these studies both ligands exhibited excellent stability. The ligand synthetic details are given elsewhere [11]. The pKa values

Acknowledgments

We are grateful to Professor Peter Sadler, Professor David Brown and Dr. Christine Cardin for helpful discussions. We are grateful to the EPSRC and the University of Brighton for financial support.

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