Quantification of single-stranded nucleic acid and oligonucleotide interactions with metal ions by affinity capillary electrophoresis--Part II

Methionine oxidation of proteins analyzed by affinity capillary electrophoresis in presence of silver(I) and gold(III) ions

Oxidative damage of biopharmaceuticals during manufacturing and storage is a key concern throughout pharmaceutical development. However, few simple and robust analytical methods are available for the determination of oxidation sites. Here, the potential of affinity capillary electrophoresis (ACE) in the separation of proteins with oxidized methionine (Met) residues is shown. Silver(I) and gold(I) ions have the attribute to selectively form complexes with thioethers over sulfoxides. The addition of these ions to the BGE leads to a selective complexation of Met residues and, thus, to a change of charge allowing separation of species according to the different oxidation states of Met. The mechanisms of these interactions are discussed and binding constants for peptides containing Met with silver(I) are calculated. Additionally, the proposed method can be used as an indicator of oxidative stress in large proteins. The presented technique is easily accessible, economical, and has rapid analysis times, adding new approaches to the analytical toolbox of Met sulfoxide detection.

Interaction study of a lysozyme-binding aptamer with mono- and divalent cations by ACE

Binding between an aptamer and its target is highly dependent on the conformation of the aptamer molecule, this latter seeming to be affected by a variety of cations. As only a few studies have reported on the interactions of monovalent or divalent cations with aptamers, we describe herein the use of ACE in its mobility shift format for investigating interactions between various monovalent (Na+, K+, Cs+ or divalent (Mg2+, Ca2+, Ba2+) cations and a 30-mer lysozyme-binding aptamer. This study was performed in BGEs of different natures (phosphate and MOPS buffers) and ionic strengths. First, the effective charges of the aptamer in 30 mM ionic strength phosphate and MOPS (pH 7.0) were estimated to be 7.4 and 3.6, respectively. Then, corrections for ionic strength and counterion condensation effects were performed for all studies. The effective mobility shift was attributed not only to these effects, but also to a possible interaction with the buffer components (binary or ternary complexes) as well as possible conformational changes of the aptamer. Finally, apparent binding constants were calculated for divalent cations with mathematical linearization methods, and the influence of the nature of the BGE was evidenced.