Résumé
As with colloids, protein aggregation is the result of a combination of long-range repulsions and short-range attractions, which, depending on the balance of these interactions , can produce clusters of finite or infinite size.[1] Short-range forces include hydrogen (or sulfur) bindings, van de Waals and hydrophobic interactions, while long-range forces are primarily electrostatic. The diversity of protein-protein interactions and the softness of the ternary structure complicates the interpretation of the major driving force(s) for protein aggregation. Chaotropic salts are typically employed to disrupt the protein's hydrogen bonding network and lessen the hydrophobic effect, which lowers the protein's native state stability but increases its water solubility.[2] We studied the physical-chemistry behavior of nanometric size and low charge density inorganic anions. We have demonstrated their ability to strongly bind to neutral hydrated organic matter in aqueous solution, increasing significantly the cloud point of non-ionic surfactant self-assemblies, [3] but also to clusterize short amphiphiles that cannot self-assemble by themselves.[4] We focus here on the cobalta-bis-(dicarbollide) anion, COSAN [Co(C2B9H11)2]-, (Fig. 1a). Besides its superchaotropic character, COSAN has surfactant properties, although lacking the classical amphiphilic structure: surface activity, self-assembly, and formation of lyotropic lamellar phases.[5] This research focuses initially on myoglobin (Mb) in an effort to better understand COSAN-protein interactions (Fig. 1b). We alter the net charge on the Mb to achieve a quasi-completely negative potential and to reduce the electrostatic interaction of the COSAN anion with the protein while working in a buffer solution at pH 10. By using SAXS (Fig. 1c), we draw the conclusion that the COSAN/protein interaction generates a protein/protein association via several steps (dimerization, quadrimerization, and so on) fully tuned by the molar ratio of the nano-ion. Moreover, spectroscopic techniques (UV, fluorescence, and circular dichroism (Fig. 1d) were used to obtain information on the local environment of COSAN and on the proteins secondary structure and folding. Beyond a critical ratio that relies on the degree of the chaotropicity, these effects are observed with various nano-ions and proteins and appear to be general to the superchaotropicity of nano-ions. The controlled protein association achieved by using nano-ions appears to be potentially important for protein stabilization, drug delivery and the control of protein assembly.