Binding of l-tryptophan and bovine serum albumin by novel gold nanoparticles capped with amphiphilic sulfonatomethylated calixresorcinarenes

Abstract

A simple, one-step synthesis of spherical gold nanoparticles (dcore=38 and 25nm) in aqueous solution has been carried out using sulfonatomethylated calixresorcinarenes with methyl (C1S) and pentyl (C5S) substituents on the lower rim both as reducing and stabilizing agents. The nanoparticles obtained, Au@C1S and Au@C5S, were characterized by absorption spectrophotometry, TEM, DLS, IR and TGA. The gold nanoparticles are stabilized by the interaction that occurs between macrocycles' sulfo-groups and the metal surface. It has been shown, that Au@C5S nanoparticles have a higher thermal stability than Au@C1S nanoparticles due to the denser packing of the macrocycle on the surface of gold nanoparticles caused by amphiphilic nature of the C5S macrocycle. C5S or Au@C5S associate cooperatively with l-tryptophan to form a nanoscale system capable of a spectral response to changing pH in aqueous solution.A detailed study of the interaction of С5S and Au@C5S with BSA in aqueous solution was carried out by spectrophotometry, DLS and fluorimetry. It was found that C5S and Au@C5S both form stable complexes with the protein, which was confirmed by the static character of the quenching of BSA fluorescence. The mechanism of the interaction of BSA with the macrocycles in aqueous solution and on the surface of gold nanoparticles was investigated. The quenching (KSV) and binding (Kas) constants, the number of binding sites (n), and thermodynamic interaction parameters (ΔG, ΔH, ΔS) were calculated. It was found that the process of protein binding by the free macrocycle and the modified gold nanoparticles is exothermic and spontaneous, and hydrogen bonding and van der Waals interactions make a major contribution into the binding. The synchronous fluorescence method showed that the binding of BSA with macrocycle influences the microenvironment of the tryptophan residues in protein molecule. The interaction of С5S and Au@C5S with BSA leads to the changes in the protein structure as well as in the physicochemical characteristics of gold nanoparticles, which is a promising aspect in the use of such systems for transport of protein molecules or their visualization-detection in biological media.