The protein−gold nanoparticle bioconjugates are playing an important role in the studies of biological systems. The nature of the interaction and the magnitude of the binding affinity together with the conformational changes in the protein upon binding are the most addressed topics in relation to the uses of the bioconjugates in different organisms. In this work, we study the human serum albumin (HSA) protein−gold nanoparticle (AuNP) interactions focusing on the nature of the gold nanoparticle surface modification. We have found that the interactions of the HSA with the AuNPs are mainly electrostatic and that the concentration of protein necessary to stabilize the conjugates decreases when the overall negative charge on the nanoparticle surface increases. The changes in the localized surface plasmon resonance (LSPR) signals of the gold nanoparticles (13 nm diameter) are used to determine the number of protein molecules necessary to stabilize the conjugates in a high ionic strength medium. Fluorescence spectroscopy (stationary and time-resolved) is used to characterize the different bioconjugates and determine the binding constants under different experimental conditions. Moreover, the use of an extrinsic fluorescence probe (1-anilino-8-naphthalenesulfonic acid, ANS) gives us some information about the existence of partial unfolding of the protein upon binding to the nanoparticle.

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A study of the reductive desorption process of 6MP-coated Hg and Au(111) electrodes in alkaline media has been carried out by means of such electrochemical techniques as cyclic voltammetry, chronoamperometry, and capacity−potential curves. The cyclic voltammograms show a single reduction peak whose peak potentials are strongly dependent on the nature of the substrate, Hg and Au(111) in this study. The full widths at half-maximum (fwhm) for Hg and Au(111) electrodes respectively are 19 and 21 mV at a scan rate of 0.1 V/s. These values are generally indicative of the existence of strong lateral interactions between molecules.The chronoamperometric curves have been recorded by single potential step experiments from a potential where the monolayer is stable to different final values within the range of potential where the monolayer is being desorbed. The curve shapes resemble those of systems that follow nucleation and growth mechanisms in the formation or dissolution of two-dimensional layers. In fact, the curve analysis using the nucleation and growth model of etching centers which was carried out in this report is actually in agreement with a mechanism of this kind for the dissolution of the 6MP monolayer at Hg and Au(111) electrodes. This study compares the properties of the 6MP monolayer at the two substrates Hg and Au(111) as well as with other thiol derivative monolayers.

The relevant biological information to take into account the design of bionanoconjugates is the structural organization of the proteins that interact with a metallic nanoparticle and form a protein corona. We have studied the interaction of the protein lysozyme (LYZ) with gold nanoparticles (AuNPs) protected by citrate, 6-mercaptopurine, and ω-mercaptoundecanoic acid monolayers in aqueous solution in a wide pH range. The bioconjugates are stable at pH higher than the isoelectric point (pI) of LYZ. At lower pH, flocculation occurs possibly by interactions between the exposed positive charges of the protein. The modification of the Lys residues of the protein by either succinylation or phosphopyridoxylation brings about important changes in the bioconjugates' flocculation behavior. By considering the location of the modified residues in the three-dimensional structure of the protein, a different orientation upon binding in comparison to that of the native protein is proposed. Finally, the high fluorescence quenching observed in the titration experiments of the bioconjugates is discussed in terms of an amplification of the quenching efficiency by energy transfer on the self-assembled protein film and a severe effect of fluorofore interactions of the proteins attached to the nanoparticle surface.

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Self-assembled monolayers (SAMs) of 6-mercaptopurine (6MP) have been prepared on a Au(111) single-crystal electrode by immersion of the metal surface in a 100 microM 6MP and 0.01 M HClO4 solution. The 6MP-SAM Au(111) single-crystal electrodes were transferred to the cell and allowed to equilibrate with the different aqueous working solutions before the electrochemical experiments. The influence of the solution pH was studied by cyclic voltammetry, double layer capacitance curves, and electrochemical impedance spectroscopy. The electrochemical behavior of the 6MP-SAM in acetic acid at pH 4 presents important differences in comparison to that obtained in 0.1 M KOH solutions. Cyclic voltammograms for the reductive desorption process in acid medium are broad and show some features that can be explained by a phase transition between a chemisorbed and a physisorbed state of the 6MP molecules. The low solubility of these molecules in acid medium could explain this phenomenon and the readsorption of the complete monolayer when the potential is scanned in the positive direction. The variation of the double-layer capacitance values in the potential range of monolayer stability with the pH suggests that the acid-base chemistry of the 6MP molecules is playing a role. This fact has been studied by following the variations of the electron-transfer rate constant of the highly charged redox probes as are Fe(CN)(6)-3/-4 and Ru(NH3)(6)+3/+2 as a function of solution pH. The apparent surface pKa value for the 6MP-SAM (pKa approximately 8) is explained by the total conversion of the different 6MP tautomers that exist in solution to the thiol species in the adsorbed state.