Effect of Gold Nanoparticle Aggregation on the Kinetic Aspect of AuNPs/DNA Interactions

Research Article

Austin J Nanomed Nanotechnol. 2021; 9(1): 1062.

Effect of Gold Nanoparticle Aggregation on the Kinetic Aspect of AuNPs/DNA Interactions

Grueso EM1*, Giraldéz-Pérez RM2 and Prado-Gotor R1

1University of Seville, Department of Physical Chemistry, Spain

2University of Cordoba, Faculty of Science, Department of Cellular Biology, Physiology and Immunology, Spain

*Corresponding author: Grueso EM, Department of Physical Chemistry, University of Seville, Spain; Email: elia@us.es

Received: February 05, 2021; Accepted: March 03, 2021; Published: March 10, 2021

Abstract

Since successful therapy for curing cancer and others genetic diseases requires the transport of DNA into the cell by delivery vehicles, the understanding of the factors that control the complexation and condensation of the DNA is a key problem. During the last decade, researchers have developed some uses of nanoparticles-DNA systems, the majority of these studies dealing with NPs, which are covalently bound to the DNA. However, the kinetic aspect of AuNPs/DNA system by non-covalent interactions is less explored. Moreover, the role of high salt concentrations in these studies is of great interest due to the majority of nanoparticles have a great tendency to aggregate upon exposure to biological medium, significantly alter the uptake extent, rate, and mechanism of AuNPs/DNA interaction. As a contribution to this field, we have studied kinetics aspects of the binding of small tiopronin gold nanoparticles, AuNPs, to double stranded DNA in at high salt concentration by using the stopped-flow technique. The kinetic curves are biexponential and reveal the presence of two kinetic steps. Moreover, AFM studies reveal AuNPs aggregation in the presence of high salt content, while the same particle are well-dispersed in water. A two-step series mechanism reaction scheme was proposed. According to the reaction scheme, the formation of an intermediate complex formed by aggregated gold nanoparticles and DNA precedes the rate-determining step of the reaction.

Keywords: DNA; Gold nanoparticles; Kinetic; AuNPs-aggregation

Introduction

The understanding and dealing of nanoparticle/DNA interactions has become an important emerging area of research due to the high number of diagnostic and therapeutic applications derived from these systems [1-4]. In particular, the comprehensive study of DNA-AuNPs affinity interactions have contributed to the promising challenge of the use of these systems to treat diseases, specifically, hereditary diseases by the insertion of genes into the human cells is so called gene therapy. Importantly, the gene transfection strategies need, as a pre-requisite, the effective complexation and the collapse of extended DNA chains into compact DNA structures [5]. Of particular note, non-covalent interactions between nanoparticles and DNA have been recognized to control physicochemical aspects of the interaction. In this sense for each nanoparticle-DNA systems a global effect is exerted in conjunction by both the metal cluster core and the capping agents that contribute to stabilize the nanosystem [6]. Regarding the metal cluster core, especially electrostatic interactions [7,8], hydrophobic forces [9], and the specific bonding between the chemical groups of DNA bases and the metal center control these interactions [10].

In relation to this, understanding the interactions of aggregates or isolated nanoparticles with DNA is determinant to their use in vivo delivery of drugs and efficacious nanomedicine design. In fact, the exploration of new nanoparticle systems that are capable of changing the aggregation state within the physiological environment constitutes an emerging concept [11]. In a previous paper, the effect of etanol on the kinetic of aggregation of AuNPs/DNA was explored, revealing not only a change in the binding mode induced by the solvent but also on the mechanism of its interaction [12]. However, the effect of high salt concentration on AuNPs/DNA mechanism is unexplored. Since nanoparticles may aggregate upon exposure to biological medium, due to the presence of aggregation-inducing molecules/species (such as salt), exploring effect of high salt concentration on the kinetic and thermodynamic of AuNPs/DNA interactions is a topic of great interest. In fact, the presence of high salt concentration may significantly alter the uptake extent, rate and mechanism of AuNPs/DNA interaction [13]. In a previous work, the interaction of small tiopronin gold nanoparticles, Au@tiopronin, with DNA biopolymer was evaluated at very low [NaCl] (0.001-0.015 M) [14]. The result demonstrated that the kinetic results are compatible with a three-step series mechanism reaction scheme, in which the groove binding interactions of DNA and gold nanoparticles was governed by solvation and viscosity factors [14]. In this study we have investigated the effect of high salt concentrations on the kinetic of Au@tiopronin with DNA. The simplest mechanism consistent with the kinetic results involves a more simple reaction scheme with two-step reactions. The first step corresponds to a very fast step that is related to a diffusion controlled formation of an external precursor complex between aggregated gold nanoparticles and DNA. The second step involves the formation of an external complex, as a result of the binding affinity between hydrophilic groups of the aggregated tiopronin nanoparticles and the DNA grooves. As a summary, this study reveals that kinetic of AuNPs/DNA interaction is controlled by AuNPs aggregation state, revealing the importance to take great care in control of the solvent media for diagnostic and therapeutic applications derived from these systems.

Materials and Methods

All chemicals were of Anal. R. grade and were used without further purification. Hydrogen tetrachloroaureate (III) trihydrate, 3-Aminopropyltriethoxilane (APTES), NaCl and BaClO4 were purchased from Sigma-Aldrich; N-(2-mercaptopropionyl)glycine from Fluky; NaBH4 from Lancaster. Calf thymus DNA was purchased from Pharmacia and used without further purification, because preliminary experiments showed that purification does not produce any changes in the experimental results. The absorbance ratio of DNA stock solutions at 260nm and 280nm was monitored and found to be between 1.8 and 1.9 (A260/A280 = 1.87), indicating no protein contamination [15]. An agarose gel electrophoresis test using ethidium bromide indicated that the average number of base pairs per DNA molecule is above 10,000 bp [16]. Polynucleotide concentrations were determined spectrophotometrically from the molar absorptivity (6600M-1cm-1 at 258nm in order to have the DNA concentration in phosphate units) [17]. Solutions were prepared with de-ionized water, its conductivity being less than 10-6Sm-1. Tiopronin gold nanoparticles, Au@tiopronin were synthesized by using Templeton et al.’s procedure [18]. Gold nanoparticles were characterized by visible absorption spectra, TEM and microanalysis (11.8% C; 1.86% H; 2.89% N; 5.80% S; Au 70.88%, C410H656O246N82S82Au197). A value of (1.6 ± 0.2) nm was obtained for the diameter of the gold nanoparticle in the absence of any added salt (Figure 1). According to these data, the relationship between the number of Au atoms and tiopronin ligands was 197/82.

Kinetics

The kinetic experiments were performed at 298.0K by using a Biologic SF 300 stopped-flow instrument and monitoring the course of the reaction in the CD detection mode. This detection mode was employed because the signal-to-noise ratio was found to be more favorable compared to the absorbance mode. The acquired signal was recorded on a PC and then analyzed by using the Jandell AISN software program. The Au@tiopronin concentration was 1.0×10-6M in all experiments, and the DNA concentration was varied. All the kinetic experiments were performed under pseudo first-order conditions ([DNA] >10 [Au@tiopronin]). Each experiment was repeated at least 10 times, and the relevant kinetic traces were accumulated in order to reduce the signal-to-noise ratio. The spread of time constants was found to be within 10%.

TEM measurements

For TEM visualization, a single drop (10mL) of an AuNPs aqueous solution was placed on a carbon film coated copper grid, which was then left to air dry for 2 hours at room temperature. TEM analysis was carried out using a Philips CM 200 electron microscope working at 200kV, and the resulting images were analyzed by using Image J free software.

Circular Dichroism (CD) spectra

Electronic CD spectra were recorded using a BioLogic Mos-450 spectropolarimeter. A standard quartz cell of 10mm path length was used. The spectra were expressed in terms of molar ellipticity. Scans were taken from 220nm to 310nm for the intrinsic region. For each spectrum, 5-10 runs were averaged at a constant temperature of 25.0ºC, with a 5min equilibration before each scan. All the spectra were recorded at a fixed concentration of double-stranded DNA, CDNA = 1.0×10-4M, and CNaCl = 0.50M.