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Polyarginine (poly-Arg) and arginine-rich peptides have been attracting enormous interest in chemical and cell biology as cell-penetrating peptides capable of direct intracellular penetration. Owing to advances in protein engineering, arginine-rich fragments are often incorporated into multifunctional bioorganic/inorganic core–shell nanoparticles, enabling them the novel unique ability to cross cells and deliver biopharmaceutical cargos. Therefore, understanding the molecular details of the adsorption, packing, and release of poly-Arg onto or from metal nanoparticles is one of the current challenges. In this work, we carry out atomistic molecular dynamics simulations to identify the most favorable location, orientation, and conformation of poly-Arg adsorbed onto a silver nanoparticle (AgNP). Herein, we utilize the constant protonation approach to identify the role of protonation of side chain arginine moieties in the adsorption of poly-Arg to AgNP as a function of pH. The strong adsorption of unprotonated poly-Arg30 onto the quasispherical surface of AgNP with an average diameter of 3.9 nm is primarily governed by multiple interactions of side chain guanidinium (Gdm) moieties, which get stacked and align flat onto the surface. The protonation of the arginine side chain enhances the protein–solvent interactions and promotes the weakening of the protein–nanoparticle binding. The formation of multiple H-bonds between the protonated Arg residues and water molecules favors exposing the charged Gdm+ moieties to the solvent. Protonated poly-Arg30 is found to be partially bound to AgNP due to some weak protein–nanoparticle contacts, maintained by binding of the amide oxygen atoms of the peptide bond. These results suggest that reversible acid–base switching between the arginine protonation states is able to drive the rearrangement of the polyarginine coating around AgNPs, which could be important for a rational design of “intelligent” multifunctional core–shell nanosystems.

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Metrics & MoreArticle Recommendations * sı Supporting Information ABSTRACT: "Smart" nanomaterials composed of inorganic nanoparticles (NPs) and synthetic polymers represent an exciting and rapidly growing interdisciplinary area of materials science and engineering. Specifically, the coating of NPs with "smart" polymers capable of responding to applied external stimuli makes them promising for various biomedical applications, including drug delivery, tissue engineering, and medical diagnostics. In this work, we present an atomistic molecular dynamics (MD) simulation study of a silver NP (AgNP) grafted with a single-chain poly(2-(N,Ndimethylamino)ethyl methacrylate) (PDMAEMA). PDMAEMA possesses ionizable side-chain groups −N(CH 3 ) 2 , which can be converted into charged moieties upon solution acidification so that this makes the whole macromolecule to be pH responsive.We examined the pH-responsive adsorption of PDMAEMA onto the AgNP as a function of the polymer chain length (220−660 units) and the degree of protonation α.We found out that, in the neutral state at α = 0, the PDMAEMA chain was collapsed and adsorbed onto the AgNP due to the weak non-covalent interactions through the neutral dimethylamino moieties. Partial protonation of these side groups at α = 0.3−0.7 triggers ionic interactions, which drive the unfolding of the polymer chain toward an aqueous solution due to the electrostatic Coulombic repulsion between the generated charges. In the charged state at α = 1, the complete ionization of PDMAEMA side groups increases its solubility that leads to the substantial decrease in the polymer−NP interactions, followed by desorption of the polymer from the inorganic core. Thus, the gradual changes in the protonation degree of PDMAEMA fine-tune a balance of hydrophilic versus hydrophobic NP−polymer interactions and govern the shape, size, and water-protecting efficiency of the polymeric shell. Finally, our MD simulations demonstrated that such pH-adaptive behavior of PDMAEMA makes it promising for the development of a wide range of new generations of "intelligent" polymer coatings for metal NPs.

The crystalline structure, the perfect face-centered cubic (fcc) atom packing and macroscopic morphological stability of sharp-edged silver nanoparticles of cubic and bipyramidal shapes were compared against quasi-spherical nanoparticles by using classical molecular dynamics (MD) simulations. A series of silver nanocubes (AgNCs) and nanobipyramides (AgNBs) of different sizes varying from 44 up to 1156 atoms were considered. Our MD simulations revealed that starting from the preformed perfect crystalline structures the initial shape was preserved for cubic and bipyramidal nanoparticles composed of more than 256 atoms. Surprisingly, the rapid loss of the cubic-shape morphology and transformation into the non-fcc-structure were found for the smaller AgNCs composed of less than 172 atoms. No such loss of the preformed crystalline structure was noticed for bipyramidal and quasi-spherical nanoparticles. The analysis of the binding energy of the outermost Ag surface atoms suggests that the loss of the perfect cubic shape, rounding and smoothing of sharp edges and corners were driven by the tendency towards the increase in their coordination number.

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