Counter ion induced irreversible denaturation of hen egg white lysozyme upon electrostatic interaction with iron oxide nanoparticles: A predicted model

Nanoparticle protein corona: from structure and function to therapeutic targeting

Nanoparticle (NP)-based therapeutics have ushered in a new era in translational medicine. However, despite the clinical success of NP technology, it is not well-understood how NPs fundamentally change in biological environments. When introduced into physiological fluids, NPs are coated by proteins, forming a protein corona (PC). The PC has the potential to endow NPs with a new identity and alter their bioactivity, stability, and destination. Additionally, the conformation of proteins is sensitive to their physical and chemical surroundings. Therefore, biological factors and protein-NP-interactions can induce changes in the conformation and orientation of proteins in vivo. Since the function of a protein is closely connected to its folded structure, slight differences in the surrounding environment as well as the surface characteristics of the NP materials may cause proteins to lose or gain a function. As a result, this can alter the downstream functionality of the NPs. This review introduces the main biological factors affecting the conformation of proteins associated with the PC. Then, four types of NPs with extensive utility in biomedical applications are described in greater detail, focusing on the conformation and orientation of adsorbed proteins. This is followed by a discussion on the instances in which the conformation of adsorbed proteins can be leveraged for therapeutic purposes, such as controlling protein conformation in assembled matrices in tissue, as well as controlling the PC conformation for modulating immune responses. The review concludes with a perspective on the remaining challenges and unexplored areas at the interface of PC and NP research.

Investigation of Aggregation and Disaggregation of Self-Assembling Nano-Sized Clusters Consisting of Individual Iron Oxide Nanoparticles upon Interaction with HEWL Protein Molecules

In this paper, iron oxide nanoparticles coated with trisodium citrate were obtained. Nanoparticles self-assembling stable clusters were ~10 and 50-80 nm in size, consisting of NPs 3 nm in size. The stability was controlled by using multi-angle dynamic light scattering and the zeta potential, which was -32 ± 2 mV. Clusters from TSC-IONPs can be destroyed when interacting with a hen egg-white lysozyme. After the destruction of the nanoparticles and proteins, aggregates are formed quickly, within 5-10 min. Their sizes depend on the concentration of the lysozyme and nanoparticles and can reach micron sizes. It is shown that individual protein molecules can be isolated from the formed aggregates under shaking. Such aggregation was observed by several methods: multi-angle dynamic light scattering, optical absorption, fluorescence spectroscopy, TEM, and optical microscopy. It is important to note that the concentrations of NPs at which the protein aggregation took place were also toxic to cells. There was a sharp decrease in the survival of mouse fibroblasts (Fe concentration ~75-100 μM), while the ratio of apoptotic to all dead cells increased. Additionally, at low concentrations of NPs, an increase in cell size was observed.

Protein-nanoparticle interactions and a new insight

The study of protein-nanoparticle interactions provides knowledge about the bio-reactivity of nanoparticles, and creates a database of nanoparticles for applications in nanomedicine, nanodiagnosis, and nanotherapy. The problem arises when nanoparticles come in contact with physiological fluids such as plasma or serum, wherein they interact with the proteins (or other biomolecules). This interaction leads to the coating of proteins on the nanoparticle surface, mostly due to the electrostatic interaction, called 'corona'. These proteins are usually partially unfolded. The protein corona can deter nanoparticles from their targeted functionalities, such as drug/DNA delivery at the site and fluorescence tagging of diseased tissues. The protein corona also has many repercussions on cellular intake, inflammation, accumulation, degradation, and clearance of the nanoparticles from the body depending on the exposed part of the proteins. Hence, the protein-nanoparticle interaction and the configuration of the bound-proteins on the nanosurface need thorough investigation and understanding. Several techniques such as DLS and zeta potential measurement, UV-vis spectroscopy, fluorescence spectroscopy, circular dichroism, FTIR, and DSC provide valuable information in the protein-nanoparticle interaction study. Besides, theoretical simulations also provide additional understanding. Despite a lot of research publications, the fundamental question remained unresolved. Can we aim for the application of functional nanoparticles in medicine? A new insight, given by us, in this article assumes a reasonable solution to this crucial question.

Research progress and application opportunities of nanoparticle-protein corona complexes

Nanoparticles (NPs) can be used to design for nanomedicines with different chemical surface properties owing to their size advantages and the capacity of specific delivery to targeted sites in organisms. The discovery of the presence of protein corona (PC) has changed our classical view of NPs, stimulating researchers to investigate the in vivo fate of NPs as they enter biological systems. Both NPs and PC have their specificity but complement each other, so they should be considered as a whole. The formation and characterization of NP-PC complexes provide new insights into the design, functionalization, and application of nanocarriers. Based on progress of recent researches, we reviewed the formation, characterization, and composition of the PC, and introduced those critical factors influencing PC, simultaneously expound the effect of PC on the biological function of NPs. Especially we put forward the opportunities and challenges when NP-PC as a novel nano-drug carrier for targeted applications. Furthermore, we discussed the pros versus cons of the PC, as well as how to make better PC in the future application of NPs.

Promising insights into the kosmotropic effect of magnetic nanoparticles on proteins: The pivotal role of protein corona formation

Increasing concerns about biosafety of nanoparticles (NPs) has raised the need for detailed knowledge of NP interactions with biological molecules especially proteins. Herein, the concentration-dependent effect of magnetic NPs (MNPs) on bovine serum albumin and hen egg white lysozyme was explored. The X-ray diffraction patterns, zeta potential, and dynamic light scattering measurements together with scanning electron microscopy images were employed to characterize MNPs synthesized through coprecipitation method. Then, we studied the behavior of two model proteins with different surface charges and structural properties on interaction with Fe₃ O₄ . A thorough investigation of protein-MNP interaction by the help of intrinsic fluorescence at different experimental conditions revealed that affinity of proteins for MNPs is strongly affected by the similarity of protein and MNP surface charges. MNPs exerted structure-making kosmotropic effect on both proteins under a concentration threshold; however, binding strength was found to determine the extent of stabilizing effect as well as magnitude of the concentration threshold. Circular dichroism spectra showed that proteins with less resistance to conformational deformations are more prone to secondary structure changes upon adsorption on MNPs. By screening thermal aggregation of proteins in the presence of Fe₃ O₄ , it was also found that like chemical stability, thermal stability is influenced to a higher extent in more strongly bound proteins. Overall, this report not only provides an integrated picture of protein-MNP interaction but also sheds light on the molecular mechanism underling this process.

An Engineered Sso7d Variant Enables Efficient Magnetization of Yeast Cells

Magnetization using cheap and minimally toxic materials, such as iron oxide nanoparticles can enable easy separation of cells from culture medium and is relevant to several industrial applications. Here, we show that cell surface expression of a mutant protein that binds iron oxide can enable efficient magnetization of yeast cells. We screened a combinatorial library of mutants derived from the Sso7d protein scaffold to isolate proteins that exhibit preferential binding to iron oxide. One of the isolated mutants, SsoFe2, was chosen for further characterization. Yeast cells expressing SsoFe2 as fusions to a cell wall protein-but not other Sso7d mutants with similar overall protein charge or amino acid composition-preferentially bind iron oxide when present in a solution with high protein concentration and in the presence of 1000-fold excess of competitor yeast cells. Moreover, coexpression of cell surface SsoFe2 enables efficient magnetic capture and separation of yeast cells expressing an enzyme (glucose oxidase) on the cell surface from yeast culture medium, and solutions with high protein concentration or containing other metal oxides. Therefore, SsoFe2-enabled magnetization can enable a range of industrial and biotechnology applications, where easy separation of cells or organelles from complex media is desirable.

Colloids versus solution state adsorption of proteins: Interaction of Myoglobin with supported lipid bilayers

This study examines adsorption of Myoglobin (Mb) in solution and as colloid on supported lipid bilayers of neutral phospholipids and a mixture of neutral+cationic lipids formed on gold coated quartz in a Quartz crystal microbalance (QCM). Results indicate that thin adsorbed films of Mb in solution and as colloids, show atleast 3 steps in the interaction with the bilayers: i) An initial strain of a viscoelastic film ii) Entrained water that moves in and out of the adsorbed film and iii) The coupled load from the bulk liquid which increases the strain of the film. These three components constitute an effective viscoelastic film which is rigidly coupled to the QCM. Grazing incidence XRD (GIXD) shows that the bilayer head group remains nearly undisturbed for Mb solution with pure (neutral) and (neutral+cationic) mixtures, whereas for the colloids there is an increase in head group thickness with neutral and decrease in the case of mixture. Unsaturation in the alkyl tails in the neutral lipid resulting in flexible disordered bilayers and more entrained water in the cationic system results in these changes. The sensitivity of QCM-D, makes it useful to study real-time monitoring of bilayer structural robustness cytotoxicity, drug delivery and lipid self-assembly.

The ambivalent effect of Fe3O4 nanoparticles on the urea-induced unfolding and dilution-based refolding of lysozyme

Due to the numerous biological applications of magnetite (Fe₃O₄) nanoparticles (MNPs), it is essential to identify the influence of these nanoparticles on basic biological processes. Therefore, in this research, the effect of MNPs on the structure and activity of hen egg white lysozyme (HEWL) (EC 3.2.1.1) as a model protein was examined using tryptophan intrinsic fluorescence, UV/Vis, and circular dichroism spectroscopy. Moreover, enzyme activities were analyzed by a turbidometric approach in the presence of MNPs at concentrations providing MNPs/HEWL ratios in the range of 0.04-1.25. As-synthesized MNPS were characterized by Fourier transform infrared spectroscopy, x-ray diffraction, scanning electron microscopy, transmission electron microscopy, vibrating sample magnetometry and the zeta potential of MNPs was measured to be -29 mV. The goal of this work was investigating the ordering or disordering effect of MNPs on protein structure at ratios lower or higher than 0.918 as concentration ratio of threshold (CRT), respectively, in order to answer the question: 'How can the denaturation and refolding of a model protein (HEWL) be affected by MNPs?' As has been reported recently, the protein folding, helicity, and half-life were improved at <CRT to make the protein more ordered and conversely, HEWL was unfolded, and the helicity and half-life were decreased at >CRT to make the protein more disordered upon interaction with MNPs. The disordering effect of urea at >CRT and even at <CRT in the denaturation buffer (urea 6 M) increased and at <CRT the MNPs can provide a significant improvement in the refolding of the unfolded urea treated protein. These observations provide a new perspective on the growing applications of MNPs in biotechnology and biomedicine.

Understanding the curvature effect of silica nanoparticles on lysozyme adsorption orientation and conformation: a mesoscopic coarse-grained simulation study

In nanobiotechnology applications, curvature of nanoparticles has a significant effect on protein activities. In this work, lysozyme adsorption on different-sized silica nanoparticles (SNPs) was simulated at the microsecond timescale by using mesoscopic coarse-grained molecular dynamics simulations. It is found that, with the increase of nanoparticle size, which indicates a decrease of surface curvature, adsorbed lysozyme shows a narrower orientation distribution and a greater conformation change, as the electrostatic attraction dominates lysozyme adsorption, and this trend is more pronounced on larger SNPs. Interestingly, the effect induced by different SNP surface curvatures is not related to the direct contact area between lysozyme and SNPs, but to the interfacial hydration layer above the silica surface, since a smaller curvature can lead to a stronger interfacial hydration and make the distribution of interfacial water molecules more ordered. Besides, at higher ionic strength, lysozyme conformation is less affected by strongly negatively charged SNPs, especially for larger nanoparticles. This work might shed some light on how to prepare protein coronas with higher bioactivities in nanobiotechnology.

Assessing safety and protein interactions of surface-modified iron oxide nanoparticles for potential use in biomedical areas

We have investigated the electrostatic interaction between bare iron oxide nanoparticles (IONPs) or low molecular weight chitosan coated iron oxide nanoparticles (LMWC-IONPs) and hen egg white lysozyme (HEWL) at different pH values using protein-nanoparticle reverse charge parity model. Physicochemical characterization of both IONPs and LMWC-IONPs were carried out using DLS, TEM, FE-SEM, XRD, TGA, XPS and VSM analysis. DLS, TEM and FE-SEM results indicated that both IONPs were monodispersed, with size ranging from 8 to 20nm. The coating of LMWC on IONPs was confirmed using zeta potential, TGA, XRD and XPS measurements. The cytotoxicity of both IONPs and LMWC-IONPs was studied in vitro in A549 human lung alveolar epithelial cells to assess their use in biomedical applications. Furthermore, the interactions between protein-nanoparticles were investigated by UV-visible, fluorescence and circular dichroism spectroscopic techniques. The present study suggests that water soluble LMWC surface modified IONPs are the promising nanomaterials. The safety and biocompatibility of these nanoparticles render them suitable for biomedical applications.

Protein adsorption onto nanoparticles induces conformational changes: Particle size dependency, kinetics, and mechanisms

The use of nanomaterials in bioapplications demands a detailed understanding of protein–nanoparticle interactions. Proteins can undergo conformational changes while adsorbing onto nanoparticles, but studies on the impact of particle size on conformational changes are scarce. We have shown that conformational changes happening upon adsorption of myoglobin and BSA are dependent on the size of the nanoparticle they are adsorbing to. Out of eight initially investigated model proteins, two (BSA and myoglobin) showed conformational changes, and in both cases this conformational change was dependent on the size of the nanoparticle. Nanoparticle sizes ranged from 30 to 1000 nm and, in contrast to previous studies, we attempted to use a continuous progression of sizes in the range found in live viruses, which is an interesting size of nanoparticles for the potential use as drug delivery vehicles. Conformational changes were only visible for particles of 200 nm and bigger. Using an optimized circular dichroism protocol allowed us to follow this conformational change with regard to the nanoparticle size and, thanks to the excellent temporal resolution also in time. We uncovered significant differences between the unfolding kinetics of myoglobin and BSA. In this study, we also evaluated the plausibility of commonly used explanations for the phenomenon of nanoparticle size‐dependent conformational change. Currently proposed mechanisms are mostly based on studies done with relatively small particles, and fall short in explaining the behavior seen in our studies.

Concentration-controlled formation of myoglobin/gold nanosphere aggregates

Gold nanoparticles are being increasingly proposed as biotechnological tools for medical diagnosis and therapy purposes. Their safety for human beings and the environment is therefore becoming an emerging issue, which calls for basic research on the interactions between nanostructured gold particles and biological materials, including physicochemical studies of model systems. In this Article, we focus on the "reaction products" of a widely known nanoparticle type, citrate-capped 30 nm gold nanospheres, with a model protein, horse myoglobin. Protein adsorption and partial denaturation were accompanied by the formation of nanoparticle aggregates with strongly distinct optical spectroscopy properties and shapes, as observed by transmission electron microscopy. We singled out the concentration of myoglobin as the determinant of these differences, and verified on this basis that surface-enhanced Raman scattering (SERS) spectra can only be obtained by aggregates with strong interparticle optical coupling, which are obtained at low protein concentration. The results can be useful both in improving the spectroscopy of biomolecules and in understanding the formation of the protein corona in biomedical applications.

Protein nanoparticle electrostatic interaction: size dependent counterions induced conformational change of hen egg white lysozyme

In our earlier paper (Ghosh et al., 2013), we have shown that (i) the positively charged hen egg white lysozyme (HEWL), dispersed in water, binds electrostatically with the negatively functionalized iron oxide nanoparticles (IONPs), and (ii) the Na(+) counterions, associated with functionalized IONPs, diffuse into bound proteins and irreversibly unfold them. Having this information, we have extended our investigation and report here the effect of the size and the charge of alkaline metal counterions on the conformational modification of HEWL. In order to obtain a negative functional 'shell' on IONPs and the counterions of different size and charge we have functionalized IONPs with different derivatives of citrate, namely, tri-lithium citrate (TLC, Li3C6H5O7), tri-sodium citrate (TSC, Na3C6H5O7), tri-potassium citrate (TKC, K3C6H5O7) and tri-magnesium citrate (TMC, Mg3C12H10O14). The size of counterions varies as Mg(2+)<Li(+)<Na(+)<K(+). After interaction with the functionalized IONPs, the unfolding of HEWL was the maximum in presence of Li(+), and was decreasing with increasing size of counterions. The UV-vis absorption measurements indicated that the unfolding of HEWL was due to modification in the hydrophobic environment around the tryptophan regions. The unfolding of HEWL was associated with the change of folding conformation from the α-helix to the β-sheet. In absence of counterions, ligand-IONPs have no effect on the native conformation of HEWL. An effective use of counterions in order to modify protein conformation (and, the functionality) via protein-nanoparticle electrostatic interaction is a new finding, and be useful for an alternative medical therapy.