Polyhydroxylated [60]fullerene binds specifically to functional recognition sites on a monomeric and a dimeric ubiquitin

Solution NMR methods for structural and thermodynamic investigation of nanoparticle adsorption equilibria

Characterization of dynamic processes occurring at the nanoparticle (NP) surface is crucial for developing new and more efficient NP catalysts and materials. Thus, a vast amount of research has been dedicated to developing techniques to characterize sorption equilibria. Over recent years, solution NMR spectroscopy has emerged as a preferred tool for investigating ligand-NP interactions. Indeed, due to its ability to probe exchange dynamics over a wide range of timescales with atomic resolution, solution NMR can provide structural, kinetic, and thermodynamic information on sorption equilibria involving multiple adsorbed species and intermediate states. In this contribution, we review solution NMR methods for characterizing ligand-NP interactions, and provide examples of practical applications using these methods as standalone techniques. In addition, we illustrate how the integrated analysis of several NMR datasets was employed to elucidate the role played by support-substrate interactions in mediating the phenol hydrogenation reaction catalyzed by ceria-supported Pd nanoparticles.

Ubiquitination of Alzheimer's-related tau protein affects liquid-liquid phase separation in a site- and cofactor-dependent manner

The formation of biomolecular condensates has emerged as a crucial player both in neuronal physiology and neurodegeneration. Phase separation of the Alzheimer's related protein tau into liquid condensates is facilitated by polyanions and is regulated by post-translational modifications. Given the central role of ubiquitination in proteostasis regulation and signaling, we investigated the behavior of monoubiquitinated tau during formation of condensates. We ubiquitinated the lysine-rich, four-repeat domain of tau either unspecifically via enzymatic conjugation or in a position-specific manner by semisynthesis. Ubiquitin conjugation at specific sites weakened multivalent tau/RNA interactions and disfavored tau/heparin condensation. Yet, heterogeneous ubiquitination was tolerated during phase separation and stabilized droplets against aggregation-linked dissolution. Thus, we demonstrated that cofactor chemistry and site of modification affect the mesoscopic and molecular signatures of ubiquitinated tau condensates. Our findings suggest that ubiquitination could influence the physiological states and pathological transformations of tau in cellular condensates.

Examining the Transient Dark State in Protein-Quantum Dot Interaction by Relaxation-Based Solution NMR

We probed the "dark" state involved in the protein-quantum dot (QD) interaction using a relaxation-based solution nuclear magnetic resonance (NMR) approach. We examined the dynamics and exchange kinetics of the ubiquitin-CdTe model system, which undergoes a fast exchange in the transverse relaxation time scale. We applied the recently developed dark-state exchange saturation transfer (DEST), lifetime line broadening (ΔR₂), and exchange-induced chemical shift (δ_ex) solution NMR techniques to obtain a residue-specific binding behavior of the protein on the QD surface. The variation in the estimated ¹⁵N-R₂^bound values clearly shows the dynamic nature of bound Ub. Upon mapping the amino acid residues showing a faster relaxation rate on the electrostatic potential surface of the protein, we have determined that the interaction is preferably electrostatic, and the amino acid residues involved in binding lie on the positively charged surface of the protein. We believe that our experimental approach should provide more in-depth knowledge to engineer new hybrid protein-QD systems in the future.

An organogel library for solution NMR analysis of nanoparticle suspensions in non-aqueous samples

Surface contrast solution NMR methods (scNMR) are emerging as powerful tools to investigate the adsorption of small molecule ligands to the surface of nanoparticles (NP), returning fundamental insight into the kinetics and thermodynamics of sorption, as well as structural information on the adsorbed species. A prerequisite for the acquisition of high quality solution NMR data is the preparation of homogeneous and stable samples that return consistent NMR spectra and allow extensive signal averaging. Unfortunately, this condition does not apply to NMR samples containing NPs that often show a tendency to sediment and accumulate at the bottom of the NMR tube over the course of the experiment. We have recently shown that preparing NMR samples in an agarose gel matrix inhibits sedimentation and allows the characterization of small molecule-NP interactions by scNMR. Unfortunately, as the agarose gel only forms in aqueous solution, this sample preparation method cannot be used to stabilize NP suspensions in a non-aqueous environment. Here, we introduce a library of 48 organogels, based on low molecular-mass organic gelators (LMOGs), to prepare NMR samples of small molecule/NP systems in a wide range of organic solvents. In addition, we present a simple method that takes advantage of ¹H transverse relaxation (¹H-R₂) measurements to screen the library and identify the best gelator to characterize the small molecule-NP interaction of interest in the solvent of choice. We expect the results of this study will enable the preparation of homogeneous and stable samples of NPs in non-aqueous environments, therefore dramatically increasing the applicability of scNMR to the characterization of heterogeneous interactions and to the investigation of the role played by solvent molecules in regulating the kinetics and thermodynamics of sorption.

Alpha-Synuclein-Nanoparticle Interactions: Understanding, Controlling and Exploiting Conformational Plasticity

Alpha-synuclein (αS) is an extensively studied protein due to its involvement in a group of neurodegenerative disorders, including Parkinson's disease, and its documented ability to undergo aberrant self-aggregation resulting in the formation of amyloid-like fibrils. In dilute solution, the protein is intrinsically disordered but can adopt multiple alternative conformations under given conditions, such as upon adsorption to nanoscale surfaces. The study of αS-nanoparticle interactions allows us to better understand the behavior of the protein and provides the basis for developing systems capable of mitigating the formation of toxic aggregates as well as for designing hybrid nanomaterials with novel functionalities for applications in various research areas. In this review, we summarize current progress on αS-nanoparticle interactions with an emphasis on the conformational plasticity of the biomolecule.

Solution NMR insights into dynamic supramolecular assemblies of disordered amyloidogenic proteins

The extraordinary flexibility and structural heterogeneity of intrinsically disordered proteins (IDP) make them functionally versatile molecules. We have now begun to better understand their fundamental role in biology, however many aspects of their behaviour remain difficult to grasp experimentally. This is especially true for the intermolecular interactions which lead to the formation of transient or highly dynamic supramolecular self-assemblies, such as oligomers, aggregation intermediates and biomolecular condensates. Both the emerging functions and pathogenicity of these structures have stimulated great efforts to develop methodologies capable of providing useful insights. Significant progress in solution NMR spectroscopy has made this technique one of the most powerful to describe structural and dynamic features of IDPs within such assemblies at atomic resolution. Here, we review the most recent works that have illuminated key aspects of IDP assemblies and contributed significant advancements towards our understanding of the complex conformational landscape of prototypical disease-associated proteins. We also include a primer on some of the fundamental and innovative NMR methods being used in the discussed studies.

Protein Interactions with Nanoparticle Surfaces: Highlighting Solution NMR Techniques

In the last decade, nanoparticles (NPs) have become a key tool in medicine and biotechnology as drug delivery systems, biosensors and diagnostic devices. The composition and surface chemistry of NPs vary based on the materials used: typically organic polymers, inorganic materials, or lipids. Nanoparticle classes can be further divided into sub-categories depending on the surface modification and functionalization. These surface properties matter when NPs are introduced into a physiological environment, as they will influence how nucleic acids, lipids, and proteins will interact with the NP surface. While small-molecule interactions are easily probed using NMR spectroscopy, studying protein-NP interactions using NMR introduces several challenges. For example, globular proteins may have a perturbed conformation when attached to a foreign surface, and the size of NP-protein conjugates can lead to excessive line broadening. Many of these challenges have been addressed, and NMR spectroscopy is becoming a mature technique for in situ analysis of NP binding behavior. It is therefore not surprising that NMR has been applied to NP systems and has been used to study biomolecules on NP surfaces. Important considerations include corona composition, protein behavior, and ligand architecture. These features are difficult to resolve using classical surface and material characterization strategies, and NMR provides a complementary avenue of characterization. In this review, we examine how solution NMR can be combined with other analytical techniques to investigate protein behavior on NP surfaces.

Stable and Biocompatible Monodispersion of C60 in Water by Peptides

The lack of solubility in water and the formation of aggregates hamper many opportunities for technological exploitation of C₆₀. Here, different peptides were designed and synthesized with the aim of monomolecular dispersion of C₆₀ in water. Phenylalanines were used as recognizing moieties, able to interact with C₆₀ through π-π stacking, while a varying number of glycines were used as spacers, to connect the two terminal phenylalanines. The best performance in the dispersion of C₆₀ was obtained with the FGGGF peptidic nanotweezer at a pH of 12. A full characterization of this adduct was carried out. The peptides disperse C₆₀ in water with high efficiency, and the solutions are stable for months both in pure water and in physiological environments. NMR measurements demonstrated the ability of the peptides to interact with C₆₀. AFM measurements showed that C₆₀ is monodispersed. Electrospray ionization mass spectrometry determined a stoichiometry of C₆₀@(FGGGF)₄. Molecular dynamics simulations showed that the peptides assemble around the C₆₀ cage, like a candy in its paper wrapper, creating a supramolecular host able to accept C₆₀ in the cavity. The peptide-wrapped C₆₀ is fully biocompatible and the C₆₀ "dark toxicity" is eliminated. C₆₀@(FGGGF)₄ shows visible light-induced reactive oxygen species (ROS) generation at physiological saline concentrations and reduction of the HeLa cell viability in response to visible light irradiation.

Proteins as supramolecular hosts for C60: a true solution of C60 in water

Hybrid systems have great potential for a wide range of applications in chemistry, physics and materials science. Conjugation of a biosystem to a molecular material can tune the properties of the components or give rise to new properties. As a workhorse, here we take a C60@lysozyme hybrid. We show that lysozyme recognizes and disperses fullerene in water. AFM, cryo-TEM and high resolution X-ray powder diffraction show that the C60 dispersion is monomolecular. The adduct is biocompatible, stable in physiological and technologically-relevant environments, and easy to store. Hybridization with lysozyme preserves the electrochemical properties of C60. EPR spin-trapping experiments show that the C60@lysozyme hybrid produces ROS following both type I and type II mechanisms. Due to the shielding effect of proteins, the adduct generates significant amounts of 1O2 also in aqueous solution. In the case of type I mechanism, the protein residues provide electrons and the hybrid does not require addition of external electron donors. The preparation process and the properties of C60@lysozyme are general and can be expected to be similar to other C60@protein systems. It is envisaged that the properties of the C60@protein hybrids will pave the way for a host of applications in nanomedicine, nanotechnology, and photocatalysis.

Specific Interaction Sites Determine Differential Adsorption of Protein Structural Isomers on Nanoparticle Surfaces

In biological systems, nanoparticles (NPs) elicit bioactivity upon interaction with proteins. As a result of post-translational modification, proteins occur in a variety of alternative covalent forms, including structural isomers, which present unique molecular surfaces. We aimed at a detailed description of the recognition of protein isomeric species by NP surfaces. The transient adsorption of isomeric ubiquitin (Ub) dimers by NPs was investigated by solution NMR spectroscopy. Lys63- and Lys48-linked Ub₂ were adsorbed by large anionic NPs with different affinities, whereas the binding strength was similar in the cases of smaller particles. After the incorporation of paramagnetic tags into NPs, the observed site-resolved paramagnetic footprints provided a high-resolution map of the different protein surfaces binding to NPs. The approach described could be extended to further protein isoforms and more specialized NP systems to allow better control of the interactions between NPs and protein targets.

Luminescent and paramagnetic properties of nanoparticles shed light on their interactions with proteins

Nanoparticles have been recognized as promising tools for targeted drug-delivery and protein therapeutics. However, the mechanisms of protein-nanoparticle interaction and the dynamics underlying the binding process are poorly understood. Here, we present a general methodology for the characterization of protein-nanoparticle interaction on a molecular level. To this end we combined biophysical techniques including nuclear magnetic resonance (NMR), circular dichroism (CD), resonance energy transfer (RET) and surface plasmon resonance (SPR). Particularly, we analyzed molecular mechanisms and dynamics of the interaction of CaF2 nanoparticles with the prototypical calcium sensor calmodulin (CaM). We observed the transient formation of an intermediate encounter complex involving the structural region linking the two domains. Specific interaction of CaM with CaF2 NPs is driven by the N-terminal EF-hands, which seem to recognize Ca2+ on the surface of the nanoparticle. We conclude that CaF2 NP-CaM interaction is fully compatible with potential applications in nanomedicine. Overall, the methods presented in this work can be extended to other systems and may be useful to quantitatively characterize structural and dynamic features of protein-NP interactions with important implications for nanomedicine and nano-biotechnology.

Fullerene derivatives act as inhibitors of leukocyte common antigen based on molecular dynamics simulations

Fullerene-based molecules are being studied as potential inhibitors of protein tyrosine phosphatases due to their unique properties and low toxicity. However, the underlying molecular mechanism remains elusive. In this study, molecular dynamics (MD) simulations in conjunction with molecular docking calculations were utilized to investigate the binding effects of C₆₀, C₆₀(NH₂)₃₀, and C₆₀(OH)₃₀ on the enzymatic activity of CD45 (a receptor-like protein tyrosine phosphatase). Our results show that all the investigated molecules can be docked into the region between D1 and D2 domains of CD45, and stabilize the protein structure. The average number of residues that directly interact with the C₆₀(NH₂)₃₀ is two more than that of C₆₀(OH)₃₀, F819 and F820 (located in the loop connects α3 and β12), resulting in different effects of C₆₀(NH₂)₃₀ and C₆₀(OH)₃₀ on protein activity. Detailed MD simulation analyses show that transformation of the interaction network caused by C₆₀(NH₂)₃₀ is completely different from that of the control simulation due to the misfolding of α3. Furthermore, the movement of D1 active pocket and KNRY motif are most severely impaired by docking with C₆₀(NH₂)₃₀. Our simulation results illustrate that fullerene derivatives modified with amino groups exhibit conspicuous tumor inhibition to protein tyrosine phosphatases, and can act as effective inhibitors. Our results give insight into the inhibitory effects of fullerene-based molecules on protein tyrosine phosphatases and providing a theoretical basis for the design of effective inhibitors.

Fullerene-based molecules are being studied as potential inhibitors of protein tyrosine phosphatases due to their unique properties and low toxicity.

Structural Basis for the Inhibitory Effects of Ubistatins in the Ubiquitin-Proteasome Pathway

The discovery of ubistatins, small molecules that impair proteasomal degradation of proteins by directly binding to polyubiquitin, makes ubiquitin itself a potential therapeutic target. Although ubistatins have the potential for drug development and clinical applications, the lack of structural details of ubiquitin-ubistatin interactions has impeded their development. Here, we characterized a panel of new ubistatin derivatives using functional and binding assays. The structures of ubiquitin complexes with ubistatin B and hemi-ubistatin revealed direct interactions with ubiquitin's hydrophobic surface patch and the basic/polar residues surrounding it. Ubistatin B binds ubiquitin and diubiquitin tighter than a high-affinity ubiquitin receptor and shows strong preference for K48 linkages over K11 and K63. Furthermore, ubistatin B shields ubiquitin conjugates from disassembly by a range of deubiquitinases and by the 26S proteasome. Finally, ubistatin B penetrates cancer cells and alters the cellular ubiquitin landscape. These findings highlight versatile properties of ubistatins and have implications for their future development and use in targeting ubiquitin-signaling pathways.

Hydrophobic Patch of Ubiquitin is Important for its Optimal Activation by Ubiquitin Activating Enzyme E1

Protein ubiquitination plays a role in essentially every process in eukaryotic cells. The attachment of ubiquitin (Ub) or Ub-like (UBL) proteins to target proteins is achieved by parallel but distinct cascades of enzymatic reactions involving three enzymes: E1, E2, and E3. The E1 enzyme functions at the apex of this pathway and plays a critical role in activating the C-terminus of ubiquitin or UBL, which is an essential step that triggers subsequent downstream transfer to their cognate E2s resulting in the fidelity of the Ub/UBL conjugation machinery. Despite the central role of the E1 enzyme in protein modification, a quantitative method to measure Ub/UBL activation by E1 is lacking. Here, we present a mass spectrometry-based assay to accurately measure the activation of Ub/UBL by E1 independent of the E2/E3 enzymes. Our method does not require radiolabeling of any components and therefore can be used in any biochemical laboratory having access to a mass spectrometer. This method allowed us to dissect the concerted process of E1-E2-catalyzed Ub conjugation in order to separately characterize the process of Ub activation and how it is affected by select mutations and other factors. We found that the hydrophobic patch of Ub is important for the optimal activation of Ub by E1. We further show that the blockers of the Ub-proteasome system such as ubistatin and fullerenol inhibit Ub activation by E1. Interestingly, our data indicate that the phosphorylation of Ub at the S65 position augments its activation by the E1 enzyme.

Mechanistic Insight into Nanoparticle Surface Adsorption by Solution NMR Spectroscopy in an Aqueous Gel

Engineering nanoparticle (NP) functions at the molecular level requires a detailed understanding of the dynamic processes occurring at the NP surface. Herein we show that a combination of dark-state exchange saturation transfer (DEST) and relaxation dispersion (RD) NMR experiments on gel-stabilized NP samples enables the accurate determination of the kinetics and thermodynamics of adsorption. We used the former approach to describe the interaction of cholic acid (CA) and phenol (PhOH) with ceria NPs with a diameter of approximately 200 nm. Whereas CA formed weak interactions with the NPs, PhOH was tightly bound to the NP surface. Interestingly, we found that the adsorption of PhOH proceeds via an intermediate, weakly bound state in which the small molecule has residual degrees of rotational diffusion. We believe the use of aqueous gels for stabilizing NP samples will increase the applicability of solution NMR methods to the characterization of nanomaterials.

Molecular Effects of Concentrated Solutes on Protein Hydration, Dynamics, and Electrostatics

Most studies of protein structure and function are performed in dilute conditions, but proteins typically experience high solute concentrations in their physiological scenarios and biotechnological applications. High solute concentrations have well-known effects on coarse protein traits like stability, diffusion, and shape, but likely also perturb other traits through finer effects pertinent at the residue and atomic levels. Here, NMR and molecular dynamics investigations on ubiquitin disclose variable interactions with concentrated solutes that lead to localized perturbations of the protein's surface, hydration, electrostatics, and dynamics, all dependent on solute size and chemical properties. Most strikingly, small polar uncharged molecules are sticky on the protein surface, whereas charged small molecules are not, but the latter still perturb the internal protein electrostatics as they diffuse nearby. Meanwhile, interactions with macromolecular crowders are favored mainly through hydrophobic, but not through polar, surface patches. All the tested small solutes strongly slow down water exchange at the protein surface, whereas macromolecular crowders do not exert such strong perturbation. Finally, molecular dynamics simulations predict that unspecific interactions slow down microsecond- to millisecond-timescale protein dynamics despite having only mild effects on pico- to nanosecond fluctuations as corroborated by NMR. We discuss our results in the light of recent advances in understanding proteins inside living cells, focusing on the physical chemistry of quinary structure and cellular organization, and we reinforce the idea that proteins should be studied in native-like media to achieve a faithful description of their function.

Characterization of fullerenol-protein interactions and an extended investigation on cytotoxicity

Fullerenols, known as polyhydroxylated derivatives of fullerene, have attracted great attention due to their distinctive material properties and potential applications in biology and medicine. As a step toward the elucidation of basic behavior in biological systems, a variety of spectroscopic measurements as well as isothermal titration calorimetry (ITC) were applied to study the interaction between fullerenol (C₆₀(OH)₄₄) and serum proteins (bovine serum albumin (BSA) and γ-globulins). The results of fluorescence spectra indicated that the intrinsic fluorescence of proteins could be effectively quenched by the dynamic mechanism. The affinity values of both proteins bound to fullerenol were of the same order of magnitude. Meanwhile, ITC results showed that the interaction between fullerenol and BSA was enthalpy favorable, while the interaction with γ-globulins was enthalpy unfavorable. Furthermore, fullerenol had little influence on the secondary structure of both proteins. Additional cytotoxicity tests showed that the presence of proteins attenuated the toxic effect of fullerenol on human normal gastric epithelial cell line (GES-1). Thus, the interaction between fullerenol and proteins is indispensable to evaluate the biosafety of fullerenol, which may in turn promotes the development of its biological applications.

Which fullerenols are water soluble? Systematic atomistic investigation

Insights to guide a choice of most soluble fullerenols.

Electrostatic Interactions and Protein Competition Reveal a Dynamic Surface in Gold Nanoparticle-Protein Adsorption

Gold nanoparticle- (AuNP-) protein conjugates are potentially useful in a broad array of diagnostic and therapeutic applications, but the physical basis of the simultaneous adsorption of multiple proteins onto AuNP surfaces remains poorly understood. Here, we investigate the contribution of electrostatic interactions to protein-AuNP binding by studying the pH-dependent binding behavior of two proteins, GB3 and ubiquitin. For both proteins, binding to 15-nm citrate-coated AuNPs closely tracks with the predicted net charge using standard pK_a values, and a dramatic reduction in binding is observed when lysine residues are chemically methylated. This suggests that clusters of basic residues are involved in binding, and using this hypothesis, we model the pK_a shifts induced by AuNP binding. Then, we employ a novel NMR-based approach to monitor the binding competition between GB3 and ubiquitin in situ at different pH values. In light of our model, the NMR measurements reveal that the net charge, binding association constant, and size of each protein play distinct roles at different stages of protein adsorption. When citrate-coated AuNPs and proteins first interact, net charge appears to dominate. However, as citrate molecules are displaced by protein, the surface chemistry changes, and the energetics of binding becomes far more complex. In this case, we observed that GB3 is able to displace ubiquitin at intermediate time scales, even though it has a lower net charge. The thermodynamic model for binding developed here could be the first step toward predicting the binding behavior in biological fluids, such as blood plasma.

Aqueous solubilization of C60 fullerene by natural protein surfactants, latherin and ranaspumin-2

C60 fullerene is not soluble in water and dispersion usually requires organic solvents, sonication or vigorous mechanical mixing. However, we show here that mixing of pristine C60 in water with natural surfactant proteins latherin and ranaspumin-2 (Rsn-2) at low concentrations yields stable aqueous dispersions with spectroscopic properties similar to those previously obtained by more vigorous methods. Particle sizes are significantly smaller than those achieved by mechanical dispersion alone, and concentrations are compatible with clusters approximating 1:1 protein:C60 stoichiometry. These proteins can also be adsorbed onto more intractable carbon nanotubes. This promises to be a convenient way to interface a range of hydrophobic nanoparticles and related materials with biological macromolecules, with potential to exploit the versatility of recombinant protein engineering in the development of nano-bio interface devices. It also has potential consequences for toxicological aspects of these and similar nanoparticles.

Global Dynamics and Exchange Kinetics of a Protein on the Surface of Nanoparticles Revealed by Relaxation-Based Solution NMR Spectroscopy

The global motions and exchange kinetics of a model protein, ubiquitin, bound to the surface of negatively charged lipid-based nanoparticles (liposomes) are derived from combined analysis of exchange lifetime broadening arising from binding to nanoparticles of differing size. The relative contributions of residence time and rotational tumbling to the total effective correlation time of the bound protein are modulated by nanoparticle size, thereby permitting the various motional and exchange parameters to be determined. The residence time of ubiquitin bound to the surface of both large and small unilamellar liposomes is ∼20 μs. Bound ubiquitin undergoes internal rotation about an axis approximately perpendicular to the lipid surface on a low microsecond time scale (∼2 μs), while simultaneously wobbling in a cone of semiangle 30-55° centered about the internal rotation axis on the nanosecond time scale. The binding interface of ubiquitin with liposomes is mapped by intermolecular paramagnetic relaxation enhancement using Gd(3+)-tagged vesicles, to a predominantly positively charged surface orthogonal to the internal rotation axis.

Paramagnetic Nanoparticles Leave Their Mark on Nuclear Spins of Transiently Adsorbed Proteins

The successful application of nanomaterials in biosciences necessitates an in-depth understanding of how they interface with biomolecules. Transient associations of proteins with nanoparticles (NPs) are accessible by solution NMR spectroscopy, albeit with some limitations. The incorporation of paramagnetic centers into NPs offers new opportunities to explore bio-nano interfaces. We propose NMR paramagnetic relaxation enhancement as a new tool to detect NP-binding surfaces on proteins with increased sensitivity, also extending the applicability of NMR investigations to heterogeneous biomolecular mixtures. The adsorption of ubiquitin on gadolinium-doped fluoride-based NPs produced residue-specific NMR line-broadening effects mapping to a contiguous area on the surface of the protein. Importantly, an identical paramagnetic fingerprint was observed in the presence of a competing protein-protein association equilibrium, exemplifying possible interactions taking place in crowded biological media. The interaction was further characterized using isothermal titration calorimetry and upconversion emission measurements. The data indicate that the used fluoride-based NPs are not biologically inert but rather are capable of biomolecular recognition.

Patent Highlights June–July 2015

A snapshot of noteworthy recent developments in the patent literature of relevance to pharmaceutical and medical research and development.

The study of transient protein-nanoparticle interactions by solution NMR spectroscopy

The rapid development of novel nanoscale materials for applications in biomedicine urges an improved characterization of the nanobio interfaces. Nanoparticles exhibit unique structures and properties, often different from the corresponding bulk materials, and the nature of their interactions with biological systems remains poorly characterized. Solution NMR spectroscopy is a mature technique for the investigation of biomolecular structure, dynamics, and intermolecular associations, however its use in protein-nanoparticle interaction studies remains scarce and highly challenging, particularly due to unfavorable hydrodynamic properties of most nanoscale assemblies. Nonetheless, recent efforts demonstrated that a number of NMR observables, such as chemical shifts, signal intensities, amide exchange rates and relaxation parameters, together with newly designed saturation transfer experiments, could be successfully employed to characterize the orientation, structure and dynamics of proteins adsorbed onto nanoparticle surfaces. This review provides the first survey and critical assessment of the contributions from solution NMR spectroscopy to the study of transient interactions between proteins and both inorganic (gold, silver, and silica) and organic (polymer, carbon and lipid based) nanoparticles. This article is part of a Special Issue entitled: Physiological Enzymology and Protein Functions.