Using hydrogen-deuterium exchange to monitor protein structure in the presence of gold nanoparticles

Exploring Residue-Level Interactions between the Biofilm-Driving R2ab Protein and Polystyrene Nanoparticles

In biological systems, proteins can bind to nanoparticles to form a "corona" of adsorbed molecules. The nanoparticle corona is of significant interest because it impacts an organism's response to a nanomaterial. Understanding the corona requires knowledge of protein structure, orientation, and dynamics at the surface. A residue-level mapping of protein behavior on nanoparticle surfaces is needed, but this mapping is difficult to obtain with traditional approaches. Here, we have investigated the interaction between R2ab and polystyrene nanoparticles (PSNPs) at the level of individual residues. R2ab is a bacterial surface protein from Staphylococcus epidermidis and is known to interact strongly with polystyrene, leading to biofilm formation. We have used mass spectrometry after lysine methylation and hydrogen-deuterium exchange (HDX) NMR spectroscopy to understand how the R2ab protein interacts with PSNPs of different sizes. Lysine methylation experiments reveal subtle but statistically significant changes in methylation patterns in the presence of PSNPs, indicating altered protein surface accessibility. HDX rates become slower overall in the presence of PSNPs. However, some regions of the R2ab protein exhibit faster than average exchange rates in the presence of PSNPs, while others are slower than the average behavior, suggesting conformational changes upon binding. HDX rates and methylation ratios support a recently proposed "adsorbotope" model for PSNPs, wherein adsorbed proteins consist of unfolded anchor points interspersed with partially structured regions. Our data also highlight the challenges of characterizing complex protein-nanoparticle interactions using these techniques, such as fast exchange rates. While providing insights into how R2ab adsorbs onto PSNP surfaces, this research emphasizes the need for advanced methods to comprehend residue-level interactions in the nanoparticle corona.

Exploring the Residue-Level Interactions between the R2ab Protein and Polystyrene Nanoparticles

In biological systems, proteins can bind to nanoparticles to form a "corona" of adsorbed molecules. The nanoparticle corona is of high interest because it impacts the organism's response to the nanomaterial. Understanding the corona requires knowledge of protein structure, orientation, and dynamics at the surface. Ultimately, a residue-level mapping of protein behavior on nanoparticle surfaces is needed, but this mapping is difficult to obtain with traditional approaches. Here, we have investigated the interaction between R2ab and polystyrene nanoparticles (PSNPs) at the level of individual residues. R2ab is a bacterial surface protein from Staphylococcus epidermidis and is known to interact strongly with polystyrene, leading to biofilm formation. We have used mass spectrometry after lysine methylation and hydrogen-deuterium exchange (HDX) NMR spectroscopy to understand how the R2ab protein interacts with PSNPs of different sizes. Through lysine methylation, we observe subtle but statistically significant changes in methylation patterns in the presence of PSNPs, indicating altered protein surface accessibility. HDX measurements reveal that certain regions of the R2ab protein undergo faster exchange rates in the presence of PSNPs, suggesting conformational changes upon binding. Both results support a recently proposed "adsorbotope" model, wherein adsorbed proteins consist of unfolded anchor points interspersed with regions of partial structure. Our data also highlight the challenges of characterizing complex protein-nanoparticle interactions using these techniques, such as fast exchange rates. While providing insights into how proteins respond to nanoparticle surfaces, this research emphasizes the need for advanced methods to comprehend these intricate interactions fully at the residue level.

Gold Nanoparticles Are an Immobilization Platform for Active and Stable Acetylcholinesterase: Demonstration of a General Surface Protein Functionalization Strategy

Immobilizing enzymes onto abiological surfaces is a key step for developing protein-based technologies that can be useful for applications such as biosensors and biofuel cells. A central impediment for the advancement of this effort is a lack of generalizable strategies for functionalizing surfaces with proteins in ways that prevent unfolding, aggregation, and uncontrolled binding, requiring surface chemistries to be developed for each surface-enzyme pair of interest. In this work, we demonstrate a significant advancement toward addressing this problem using a gold nanoparticle (AuNP) as an initial scaffold for the chemical bonding of the enzyme acetylcholinesterase (AChE), forming the conjugate AuNP-AChE. This can then be placed onto chemically and structurally distinct surfaces (e.g., metals, semiconductors, plastics, etc.), thereby bypassing the need to develop surface functionalization strategies for every substrate or condition of interest. Carbodiimide crosslinker chemistry was used to bind surface lysine residues in AChE to AuNPs functionalized with ligands containing carboxylic acid tails. Using amino acid analysis, we found that on average, 3.3 ± 0.1 AChE proteins were bound per 5.22 ± 1.25 nm AuNP. We used circular dichroism spectroscopy to measure the structure of the bound protein and determined that it remained essentially unchanged after binding. Finally, we performed Michaelis-Menten kinetics to determine that the enzyme retained 18.2 ± 2.0% of its activity and maintained that activity over a period of at least three weeks after conjugation to AuNPs. We hypothesize that structural changes to the peripheral active site of AChE are responsible for the differences in activity of bound AChE and unbound AChE. This work is a proof-of-concept demonstration of a generalizable method for placing proteins onto chemically and structurally diverse substrates and materials without the need for surface functionalization strategies.

Scratching the Surface of the Protein Corona: Challenging Measurements and Controversies

This Review aims to provide a systematic analysis of the literature regarding ongoing debates in protein corona research. Our goal is to portray the current understanding of two fundamental and debated characteristics of the protein corona, namely, the formation of mono- or multilayers of proteins and their binding (ir)reversibility. The statistical analysis we perform reveals that these characterisitics are strongly correlated to some physicochemical factors of the NP-protein system (particle size, bulk material, protein type), whereas the technique of investigation or the type of measurement (in situ or ex situ) do not impact the results, unlike commonly assumed. Regarding the binding reversibility, the experimental design (either dilution or competition experiments) is also shown to be a key factor, probably due to nontrivial protein binding mechanisms, which could explain the paradoxical phenomena reported in the literature. Overall, we suggest that to truly predict and control the protein corona, future efforts should be directed toward the mechanistic aspects of protein adsorption.

Designing Functional Bionanoconstructs for Effective In Vivo Targeting

The progress achieved over the last three decades in the field of bioconjugation has enabled the preparation of sophisticated nanomaterial–biomolecule conjugates, referred to herein as bionanoconstructs, for a multitude of applications including biosensing, diagnostics, and therapeutics. However, the development of bionanoconstructs for the active targeting of cells and cellular compartments, both in vitro and in vivo, is challenged by the lack of understanding of the mechanisms governing nanoscale recognition. In this review, we highlight fundamental obstacles in designing a successful bionanoconstruct, considering findings in the field of bionanointeractions. We argue that the biological recognition of bionanoconstructs is modulated not only by their molecular composition but also by the collective architecture presented upon their surface, and we discuss fundamental aspects of this surface architecture that are central to successful recognition, such as the mode of biomolecule conjugation and nanomaterial passivation. We also emphasize the need for thorough characterization of engineered bionanoconstructs and highlight the significance of population heterogeneity, which too presents a significant challenge in the interpretation of in vitro and in vivo results. Consideration of such issues together will better define the arena in which bioconjugation, in the future, will deliver functional and clinically relevant bionanoconstructs.

Biomolecular interactions of ultrasmall metallic nanoparticles and nanoclusters

The use of nanoparticles (NPs) in biomedicine has made a gradual transition from proof-of-concept to clinical applications, with several NP types meeting regulatory approval or undergoing clinical trials. A new type of metallic nanostructures called ultrasmall nanoparticles (usNPs) and nanoclusters (NCs), while retaining essential properties of the larger (classical) NPs, have features common to bioactive proteins. This combination expands the potential use of usNPs and NCs to areas of diagnosis and therapy traditionally reserved for small-molecule medicine. Their distinctive physicochemical properties can lead to unique in vivo behaviors, including improved renal clearance and tumor distribution. Both the beneficial and potentially deleterious outcomes (cytotoxicity, inflammation) can, in principle, be controlled through a judicious choice of the nanocore shape and size, as well as the chemical ligands attached to the surface. At present, the ability to control the behavior of usNPs is limited, partly because advances are still needed in nanoengineering and chemical synthesis to manufacture and characterize ultrasmall nanostructures and partly because our understanding of their interactions in biological environments is incomplete. This review addresses the second limitation. We review experimental and computational methods currently available to understand molecular mechanisms, with particular attention to usNP-protein complexation, and highlight areas where further progress is needed. We discuss approaches that we find most promising to provide relevant molecular-level insight for designing usNPs with specific behaviors and pave the way to translational applications.

Using NMR Spectroscopy To Measure Protein Binding Capacity on Gold Nanoparticles

A simple one-dimensional 1H NMR experiment that quantifies protein bound to gold nanoparticles has been developed for upper-division biochemistry and physical chemistry students. This laboratory experiment teaches the basics of NMR techniques, which is a highly effective tool in protein studies and supports students to understand the concepts of NMR spectroscopy and nanoparticle-protein interactions. Understanding the interactions of gold nanoparticles (AuNPs) with biological macromolecules is becoming increasingly important as interest in the clinical use of nanoparticles has been on the rise. Applications in drug delivery, biosensing, diagnostics, and enhanced imaging are all tangible possibilities with a better understanding of AuNP-protein interactions. The ability to use AuNPs as biosensors for drug delivery methods in cellular uptake is dependent on the amount of protein that is able to bind to the surface of the nanoparticle. This laboratory experiment solidifies concepts such as quantitative NMR spectroscopy while reinforcing precision laboratory titrations. Students learn how 1H proton NMR spectra can be used to measure free protein in solution and protein bound to AuNPs. A simple formula is used to determine the binding capacity of the nanoparticle. This analysis helps students to understand the impact of nanoparticle-protein interactions, and it allows them to conceptualize macromolecular binding using NMR spectroscopy.

Influence of Structured Water Layers on Protein Adsorption Process: A Case Study of Cytochrome c and Carbon Nanotube Interactions and Its Implications

Cytochrome c, an essential protein of the electron transport chain, is known to be capable of amplifying the toxicity of carbon nanomaterials via free-radical generation. To understand their interaction, as well as the more general protein-nanoparticle interaction at molecular levels, we investigate the adsorptions between cytochrome c and carbon nanotubes (CNTs) in dynamic and thermodynamic ways using molecular dynamics simulations. The results reveal a well-defined three-phase process separated by two transition points: the diffusion phase where the protein diffuses in the water box, the lockdown phase I where the protein inserts into the surface-bound water layers and rearranges its conformation to fit to the surface of the CNT, and the lockdown phase II where cytochrome c repels the water molecules standing in its way to the surface of CNT and reaches stable adsorption states. The structured water layers affect the movement of atoms by electrostatic forces. In lockdown phase I, the conformation adjustment of the protein dominates the adsorption process. The most thermally favorable adsorption conformation is determined. It shows that except for the deformation of short β sheets and some portions of α helixes, most of the secondary structures of cytochrome c remain unchanged, implying that most of the functions of cytochrome c are preserved. During these processes, the energy contributions of the hydrophilic residues of cytochrome c are much larger than those of hydrophobic residues. Interestingly, the structured water layers at the CNT surface allow more hydrophilic residues such as Lys to get into close contact with the CNT, which plays a significant role during the anchoring process of adsorption. Our results demonstrate that the heme group is in close contact with the CNT in some of the adsorbed states, which hence provides a way for electron transfer from cytochrome c to the CNT surface.

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.

Protein corona formed on silver nanoparticles in blood plasma is highly selective and resistant to physicochemical changes of the solution

Nanoparticles (NPs) in contact with protein-containing media such as biological fluids rapidly acquire a surface layer of proteins, known as the protein corona. The protein composition and structural properties of the protein corona are crucial for NP interactions with living cells. Although much has been learned about the protein corona phenomenon, further elucidation could benefit from extensive quantitative proteomics analysis. Herein we report a comprehensive quantitative characterization (>350 proteins) of the corona that formed on 60 nm silver NPs via interaction with human blood plasma, as a function of pH and temperature. By varying the pH and temperature one can access different conformational spaces and charge localizations of the plasma proteins, which in turn provide knowledge pertinent to how the proteome corresponds to binding affinity. Thirty-eight percent of the quantified proteins bind at all temperatures, 47% at all pH values, and of these most persistent proteins, approximately 60% do not significantly change in abundance within the protein corona. Evaluation of 544 protein properties (present in the Kyoto databank) suggests that binding of these proteins to NPs is determined by the extent of hydrophobicity, β-sheet propensity, α-helical structure (and turns), and amino acid composition. Protein binding is promoted by a larger amount of β-sheets, higher hydrophobicity, and a smaller amount of α-helices. Our work enhances researchers' knowledge of a long-standing, vexing aspect of the nano-bio interface.

Allosteric inhibition of α-thrombin enzymatic activity with ultrasmall gold nanoparticles

The catalytic activity of enzymes can be regulated by interactions with synthetic nanoparticles (NPs) in a number of ways. To date, however, the potential use of NPs as allosteric effectors has not been investigated in detail. Importantly, targeting allosteric (distal) sites on the enzyme surface could afford unique ways to modulate the activity, allowing for either enzyme activation, partial or full inhibition. Using p-mercaptobenzoic acid-coated ultrasmall gold NPs (AuMBA) and human α-thrombin as a model system, here we experimentally tested the hypothesis that enzyme activity could be regulated through ultrasmall NP interactions at allosteric sites. We show that AuMBA interacted selectively and reversibly around two positively charged regions of the thrombin surface (exosites 1 and 2) and away from the active site. NP complexation at the exosites transmitted long-range structural changes over to the active site, altering both substrate binding affinity and catalysis. Significantly, thrombin activity was partially reduced - but not completely inhibited - by interactions with AuMBA. These findings indicate that interactions of proteins with ultrasmall NPs may mimic a typical biomolecular complexation event, and suggest the prospect of using ultrasmall particles as synthetic receptors to allosterically regulate protein function.

Interaction between cyanine dye IR-783 and polystyrene nanoparticles in solution

The interactions between small molecule drugs or dyes and nanoparticles are important to the use of nanoparticles in medicine. Noncovalent adsorption of dyes on nanoparticle surfaces is also important to the development of nanoparticle dual-use imaging contrast agents. In this work, solution-state NMR is used to examine the noncovalent interaction between a near-infrared cyanine dye and the surface of polystyrene nanoparticles in solution. Using 1D proton NMR, we can approximate the number of dye molecules that associate with each nanoparticle for different sized nanoparticles. Saturation-Transfer Difference NMR was also used to show that protons near the positively charged nitrogen in the dye are more strongly associated with the negatively charged nanoparticle surface than protons near the negatively charged sulfate groups of the dye. The methods described here can be used to study similar drug or dye molecules interacting with the surface of organic nanoparticles.

Adsorption Induced Changes of Human Hemoglobin on Ferric Pyrophosphate Nanoparticle Surface Probed by Isotope Exchange Mass Spectrometry: An Implication on Structure-Function Correlation

In general, proteins in the biological system interact with nanoparticles (NPs) via adsorption on the particle surface. Understanding the adsorption at the molecular level is crucial to explore NP-protein interactions. The increasing concerns about the risk to human health on NP exposure have been explored through the discovery of a handful protein biomarkers and biochemical analysis. However, detailed information on structural perturbation and associated functional changes of proteins on interaction with NPs is limited. Erythrocytes (red blood cells) are devoid of defense mechanism of protecting NP penetration through endocytosis. Therefore, it is important to investigate the interaction of erythrocyte proteins with NPs. Hemoglobin, the most abundant protein of human erythrocyte, is a tetrameric molecule consisting of α- and β-globin chains in duplicate. In the present study, we have used hemoglobin as a model system to investigate NP-protein interaction with ferric pyrophosphate NPs [NP-Fe₄(P₂O₇)₃]. We report the formation of a bioconjugate of hemoglobin upon adsorption to NP-Fe₄(P₂O₇)₃ surface. Analysis of the bioconjugate indicated that Fe³⁺ ion of NP-Fe₄(P₂O₇)₃ contributed in the bioconjugate formation. Using hydrogen/deuterium exchange based mass spectrometry, it was observed that the amino termini of α- and β-globin chains of hemoglobin were involved in the adsorption on NP surface whereas the carboxy termini of both chains became more flexible in its conformation compared to the respective regions of the normal hemoglobin. Circular dichroism spectra of desorbed hemoglobin indicated an adsorption induced localized structural change in the protein molecule. The formation of bioconjugate led to functional alteration of hemoglobin, as probed by oxygen binding assay. Thus, we hypothesize that the large amount of energy released upon adsorption of hemoglobin to NP surface might be the fundamental cause of structural perturbation of human hemoglobin and subsequent formation of the bioconjugate with an altered function.

Understanding Protein Structure Deformation on the Surface of Gold Nanoparticles of Varying Size

Gold nanoparticles (AuNPs) have been of recent interest due to their unique optical properties and their biocompatibility. Biomolecules spontaneously adsorb to their surface, a trait that could potentially be exploited for drug targeting. Currently, it is unclear whether protein-AuNP interactions at the nanoparticle surface are dependent on nanoparticle size. In this work, we investigate whether varying surface curvature can induce protein unfolding and multilayer binding in citrate-coated AuNPs of various sizes. A recently developed NMR-based approach was utilized to determine the adsorption capacity, and protein NMR spectra were compared to determine whether nanoparticle size influences protein interactions at the surface. In addition, transmission electron microscopy (TEM) and dynamic light scattering (DLS) were employed to corroborate the NMR studies. Over a broad range of AuNP sizes (14-86 nm), we show that adsorption capacity can be predicted by assuming that proteins are compact and globular on the nanoparticle surface. Additionally, roughly one layer of proteins is adsorbed regardless of AuNP size. Our results hold for two proteins of significantly different sizes, GB3 (6 kDa) and bovine carbonic anhydrase (BCA, 29 kDa). However, the unstable drkN SH3 domain (ΔḠ0 ≈ 0, 7 kDa) does not appear to follow the same trend seen for stable, globular proteins. This observation suggests that unstable proteins can deform significantly when bound to AuNP surfaces. Taken together, the results of this work can be used to improve our knowledge of the mechanism of protein-AuNP interactions to optimize their use in the biomedical field.

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.

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.

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.

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

The use of nanoparticles (NPs) in biomedical applications requires an in-depth understanding of the mechanisms by which NPs interact with biomolecules. NPs associating with proteins may interfere with protein-protein interactions and affect cellular communication pathways, however the impact of NPs on biomolecular recognition remains poorly characterized. In this respect, particularly relevant is the study of NP-induced functional perturbations of proteins implicated in the regulation of key biochemical pathways. Ubiquitin (Ub) is a prototypical protein post-translational modifier playing a central role in numerous essential biological processes. To contribute to the understanding of the interactions between this universally distributed biomacromolecule and NPs, we investigated the adsorption of polyhydroxylated [60]fullerene on monomeric Ub and on a minimal polyubiquitin chain in vitro at atomic resolution. Site-resolved chemical shift and intensity perturbations of Ub's NMR signals, together with (15)N spin relaxation rate changes, exchange saturation transfer effects, and fluorescence quenching data were consistent with the reversible formation of soluble aggregates incorporating fullerenol clusters. The specific interaction epitopes were identified, coincident with functional recognition sites in a monomeric and lysine48-linked dimeric Ub. Fullerenol appeared to target the open state of the dynamic structure of a dimeric Ub according to a conformational selection mechanism. Importantly, the protein-NP association prevented the enzyme-catalyzed synthesis of polyubiquitin chains. Our findings provide an experiment-based insight into protein/fullerenol recognition, with implications in functional biomolecular communication, including regulatory protein turnover, and for the opportunity of therapeutic intervention in Ub-dependent cellular pathways.

Protein structural dynamics at the gas/water interface examined by hydrogen exchange mass spectrometry

Gas/water interfaces (such as air bubbles or foam) are detrimental to the stability of proteins, often causing aggregation. This represents a potential problem for industrial processes, for example, the production and handling of protein drugs. Proteins possess surfactant-like properties, resulting in a high affinity for gas/water interfaces. The tendency of previously buried nonpolar residues to maximize contact with the gas phase can cause significant structural distortion. Most earlier studies in this area employed spectroscopic tools that could only provide limited information. Here we use hydrogen/deuterium exchange (HDX) mass spectrometry (MS) for probing the conformational dynamics of the model protein myoglobin (Mb) in the presence of N(2) bubbles. HDX/MS relies on the principle that unfolded and/or highly dynamic regions undergo faster deuteration than tightly folded segments. In bubble-free solution Mb displays EX2 behavior, reflecting the occurrence of short-lived excursions to partially unfolded conformers. A dramatically different behavior is seen in the presence of N(2) bubbles; EX2 dynamics still take place, but in addition the protein shows EX1 behavior. The latter results from interconversion of the native state with conformers that are globally unfolded and long-lived. These unfolded species likely correspond to Mb that is adsorbed to the surface of gas bubbles. N(2) sparging also induces aggregation. To explain the observed behavior we propose a simple model, that is, "semi-unfolded" ↔ "native" ↔ "globally unfolded" → "aggregated". This model quantitatively reproduces the experimentally observed kinetics. To the best of our knowledge, the current study marks the first exploration of surface denaturation phenomena by HDX/MS.