Marc R Knecht

University of Miami, كورال غيبلز، فلوريدا, Florida, United States

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Publications (53)235.96 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Adsorption of small biomolecules onto the surface of nanoparticles offers a novel route to the generation of nanoparticle assemblies with predictable architectures. Previously, ligand exchange experiments on citrate-capped gold nanoparticles with the amino acid arginine were reported to support linear nanoparticle assemblies. Here, we use a combination of atomistic modelling with experimental characterization to explore aspects of the assembly hypothesis for these systems. Using molecular simulation, we probe the structural and energetic characteristics of arginine overlayers on the Au(111) surface under aqueous conditions, at both low and high coverage regimes. In the low density regime, the arginines lie flat on the surface. At constant composition, these overlayers are found to be lower in energy than the densely-packed films, although the latter case appears kinetically stable when arginine is adsorbed via the zwitterion group, exposing the charged guanidinium group to the solvent. Our findings suggest that zwitterion-zwitterion hydrogen-bonding at the gold surface, and minimization of the electrostatic repulsion between adjacent guanidinium groups, play key roles in determining arginine overlayer stability at the aqueous gold interface. Ligand-exchange experiments of citrate-capped gold nanoparticles with arginine derivatives agmatine and N-methyl-L-arginine reveal that modification at the guanidinium group significantly diminishes the propensity for linear assembly of the nanoparticles.
    ACS Applied Materials & Interfaces 06/2014; · 5.01 Impact Factor
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    ABSTRACT: Biomimetic nanotechnologies that use peptides to guide the growth and assembly of nanostructures offer new avenues for the creation of functional nanomaterials and manipulation of their physicochemical properties. However, the impacts of peptide sequence and binding motif upon the surface characteristics and physicochemical properties of nanoparticles remain poorly understood. The configurations of the biomolecules are expected to be extremely important for directing the synthesis and achieving desired material functionality, and these binding motifs will vary with the peptide sequence. Here, we have prepared a series of Au nanoparticles capped with a variety of materials-directing peptides with known affinity for metal surfaces. These nanomaterials were characterized by UV-vis and circular dichroism spectroscopies, transmission electron microscopy, and ζ-potential measurement. Then their catalytic activity for 4-nitrophenol reduction was analyzed. The results indicate that substantially different Au-peptide interfaces are generated using different peptide sequences, even when these sequences have similar binding affinity. This is consistent with recent work showing that Au-peptide binding affinity can have varying entropic and enthalpic contributions, with enthalpically- and entropically-driven binders exhibiting quite different ensembles of configurations on the Au surface. The catalytic activity, as reflected by the measured activation energy, did not correlate with the particle size or with the binding affinity of the peptides, suggesting that the reactivity of these materials is governed by the more subtle details of the conformation of the bound peptide and on the nanoparticle surface reconstruction as dictated by the peptide structure. Such variations in both nanoparticle surface reconstruction and peptide configuration could potentially be used to program specific functionality into the peptide-capped nanomaterials.
    Nanoscale 02/2014; · 6.73 Impact Factor
  • Dennis B. Pacardo, Eric Ardman, Marc R. Knecht
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    ABSTRACT: Advancing catalytic processes toward sustainable conditions is necessary to maintain current production levels in light of dwindling natural resources. Nanomaterial-based catalysts have been suggested as a possible route to achieve this goal; however, the effects of particle structure on the reaction remain unclear. Furthermore, for each reaction, different substrates are likely to be used that vary the molecular size, functional group composition, and reactive moiety site that could significantly alter the reactivity of nanomaterial-based catalysts. In this contribution, we have studied the effects of the molecular substrate structure on the reactivity of peptide-templated Pd nanomaterials with selectable morphologies. In this regard, spherical, ribbon-like, and networked metallic nanomaterials were studied that demonstrated significant degrees of reactivity of olefin hydrogenation using the substrates that varied the molecular size and reactive group position. The results demonstrated that substrate isomerization, rather than molecular structure, plays a significant role in attenuating the reactivity of the materials. Furthermore, the Pd structures demonstrated the ability to drive multistep reactivity for the complete hydrogenation of substrates with multiple reactive groups. Such results advance the structure/function relationship of nanocatalysis that could be important in addressing future sustainability goals.
    The Journal of Physical Chemistry C. 01/2014; 118(5):2518–2527.
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    ABSTRACT: Transitioning energy-intensive and environmentally intensive processes toward sustainable conditions is necessary in light of the current global condition. To this end, photocatalytic processes represent new approaches for H2 generation; however, their application toward tandem catalytic reactivity remains challenging. Here, we demonstrate that metal oxide materials decorated with noble metal nanoparticles advance visible light photocatalytic activity toward new reactions not typically driven by light. For this, Pd nanoparticles were deposited onto Cu2O cubes to generate a composite structure. Once characterized, their hydrodehalogenation activity was studied via the reductive dechlorination of polychlorinated biphenyls. To this end, tandem catalytic reactivity was observed with H2 generation via H2O reduction at the Cu2O surface, followed by dehalogenation at the Pd using the in situ generated H2. Such results present methods to achieve sustainable catalytic technologies by advancing photocatalytic approaches toward new reaction systems.
    Journal of the American Chemical Society 01/2014; 136(1):32-5. · 10.68 Impact Factor
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    ABSTRACT: Bionanocombinatorics is an emerging field that aims to use combinations of positionally encoded biomolecules and nanostructures to create materials and devices with unique properties or functions. The full potential of this new paradigm could be accessed by exploiting specific noncovalent interactions between diverse palettes of biomolecules and inorganic nanostructures. Advancement of this paradigm requires peptide sequences with desired binding characteristics that can be rationally designed, based upon fundamental, molecular-level understanding of biomolecule-inorganic nanoparticle interactions. Here, we introduce an integrated method for building this understanding using experimental measurements and advanced molecular simulation of the binding of peptide sequences to gold surfaces. From this integrated approach, the importance of entropically driven binding is quantitatively demonstrated, and the first design rules for creating both enthalpically and entropically driven nanomaterial-binding peptide sequences are developed. The approach presented here for gold is now being expanded in our laboratories to a range of inorganic nanomaterials and represents a key step toward establishing a bionanocombinatorics assembly paradigm based on noncovalent peptide-materials recognition.
    ACS Nano 10/2013; · 12.03 Impact Factor
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    ABSTRACT: With the rapid development of bionanotechnology, there has been a growing interest recently in identifying the affinity classes of the inorganic materials binding peptide sequences. However, there are some distinct characteristics of inorganic materials binding sequence data that limit the performance of many widely-used classification methods. In this paper, we propose a novel framework to predict the affinity classes of peptide sequences with respect to an associated inorganic material. We first generate a large set of simulated peptide sequences based on our new amino acid transition matrix, and then the probability of test sequences belonging to a specific affinity class is calculated through solving an objective function. In addition, the objective function is solved through iterative propagation of probability estimates among sequences and sequence clusters. Experimental results on a real inorganic material binding sequence dataset show that the proposed framework is highly effective on identifying the affinity classes of inorganic material binding sequences.
    Proceedings of the International Conference on Bioinformatics, Computational Biology and Biomedical Informatics; 09/2013
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    ABSTRACT: Diverse classes of metallic nanostructures have been explored recently for a variety of applications, including energy efficient catalytic transformations. The morphology, size, and local chemical environment of the catalytic nanomaterials can have dramatic effects on their reactivity. Herein, we demonstrate a peptide-template-based approach for the synthesis of Pd and Pt nanostructures of varying morphologies under ambient conditions. In this report, we examine the effect of the metal/peptide ratio over an expansive range to demonstrate the stepwise production of materials ranging from nanospheres to nanoparticle networks for the Pd structures. Interestingly, when the metallic composition was changed to Pt, only spherical materials were generated, indicating that the metallic composition of the nanostructures plays a key role in the final morphology. The hydrogenation of allyl alcohol was then employed as a model reaction to examine the catalytic reactivity of these metallic nanomaterials. Under environmentally benign reaction conditions, high turnover frequency values were observed for the metallic Pd and Pt nanocatalysts that was independent of the material morphology. Given their high degree of reactivity and facile synthetic preparation, these materials could prove to be versatile and efficient catalysts for a variety of industrially and environmentally important reactions.
    The Journal of Physical Chemistry C. 08/2013; 117(35):18053–18062.
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    ABSTRACT: Au nanomaterials are well-known for their optical properties, where Au nanorods have demonstrated unique capabilities because of their readily tunable size and shape. Unfortunately, functionalization of the material surface is challenging because of their lack of stability after only a few purification cycles. Here, we demonstrate that enhanced Au-nanorod stability can be achieved by purifying the materials using dilute cetyltrimethylammonium bromide (CTAB) wash solutions. To this end, purifying the materials in such a manner shifts the passivant on/off equilibrium to maintain surfactant adsorption to the metal surface, leading to enhanced stability. Interestingly, from this study, a bimodal distribution of Au nanorods was evident, where one species was prone to bulk aggregation, whereas the second population remained stable in solution. This likely arose from defects within the CTAB bilayer at the nanorod surface, resulting in selective material aggregation. For this, those structures with high numbers of defects aggregated, whereas nanorods with a more pristine bilayer remained stable. Coating of the Au nanorods using polyelectrolytes was also explored for enhanced stability, where the composition of the anionic polymer played an important role in controlling materials stability. Taken together, these results demonstrate that the stability of Au nanorods can be directly tuned by the solvent-exposed surface structure, which could be manipulated to allow for the extensive material functionalization that is required for the generation of nanoplatforms with multiple applications.
    ACS Applied Materials & Interfaces 08/2013; · 5.01 Impact Factor
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    ABSTRACT: Peptide-based methods represent new approaches to selectively produce nanostructures with potentially important functionality. Unfortunately, biocombinatorial methods can only select peptides with target affinity and not for the properties of the final material. In this work, we present evidence to demonstrate that materials-directing peptides can be controllably modified to substantially enhance particle functionality without significantly altering nanostructural morphology. To this end, modification of selected residues to vary the site-specific binding strength and biological recognition can be employed to increase the catalytic efficiency of peptide-capped Pd nanoparticles. These results represent a step toward the de novo design of materials-directing peptides that control nanoparticle structure/function relationships.
    Journal of the American Chemical Society 07/2013; · 10.68 Impact Factor
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    ABSTRACT: Surfactant-stabilized metal nanoparticles have shown promise as catalysts although specific surface features and their influence on catalytic performance have not been well understood. We quantify the thermodynamic stability, the facet composition of the surface, and distinct atom types that affect rates of atom leaching for a series of twenty near-spherical Pd nanoparticles of 1.8 to 3.1 nm size using computational models. Cohesive energies indicate higher stability of certain particles that feature an approximate 60/20/20 ratio of {111}, {100}, and {110} facets while less stable particles exhibit widely variable facet composition. Unique patterns of atom types on the surface cause apparent differences in binding energies and changes in reactivity. Estimates of the relative rate of atom leaching as a function of particle size were obtained by the summation of Boltzmann-weighted binding energies over all surface atoms. Computed leaching rates are in good qualitative correlation with the measured catalytic activity of peptide-stabilized Pd nanoparticles of the same shape and size in Stille coupling reactions. The agreement supports rate-controlling contributions by atom leaching in the presence of reactive substrates. The computational approach provides a pathway to estimate the catalytic activity of metal nanostructures of engineered shape and size, and possible further refinements are described.
    Physical Chemistry Chemical Physics 03/2013; · 3.83 Impact Factor
  • Dennis B. Pacardo, Marc R. Knecht
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    ABSTRACT: Herein we systematically probed the atom-leaching mechanism of Pd nanoparticle-driven Stille coupling to further elucidate the fate of the highly active Pd0 atoms released in solution. In this regard, initial oxidative addition at the particle surface results in Pd atom abstraction for reactivity in solution. As a result, two reaction sites are present, the particle surface and pre-leached Pd atoms, thus different degrees of reactivity are possible. This effect was probed via aryl halide combinations that varied the halogen identity allowing for oxidative addition of two substrates simultaneously. The results demonstrate that the system was highly reactive for iodo-based compounds in the mixture at room temperature; however, reactivity at bromo-based substrates was only observed at slightly elevated temperatures of 40.0 °C. As such, substrate selectivity was evident from the catalytic materials that can be controlled based upon the aryl halide composition and reaction temperature. Furthermore, both intermolecular and intramolecular selectivity is possible, thus raising the degree of reaction complexity that can be achieved.
    Catal. Sci. Technol. 02/2013; 3(3):745-753.
  • Manish Sethi, Marc R Knecht
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    ABSTRACT: Nanoparticles possess unique properties that are enhanced due to their small size and varied shapes. These properties can be directly manipulated by controlling the aggregation state, which can further be exploited for applications in bio/chemical sensing, plasmonics, and as supports for catalysts. While the advantages of controlled aggregates of nanomaterials are great, synthetic strategies to achieve such structures with precision over the final arrangement of the materials in three-dimensional space remain limited. We have shown that ligand exchange reactions on Au nanomaterials of various shapes using simple amino acids can induce the formation of linear aggregates of the materials. The assembly process is mediated by partial ligand exchange on the particle surface, followed by the surface segregation of the two ligands that produces an electric dipole across the nanomaterial from which alignment occurs in solution via dipole-dipole interactions. This linear-based assembly can be used to tune the optical properties of the materials and could represent new pathways to study the interactions between biological molecules and inorganic nanomaterials.
    Methods in molecular biology (Clifton, N.J.) 01/2013; 1026:149-61. · 1.29 Impact Factor
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    ABSTRACT: We have shown previously that the function of Ycf1p, yeast ortholog of multidrug resistance-associated protein 1 (MRP1), is regulated by yeast casein kinase 2α (Cka1p) via phosphorylation at Ser251. In this study, we explored whether casein kinase 2α (CK2α), the human homolog of Cka1p, regulates MRP1 by phosphorylation at the semiconserved site Thr249. Knockdown of CK2α in MCF7-derived cells expressing MRP1 [MRP1 CK2α(-)] resulted in increased doxorubicin sensitivity. MRP1-dependent transport of leukotriene C(4) and estradiol-17β-d-glucuronide into vesicles derived from MRP1 CK2α(-) cells was decreased compared with MRP1 vesicles. Moreover, mutation of Thr249 to alanine (MRP1-T249A) also resulted in decreased MRP1-dependent transport, whereas a phosphomimicking mutation (MRP1-T249E) led to dramatic increase in MRP1-dependent transport. Studies in tissue culture confirmed these findings, showing increased intracellular doxorubicin accumulation in MRP1 CK2α(-) and MRP1-T249A cells compared with MRP1 cells. Inhibition of CK2 kinase by 2-dimethylamino-4,5,6,7-tetrabromo-1H-benzimidazole resulted in increased doxorubicin accumulation in MRP1 cells, but not in MRP1 CK2α(-), MRP1-T249A, or MRP1-T249E cells, suggesting that CK2α regulates MRP1 function via phosphorylation of Thr249. Indeed, CK2α and MRP1 interact physically, and recombinant CK2 phosphorylates MRP1-derived peptide in vitro in a Thr249-dependent manner, whereas knockdown of CK2α results in decreased phosphorylation at MRP1-Thr249. The role of CK2 in regulating MRP1 was confirmed in other cancer cell lines where CK2 inhibition decreased MRP1-mediated efflux of doxorubicin and increased doxorubicin cytotoxicity. This study supports a model in which CK2α potentiates MRP1 function via direct phosphorylation of Thr249.
    Molecular pharmacology 06/2012; 82(3):488-99. · 4.53 Impact Factor
  • Rohit Bhandari, Marc R. Knecht
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    ABSTRACT: Bio-inspired-based methods represent new approaches for the fabrication and activation of nanomaterials, all under ambient and energy-neutral conditions. Recent advances have demonstrated the production of non-spherical materials of Pd and Pt; however, the production of similar Au materials remains challenging. Such fabrication routes are highly important as Au-based nanomaterials of selectable morphologies could have immediate applications in catalysis and energy storage. In this contribution, we demonstrate a peptide template-based methodology for the fabrication of Au nanoparticle networks, which are highly branched linear structures that are prepared in water at room temperature. The materials were fully characterized using UV-vis, TEM, XRD, and DLS, from which their catalytic activity was subsequently studied for the reduction of 4-nitrophenol. Using this approach, the materials were shown to be highly reactive as compared to comparable structures, which is likely due to their unique biological template. Together, this research represents a step forward in bio-based methodologies for the fabrication of functional and potentially sustainable materials.
    Catal. Sci. Technol. 06/2012; 2(7):1360-1366.
  • Rohit Bhandari, Marc R Knecht
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    ABSTRACT: Advances in nanotechnology have indicated that the passivant and the inorganic surface play a pivotal role in controlling the structure/function relationship of materials. Beyond standard materials-based methods, bioligands have recently demonstrated the production of unique nanomaterial morphologies for application under ambient conditions for multiple activities, such as catalysis and biosensing. We have recently demonstrated that a biotemplate technique could be employed to produce spherical and linear Pd nanostructures in water using a self-assembling peptide framework. The materials possessed high catalytic reactivity that was controlled by the three-dimensional structure of the composite materials. To investigate the effect of the peptide template on the reactivity of Pd nanostructures, an in depth analysis of the catalytic activity of Pd nanostructures fabricated via truncated templates is presented. The new templates were designed from portions of the original framework, which demonstrated unique synthetic and functionality control. Two different reactions, Stille C-C coupling and 4-nitrophenol reduction, were employed to ascertain the effect of template structure on the reactivity of synthesized Pd nanomaterials via changes in reagent diffusion through the bioscaffold. The results indicate that the peptide framework plays an important role and could be used to tune and optimize the functionality of the final composite materials for the target application.
    Langmuir 05/2012; 28(21):8110-9. · 4.38 Impact Factor
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    ABSTRACT: The ability to tune the size, shape, and composition of nanomaterials at length scales <10 nm remains a challenging task. Such capabilities are required to fully realize the application of nanotechnology for catalysis, energy storage, and biomedical technologies. Conversely, nature employs biomacromolecules such as proteins and peptides as highly specific nanoparticle ligands that demonstrate exacting precision over the particle morphology through controlling the biotic/abiotic interface. Here we demonstrate the ability to finely tune the size, surface structure, and functionality of single-crystal Pd nanoparticles between 2 and 3 nm using materials directing peptides. This was achieved by selectively altering the peptide sequence to change the binding motif, which in turn modifies the surface structure of the particles. The materials were fully characterized before and after reduction using atomically resolved spectroscopic and microscopic analyses, which indicated that the coordination environment prior to reduction significantly affects the structure of the final nanoparticles. Additionally, changes to the particle surface structure, as a function of peptide sequence, can allow for chloride ion coordination that alters the catalytic abilities of the materials for the C-C coupling Stille reaction. These results suggest that peptide-based approaches may be able to achieve control over the structure/function relationship of nanomaterials where the peptide sequence could be used to selectivity tune such capabilities.
    ACS Nano 02/2012; 6(2):1625-36. · 12.03 Impact Factor
  • Rohit Bhandari, Ryan Coppage, Marc R. Knecht
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    ABSTRACT: Recent developments in bionanotechnology have produced a knowledge pool that enables the fabrication, functionalization, and activation of inorganic nanostructures. Continued progress in this field has led to advances in inorganic nanomaterial control, providing for the generation of catalysts that operate under biologically influenced conditions of temperature, pressure, and solvent. Outlined in this Perspective are a selection of catalysts active for a variety of reactions including C–C coupling, chemical reduction, electrocatalysis, and bond cleavage reactions, where a combination of both the inorganic core and biological surface work in concert to achieve the final functionality. By fully understanding the total structure/function relationship of these bio-inspired nanomaterials, new catalytic structures could be designed using biological principles that are energy neutral, eco-friendly, and selective, all of which represent grand challenges in light of the current global condition.
    Catal. Sci. Technol. 02/2012; 2(2):256-266.
  • Beverly D. Briggs, Marc R. Knecht
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    ABSTRACT: Nature exploits sustainable methods for the creation of inorganic materials on the nanoscale for a variety of applications. To achieve such capabilities, biomolecules such as peptides and proteins have been developed that recognize and bind the different compositions of materials. While a diverse set of materials binding sequences are present in the biosphere, biocombinatorial techniques have been used to rapidly identify peptides that facilitate the formation of new materials of technological importance. Interestingly, the binding motif of the peptides at the inorganic surface is likely to control the size, structure, composition, shape, and functionality of the final materials. In order to advance these intriguing new biomimetic approaches, a complete understanding of this biotic/abiotic interface is required. In this Perspective, we highlight recent advances in the biofunctionalization of nanoparticles with potential applications ranging from catalysis and energy storage to plasmonics and biosensing. We specifically focus on the physical characterization of the peptide-based surface from which specificity and activity are likely embedded.
    The Journal of Physical Chemistry Letters. 01/2012; 3(3).
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    ABSTRACT: The ability to control the size, shape, composition, and activity of nanomaterials presents a formidable challenge. Peptide approaches represent new avenues to achieve such control at the synthetic level; however, the critical interactions at the bio/nano interface that direct such precision remain poorly understood. Here we present evidence to suggest that materials-directing peptides bind at specific time points during Pd nanoparticle (NP) growth, dictated by material crystallinity. As such surfaces are presented, rapid peptide binding occurs, resulting in the stabilization and size control of single-crystal NPs. Such specificity suggests that peptides could be engineered to direct the structure of nanomaterials at the atomic level, thus enhancing their activity.
    Journal of the American Chemical Society 08/2011; 133(32):12346-9. · 10.68 Impact Factor
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    ABSTRACT: Biomolecule-directed growth and assembly of nanomaterials utilizes highly specific interactions to provide the exciting prospect of producing a new generation of precisely arranged, stimuli responsive, and reconfigurable nanoarrayed structures for a wide range of applications from catalysis to energy storage. With an objective to create a much needed fundamental understanding of the complex biotic/abiotic interfacial interactions, this paper presents a systematic study of surface interactions of a series of amino acids with Aunanoparticles. We have employed a self-assembly based method that monitors changes in the optical properties and aggregate size of Aunanoparticles in response to their binding with selected amino acid residues. The observations were used to derive information on the binding strength and ligand surface arrangement. Our experimental results follow previously derived computational trends in the surface affinities of the residues, thus suggesting that our approach may be used to assess the binding abilities and interligand interactions of biomacromolecules on nanomaterial surfaces.
    Soft Matter 07/2011; 7(14):6532-6541. · 4.15 Impact Factor

Publication Stats

436 Citations
235.96 Total Impact Points


  • 2011–2014
    • University of Miami
      • Department of Chemistry
      كورال غيبلز، فلوريدا, Florida, United States
  • 2008–2013
    • University of Kentucky
      • Department of Chemistry
      Lexington, KY, United States
  • 2006–2009
    • University of Texas at Austin
      Austin, Texas, United States
    • Texas A&M University
      • Department of Chemistry
      College Station, TX, United States
  • 2004–2006
    • Vanderbilt University
      • Department of Chemistry
      Nashville, MI, United States