Papers by Daniel Schwartz
Journal of Pharmaceutical Sciences, 2019
Silicone oil, used as a lubricating coating in pharmaceutical containers, has been implicated as ... more Silicone oil, used as a lubricating coating in pharmaceutical containers, has been implicated as a cause of therapeutic protein aggregation. After adsorbing to silicone oil-water interfaces, proteins may form interfacial gels, which can be transported into solution as insoluble aggregates if the interfaces are perturbed. Mechanical interfacial perturbation of both monomeric recombinant human interleukin-1 receptor antagonist (rhIL-1ra) and PEGylated rhIL-1ra (PEG rhIL-1ra) in siliconized syringes resulted in losses of soluble monomeric protein. However, the loss of rhIL-1ra was twice that for PEG rhIL-1ra; even though in solution, PEG rhIL-1ra had a lower DG unf and exhibited a more perturbed tertiary structure at the interface. Net protein-protein interactions in solution for rhIL-1ra were attractive but increased steric repulsion because of PEGylation led to net repulsive interactions for PEG rhIL-1ra. Attractive interactions for rhIL-1ra were associated with increases in intermolecular b-sheet content at the interface, whereas no intermolecular b-sheet structures were observed for adsorbed PEG rhIL-1ra. rhIL-1ra formed interfacial gels that were 5 times stronger than those formed by PEG rhIL-1ra. Thus, the steric repulsion contributed by the PEGylation resulted in decreased interfacial gelation and in the reduction of aggregation, in spite of the destabilizing effects of PEGylation on the protein's conformational stability.
Physical Review Letters, Aug 14, 2000
We have studied the structural changes that fatty acid monolayers in the Ov phase undergo when a ... more We have studied the structural changes that fatty acid monolayers in the Ov phase undergo when a simple shear flow is imposed. A strong coupling is revealed by the changes in domain structure that are observable using Brewster angle microscopy, suggesting the possibility of shear alignment. The dependence of the alignment on the molecular polar tilt proves that the mechanism is different than in nematic liquid crystals. We argue that the degenerate lattice symmetry lines of the underlying pseudohexagonal lattice align in the flow direction, and we explain the observed alignment angle using geometrical arguments.
ACS Nano, Apr 10, 2014
urface hybridization involves a target single-stranded DNA (ssDNA) engaging in specific interacti... more urface hybridization involves a target single-stranded DNA (ssDNA) engaging in specific interactions (hybridization) with immobilized probe ssDNA molecules.
Biomacromolecules, Nov 13, 2012
Single-molecule total internal reflection fluorescence microscopy was used to observe the dynamic... more Single-molecule total internal reflection fluorescence microscopy was used to observe the dynamic behavior of (poly)-cytosine ssDNA (1-50 nucleotides long) at the interface between aqueous solution and hydrophilic (oligoethylene oxide-modified fused silica, OEG) and hydrophobic (octadecyltriethoxysilane-modified fused silica, OTES) solid surfaces. High throughput molecular tracking was used to determine >75,000 molecular trajectories for each molecular length, which were then used to calculate surface residence time and squared displacement (i.e. "step-size") distributions. On hydrophilic OEG surfaces, the surface residence time increased systematically with ssDNA chain length, as expected due to increasing moleculesurface interactions. Interestingly, the residence time decreased with increasing ssDNA length on the hydrophobic OTES surface, particularly for longer chains. Similarly, the interfacial mobility of polynucleotides slowed with increasing chain length on OEG, but became faster on OTES. On OTES surfaces, the rates associated with desorption and surface diffusion exhibited the distinctive anomalous temperature dependence that is characteristic of hydrophobic interactions for short chain species but not for longer chains. These combined observations suggest that long oligonucleotides adopt conformations minimizing hydrophobic interactions, e.g. by internal sequestration of hydrophobic nucleobases.
The Journal of Physical Chemistry Letters, 2019
In this Letter, we report that surface-bound nanobubbles reduce protein denaturation on methylate... more In this Letter, we report that surface-bound nanobubbles reduce protein denaturation on methylated glass by irreversible protein shell formation. Single-molecule total internal reflection fluorescence (SM-TIRF) microscopy was combined with intramolecular Forster resonance energy transfer (FRET) to study the conformational dynamics of nitroreductase (NfsB) on nanobubble-laden methylated glass surfaces, using reflection brightfield microscopy to register nanobubble locations with NfsB adsorption. First, NfsB adsorbed irreversibly to nanobubbles with no apparent desorption after 5 h. Moreover, virtually all (96%) of the NfsB molecules that interacted with nanobubbles remained folded, whereas less than 50% of NfsB molecules remained folded in the absence of nanobubbles on unmodified silica or methylated glass surfaces. This trend was confirmed by ensemble-average fluorometer TIRF experiments. We hypothesize that nanobubbles reduce protein damage by passivating strongly denaturing topographical surface defects. Thus, nanobubble stabilization on surfaces may have important implications for antifouling surfaces and improving therapeutic protein storage. P roteins are ubiquitous in biomedical and biotechnological applications, including protein therapeutics, biosensing, biocatalysis, food processing, and detergents. [1][2][3] Unfortunately, proteins are notoriously unstable over long-term storage as a result of air oxidation, thermal unfolding, mechanical shearing by ice crystals, and many other potential processes. 4 Beyond simple loss of activity, in some cases therapeutic proteins form large aggregates that can provoke an immune response, diminishing long-term efficacy and potentially even endangering the patient's life. Thus, a considerable amount of research and effort has been devoted to maintaining protein stability during storage. Even under optimal conditions, proteins still adsorb and desorb from surfaces that contact liquid formulations, in some cases desorbing from the surface in an unfolded state. Recent advances using single-molecule (SM) fluorescence microscopy methods have led to the identification of anomalous surface sites, or "hot spots," that are responsible for the majority of unfolding events. 12 These anomalous denaturing sites are thought to be caused by random heterogeneities in the surface (e.g., glass) structure such as step edges or grain boundaries. Even nominally smooth glass exhibits topographical nonuniformities with protruding or depressed surface features, which can, in turn, alter the interaction of the protein with the surface relative to homogeneous surfaces. 12-14 Because these features may arise from many sources, such as demixing, crystallization, contamination, and/or the processing history of the glass, 15 and are thus generally unavoidable, an understanding of these sites and strategies to mediate their impact on protein stability are critical.
Applied Catalysis A: General, 2018
Controlling the near-surface environment of heterogeneous catalysts is of fundamental importance ... more Controlling the near-surface environment of heterogeneous catalysts is of fundamental importance for high selectivity and activity. Self-assembled monolayers (SAMs) are effective tools to control reaction selectivity and activity for both supported noble metal and metal oxide catalysts. We previously demonstrated tunable dehydration activity of alcohols on phosphonic acidmodified, anatase-phase TiO2. In this work, we investigated the generality of this approach by studying the modification of other metal oxides including Al2O3,
Acta Physica Polonica A, 1998
The atomic force microscope was used to show that Langmuir-Blodgett films are unstable to reorgan... more The atomic force microscope was used to show that Langmuir-Blodgett films are unstable to reorganization via a folding mechanism by which uniformly thick films spontaneously form holes and multilayer steps. These bilayer step defects originate at isolated sites and quickly spread to cover the entire film. The defects retain the sixfold symmetry of the underlying molecular lattice; after sufficient time, straight edges begin to form and the entire film is comprised of high islands with straight edges aligned with sixfold symmetry. The kinetics of the reorganization depend strongly on the chain length of the fatty acid used and the nature of the substrate. The results suggest that the reorganization is driven by an interconversion of strained, asymmetric monolayers to unstrained, centrosymmetric bilayers.
ACS applied materials & interfaces, Jan 13, 2017
As particles flow through porous media, they follow complex pathways and experience heterogeneous... more As particles flow through porous media, they follow complex pathways and experience heterogeneous environments that are challenging to characterize. Tortuosity is often used as a parameter to characterize the complexity of pathways in porous materials and is useful in understanding hindered mass transport in industrial filtration and mass separation processes. However, conventional calculations of tortuosity provide only average values under static conditions; they are insensitive to the intrinsic heterogeneity of porous media and do not account for potential effects of operating conditions. Here, we employ a high-throughput nanoparticle tracking method which enables the observation of actual particle trajectories in polymer membranes under relevant operating conditions. Our results indicate that tortuosity is not simply a structural material property but is instead a functional property that depends on flow rate and particle size. We also resolved the spatiotemporal heterogeneity o...
Langmuir, May 20, 1999
Brewster angle microscopy was used to observe the surface pressure-driven flow of various fatty a... more Brewster angle microscopy was used to observe the surface pressure-driven flow of various fatty acid Langmuir monolayers through a narrow channel. In the tilted liquid crystalline phases (L2, L2′, and Ov), the velocity profile was generally parabolic at low flow rates, evolved to a nearly triangular shape at higher flow rates, and often returned to a parabolic shape at very high flow rates (when it was possible to observe them). In the L2 phase, the critical flow rates for the transitions between flow profiles showed a marked decrease with increasing temperature. However, the critical flow rates were insensitive to temperature in the L2′ and Ov phases. The temperature dependence of these critical flow rates for each of the three chain lengths studied (octadecanoic, eicosanoic, and docosanoic acids) is nearly identical if one adjusts the temperatures by the typical 5°C per methylene group in order to consider equivalent positions in the generalized fatty acid monolayer phase diagram. This suggests that the non-Newtonian flow behavior is specifically linked to the structures of the mesophases themselves and not due to effects such as coupling to the subphase or a simple correlation to the polydomain structure of the monolayers.
A technique for direct observation of particle motion under shear in a Langmuir monolayer
Journal of Rheology, Sep 1, 1997
We have used fluorescence microscopy to observe the effect of shear on rigid two-dimensional stru... more We have used fluorescence microscopy to observe the effect of shear on rigid two-dimensional structures residing in a Langmuir monolayer. Monolayers of 12-NBD stearic acid undergo a two-dimensional liquid-to-solid transition on the water surface. The domains of the solid phase that form in the coexistence region are elongated (needle-shaped) crystallites. We observe the effect of shear within the monolayer in two geometries: two-dimensional Poiseuille flow and simple shear flow created by moving bands. The technique allows us to determine the average orientational order parameter as well as the details of the rotational kinematics, which are, as expected, well-described by a Jeffery orbit. We propose that this technique of direct observation in Langmuir monolayers will be a useful method for the study of systems of rigid particles under flow.
PLOS ONE
The activity of antimicrobial peptides (AMPs) has significant bacterial species bias, the mechani... more The activity of antimicrobial peptides (AMPs) has significant bacterial species bias, the mechanisms of which are not fully understood. We employed single-molecule tracking to measure the affinity of three different AMPs to hybrid supported bilayers composed of lipid A extracted from four different Gram negative bacteria and observed a strong empirical anticorrelation between the affinity of a particular AMP to a given lipid A layer and the activity of that AMP towards the bacterium from which that lipid A was extracted. This suggested that the species bias of AMP activity is directly related to AMP interactions with bacterial outer membranes, despite the fact that the mechanism of antimicrobial activity occurs at the inner membrane. The trend also suggested that the interactions between AMPs and the outer membrane lipid A (even in the absence of other components, such as lipopolysaccharides) capture effects that are relevant to the minimum inhibitory concentration.
Accounts of Chemical Research, 2020
My graduate school experience at University of Colorado Boulder has been incredibly enlightening ... more My graduate school experience at University of Colorado Boulder has been incredibly enlightening and enjoyable. First and foremost, I would like to thank my thesis advisor, Daniel K. Schwartz, for always being extremely supportive since I joined the group in 2017. I have been deeply influenced by his enthusiasm in research, he exemplified to me the qualities that a good researcher should have, patience and persistent dedication, which I shall remember forever. His advising has been instrumental in inspiring critical thinking and problem solving, which ultimately helped me to evolve as an independent scientist. In addition, Dan has always been more than a good advisor but as a great friend as well, providing us with various professional resources, opportunities, and networks, and sharing his life experience gleaned over the years. It was one of the best decisions I have made to come to Boulder and join the Schwartz research group. I have had the opportunity to work or interact with many excellent scientists, engineers and staff in the Department of Chemical and Biological Engineering at University of Colorado Boulder. In particular, I would like to acknowledge professors Robert Davis, Andrew Goodwin, Yifu Ding, Wyatt Shields and Orit Peleg for serving as my thesis committee members. Thank you all for selflessly giving your time and providing valuable guidance and insights to my thesis. I also want to thank Dominique de Vangel, who is the advisor of our graduate program, for his kind and prompt assistance throughout these years.
Journal of the American Chemical Society, 2021
During integration into materials, the inactivation of enzymes as a result of their interaction w... more During integration into materials, the inactivation of enzymes as a result of their interaction with nanometer size denaturing "hotspots" on surfaces represents a critical challenge. This challenge, which has received far less attention than improving the long-term stability of enzymes, may be overcome by limiting the exploration of surfaces by enzymes. One way this may be accomplished is through increasing the rate constant of the surface ligation reaction and thus probability of immobilization with reactive surface sites (i.e., ligation efficiency). Here, the connection between ligation reaction efficiency and the retention of enzyme structure and activity was investigated by leveraging the extremely fast reaction of strained trans-cycopropenecyclooctenes (sTCOs) and tetrazines (Tet). Remarkably, upon immobilization via Tet-sTCO chemistry, carbonic anhydrase (CA) retained 77% of its solution-phase activity, while immobilization via less efficient reaction chemistries, such as thiol-maleimide and azide-dibenzocyclooctyne, led to activity retention of only 46% and 27%, respectively. Dynamic single-molecule (SM) fluorescence tracking methods further revealed that longer surface search distances prior to immobilization (>0.5 μm) dramatically increased the probability of CA unfolding. Notably, CA distance to immobilization was significantly reduced through the use of tetrazine-sTCO chemistry, which correlated with the increased retention of structure and activity of immobilized CA compared to the use of slower ligation chemistries. These findings provide unprecedented insight into the role of ligation reaction efficiency in mediating the exploration denaturing hotspots on surfaces by enzymes, which, in turn, may have major ramifications in the creation of functional biohybrid materials.
The Journal of Physical Chemistry Letters, 2019
While both cis and trans (adhesive)-interactions cooperate in the assembly of intercellular adhes... more While both cis and trans (adhesive)-interactions cooperate in the assembly of intercellular adhesions, computational simulations have predicted that two-dimensional confinement may promote cis-oligomerization, in the absence of trans-interactions. Here, single-molecule tracking of cadherin extracellular domains on supported lipid bilayers revealed the density-dependent formation of oligomers and cis-clusters in the absence of trans-interactions. Lateral oligomers were virtually eliminated by mutating a putative cis (lateral) binding interface. At low cadherin surface coverage, wild-type and mutant cadherin diffused rapidly, consistent with the motion of a lipid molecule within a cadherin-free supported bilayer and with cadherins diffusing as monomers. Although the diffusion of mutant cadherin did not change appreciably with increasing surface coverage, the average short-time diffusion coefficient of wild-type cadherin slowed significantly above a fractional surface coverage of ∼0.01 (∼1100 molecules/μm 2). A detailed analysis of molecular trajectories suggested the presence of a broad size distribution of cis-cadherin oligomers. These findings verify predictions that two-dimensional confinement promotes cis-oligomerization, in the absence of trans-interactions.
ACS Nano, 2019
Adsorption of soluble DNA to surfaces decorated with complementary DNA plays an important role in... more Adsorption of soluble DNA to surfaces decorated with complementary DNA plays an important role in many bionanotechnology applications, and previous studies have reported complex dependencies of the surface density of immobilized DNA on hybridization. While these effects have been speculatively ascribed to steric and/or electrostatic effects, the influence of surfacemediated molecular transport (i.e. intermittent "hopping diffusion") has not been fully appreciated. Here, single-molecule tracking and Förster Resonance Energy Transfer (FRET) were employed to characterize the mobility and the hybridization efficiency of adsorbed ssDNA oligonucleotides ("target") at solid-liquid interfaces exhibiting surface-immobilized ssDNA ("probe") over a wide range of surface grafting densities. Two distinct regimes were observed, with qualitatively different transport and hybridization behaviors. At dilute grafting density, only 1-3% of target molecules were observed to associate with probes (i.e. to hybridize). Adsorbing target molecules often searched unsuccessfully and "flew", via desorption-mediated diffusion, to secondary locations before hybridizing. In contrast, at high probe grafting density, approximately 20% of target DNA hybridized to immobilized probes, and almost always in the vicinity of initial adsorption. Moreover, following a dehybridization event, target molecules rehybridized at high probe density, but rehybridization was infrequent in the dilute density regime. Interestingly, the intermittent interfacial transport of mobile target molecules was suppressed by the presence of immobilized probe DNA, presumably due to an increased probability of re-adsorption following each "hop". Together, these findings suggested that many salient effects of grafting density on surface-mediated DNA hybridization can be directly related to the mechanisms of surfacemediated intermittent diffusion.
Macromolecules, 2018
Single-molecule tracking was used to characterize the mobility of poly(ethylene glycol) chains at... more Single-molecule tracking was used to characterize the mobility of poly(ethylene glycol) chains at a solid−liquid interface over a wide range of surface coverage. Trajectories exhibited intermittent motion consistent with a generalized continuous time random walk (CTRW) model, where strongly confined "waiting times" alternated with rapid flights. The presence of three characteristic regimes emerged as a function of surface coverage, based on an analysis of effective short-time diffusion coefficients, mean-squared displacement, and CTRW distributions. The dilute "site-blocking" regime exhibited increasing short-time diffusion, less confined behavior, and shorter waiting times with higher surface coverage, as anomalously strong adsorption sites were increasingly passivated. At intermediate values of surface coverage, the "crowding" regime was distinguished by the exact opposite trends (slower, more confined mobility), presumably due to increasing intermolecular interactions. The trends reversed yet again in the "brush" regime, where adsorbing molecules interacted weakly with a layer of extended overlapping chains.
The Journal of Physical Chemistry C, 2018
Cooperative catalysts containing a combination of noble metal hydrogenation sites and Brønsted ac... more Cooperative catalysts containing a combination of noble metal hydrogenation sites and Brønsted acid sites are critical for many reactions, including the deoxygenation (DO) of biomass-derived oxygenates in the upgrading of pyrolysis oil. One route toward design of cooperative catalysts is to tether two different catalytically active functions so that they are in close proximity while avoiding undesirable interactions that can block active sites. Here, we deposited carboxylic acid-functionalized organophosphonate monolayers onto Al 2 O 3-supported Pd nanoparticle catalysts to prepare bifunctional catalysts containing both Brønsted acid and metal sites. Modification with phosphonic acids improved activity and selectivity for gas-phase DO reactions, but the degree of improvement was highly sensitive to both the presence and positioning of the carboxylic acid group, suggesting a significant contribution from both the phosphonic and carboxylic acid sites. Short spacer lengths of 1-2 methylene groups between the phosphonate head and carboxylic acid tail were found to yield the best DO rates and selectivities, while longer chains performed similarly to SAMs having alkyl tails. Results from a combination of density functional theory and Fourier-transform infrared spectroscopy suggested that the enhanced catalyst performance on the optimally positioned carboxylic acids was due to the generation of strong acid sites on the Al 2 O 3 support adjacent to the metal. Furthermore, the high activity of these sites was found to result from a hydrogen-bonded cyclic structure involving cooperativity between the phosphonate head group and carboxylic acid tail function. More broadly, these results indicate that functional groups tethered to supports via organic ligands can influence catalytic chemistry on metal nanoparticles. Recent work has demonstrated that the DO of many biomass-derived oxygenates can be dramatically improved over multifunctional catalysts having both noble metal and Brønsted acid sites, occurring through a combination of hydrogenation and dehydration steps. 10,15-18 Noble metal nanoparticles (e.g. supported Pd, Pt, Rh) are typical hydrogenation catalysts since they readily activate H 2 , while Brønsted acid catalysts are known to be efficient for many reactions including isomerization, cracking, and dehydration. 19-21 In heterogeneous catalysis, many studies have highlighted the importance of the metal-support interface for various reactions, which offers the opportunity to combine adjacent Brønsted acid and metal active sites. 15,17,22-24 Moreover, it has been proposed that multifunctional interfaces have the potential to overcome catalyst
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Papers by Daniel Schwartz