DNA hairpin stabilization on a hydrophobic surface

Integrating Nanosensors into Macroporous Hydrogels for Implantation

Macroporous hydrogels are an attractive platform for implantable sensors because the network of interconnected macropores facilitates tissue integration. Embedded sensing elements, in our case, plasmonic gold nanoparticles, can transduce the presence, absence, and concentration of biochemical markers to the outside. We present here how to integrate such nanosensors into a macroporous hydrogel while preserving the nanosensor functionality in order to produce implantable sensors. We demonstrate that out of four different polymers, the poly(2-hydroxyethyl methacrylate-poly(ethylene glycole)diacrylate copolymer (pHEMA-PEGDA) results in a working sensor. Our approach of incorporating nanosized sensor elements into a hydrogel matrix generally identifies suitable polymers for implantable sensor systems.

Recent developments in the characterization of nucleic acid hybridization kinetics

Hybridization of nucleic acids (NAs) is a fundamental molecular mechanism that drives many cellular processes and enables new biotechnologies as well as therapeutics. However, existing methods that measure hybridization kinetics of nucleic acids are either performed at the ensemble level or constrained to non-native physiological conditions. Recent advances in 3D single-molecule tracking techniques break these limitations by allowing multiple annealing and melting events to be observed on a single oligonucleotide freely diffusing inside a live mammalian cell. This review provides an overview of diverse approaches to measuring NA hybridization kinetics at the single-molecule level and in live cells, and concludes with a synopsis of unresolved challenges and opportunities in the live-cell hybridization kinetics measurements. Important discoveries made by NA kinetics measurements and biotechnologies that can be improved with a deeper understanding of hybridization kinetics are also described.

Confocal Raman Microscopy Enables Label-Free, Quantitative, and Structurally Informative Detection of DNA Hybridization at Porous Silica Surfaces

Characterization of DNA at solid/liquid interfaces remains a challenge because most surface-sensitive techniques are unable to provide quantitative insight into the base content, length, or structure. Surface-enhanced Raman scattering measurements of DNA hybridization on plasmonic-metal substrates have been used to overcome small Raman-scattering cross-sections; however, surface-enhanced Raman spectroscopy measurements are not generally quantitative due to the fall-off in the scattering signal with the decay of the electric field enhancement from the surface, which also limits the length of oligonucleotides that can be investigated. In this work, we introduce an experimental methodology in which confocal Raman microscopy is used to characterize hybridization reactions of ssDNA immobilized at the solid/liquid interface of porous silica particles. By focusing the femtoliter confocal probe volume within a single porous particle, signal enhancement arises from the ∼1500-times greater surface area detected compared to a planar substrate. Because the porous support is a purely dielectric material, the scattering signal is independent of the proximity of the oligonucleotide to the silica surface. With this technique, we characterize a 19-mer capture strand and determine its hybridization efficiency with 9-mer and 16-mer target sequences from the scattering of a structurally insensitive phosphate-stretching mode. Changes in polarizability and frequency of scattering from DNA bases were observed, which are consistent with Watson-Crick base pairing. Quantification of base content from their duplex scattering intensities allows us to discriminate between hybridization of two target strands of equivalent length but with different recognition sequences. A duplex having a single-nucleotide polymorphism could be distinguished from hybridization of a fully complementary strand based on differences in base content and duplex conformation.

How Surfaces Affect Hybridization Kinetics

Hybridization between nucleic acid strands immobilized on a solid support with partners in solution is widely practiced in bioanalytical technologies and materials science. An important fundamental aspect of understanding these reactions is the role played by immobilization in the dynamics of duplex formation and disassembly. This report reviews and analyzes literature kinetic data to identify commonly observed trends and to correlate them with probable molecular mechanisms. The analysis reveals that while under certain conditions impacts from immobilization are minimal so that surface and solution hybridization kinetics are comparable, it is more typical to observe pronounced offsets between the two scenarios. In the forward (hybridization) direction, rates at the surface commonly decrease by one to two decades relative to solution, while in the reverse direction rates of strand separation at the surface can exceed those in solution by tens of decades. By recasting the deviations in terms of activation barriers, a consensus of how immobilization impacts nucleation, zipping, and strand separation can be conceived within the classical mechanism in which duplex formation is rate limited by preassembly of a nucleus a few base pairs in length, while dehybridization requires the cumulative breakup of base pairs along the length of a duplex. Evidence is considered for how excess interactions encountered on solid supports impact these processes.

Modeling Effects of Surface Properties and Probe Density for Nanoscale Biosensor Design: A Case Study of DNA Hybridization near Surfaces

Electrochemical biosensors have extremely robust applications while offering ease of preparation, miniaturization, and tunability. By adjusting the arrangement and properties of immobilized probes on the sensor surface to optimize target-probe association, one can design highly sensitive and efficient sensors. In electrochemical nucleic acid biosensors, a self-assembled monolayer (SAM) is widely used as a tunable surface with inserted DNA or RNA probes to detect target sequences. The effects of inhomogeneous probe distribution across surfaces are difficult to study experimentally due to inadequate resolution. Regions of high probe density may inhibit hybridization with targets, and the magnitude of the effect may vary depending on the hybridization mechanism on a given surface. Another fundamental question concerns diffusion and hybridization of DNA taking place on surfaces and whether it speeds up or hinders molecular recognition. We used all-atom Brownian dynamics simulations to help answer these questions by simulating the hybridization process of single-stranded DNA (ssDNA) targets with a ssDNA probe on polar, nonpolar, and anionic SAMs at three different probe surface densities. Moreover, we simulated three tightly packed probe clusters by modeling clusters with different interprobe spacing on two different surfaces. Our results indicate that hybridization efficiency depends strongly on finding a balance that allows attractive forces to steer target DNA toward probes without anchoring it to the surface. Furthermore, we found that the hybridization rate becomes severely hindered when interprobe spacing is less than or equal to the target DNA length, proving the need for a careful design to both enhance target-probe association and avoid steric hindrance. We developed a general kinetic model to predict hybridization times and found that it works accurately for typical probe densities. These findings elucidate basic features of nanoscale biosensors, which can aid in rational design efforts and help explain trends in experimental hybridization rates at different probe densities.

Super-Resolution Imaging of Competitive Unlabeled DNA Hybridization Reveals the Influence of Fluorescent Labels on Duplex Formation and Dissociation Kinetics

Single-molecule fluorescence imaging is a powerful method to measure reversible reaction kinetics, allowing one to monitor the bound state of individual probe molecules with fluorescently labeled targets. In the case of DNA hybridization, previous studies have shown that the presence of a fluorescent label on a target strand can exhibit significant influence on the stability of a DNA duplex that is formed. In this work, we have developed a super-resolution imaging method to measure the hybridization kinetics of unlabeled target DNA that compete with a fluorescently labeled tracer DNA strand to hybridize with an unlabeled probe DNA immobilized at a surface. The hybridization of an unlabeled DNA target cannot be detected directly, but its presence blocks the immobilized probe DNA, influencing the measured time intervals between labeled DNA hybridization events. We derive a model for competitive hybridization kinetics to extract the association and dissociation rate constants of the unlabeled species from the distribution of time intervals between hybridization events of the labeled tracer DNA at individual localized DNA probe sites. We use this methodology to determine the hybridization kinetics of a model 11-mer unlabeled target DNA strand and then determine how five different fluorescent labels attached to the same target DNA strand impact the hybridization kinetics. Compared to the unlabeled target, these labels can slow the association and dissociation rates by as much as a factor of 5. The super-resolution time-interval methodology provides a unique approach to determining fundamental (label-free) rates of DNA hybridization, revealing the significant influence of fluorescent labels on these kinetics. This measurement concept can be extended to studies of other reversible reaction systems, where kinetics of unlabeled species can be determined from their influence on the reaction of a labeled species with localized probe molecules on a surface.

Dynamics and molecular interactions of single-stranded DNA in nucleic acid biosensors with varied surface properties

Electrochemical DNA biosensors utilizing self-assembled monolayers (SAMs) with inserted DNA probes are promising biosensor designs because of their ease of preparation, miniaturization, and tunability. However, much is still unknown about the interactions between biomolecules such as DNA and various surfaces. A fundamental question regarding these sensors concerns the nature of diffusion of target molecules taking place on sensor surfaces and whether it speeds up the molecular recognition process. Lack of understanding of molecular interaction and surface diffusion in addition to questions regarding the behavior of DNA probes immobilized on these surfaces currently limits the rational design of nucleic acid biosensors. Using all-atom unbiased molecular dynamics (MD) simulations we found that single-stranded DNA (ssDNA) behavior on SAMs is drastically altered by different surface chemistries, with ssDNA adopting very different orientations upon adsorption and surface diffusivity varying over an order of magnitude. Probe behavior varies equally broadly as probes are considerably more stable in certain SAMs than others, which affects the accessibility of probes to the target molecules and likely changes DNA hybridization kinetics in multiple ways. We also found that nearby probes can alter each other's orientations substantially, which highlights the importance of surface density control. Our results elucidate nucleic acid biosensor dynamics vital to rational design and offer insights that can aid in the design of surface properties and patterning for specific applications.

Surface-Mediated DNA Hybridization: Effects of DNA Conformation, Surface Chemistry, and Electrostatics

Single-molecule Förster Resonance Energy Transfer (FRET) was used to study the dynamic association of mobile donor-labeled ssDNA oligonucleotides ("target") with covalently immobilized complementary acceptor-labeled ssDNA oligonucleotides ("probe"). While probe-target association events were resolved for all experiments, such FRET events were far more likely to occur in systems with complementarity and on hydrophobic, as compared to hydrophilic, surfaces. The distribution of donor-acceptor association-time intervals did not exhibit simple first-order kinetics, and when decomposed into a superposition of first-order processes, only a small fraction of events corresponded to a long-lived state that was presumed to represent true DNA hybridization, while the majority of association events were transient, representing nonspecific associations or partial hybridization. The structure of the DNA target and probe affected both the stability of the hybridized state, as well as the likelihood that an association between the two led to hybridization. In particular, the likelihood of hybridization decreased for longer target strands and for targets with stem-loop secondary structure. The presence of oligonucleotide secondary structure reduced the stability of hybridization, while greater complementarity increased stability of the hybridized state. Interestingly, increased ionic strength (i.e., greater electrostatic screening) increased the probability of hybridization but did not influence the lifetime of the hybridized state. Combined, these observations provide a nuanced view of surface-mediated DNA hybridization, where various factors independently influence the probability and stability of hybridization.

Connecting Protein Conformation and Dynamics with Ligand-Receptor Binding Using Three-Color Förster Resonance Energy Transfer Tracking

Specific binding between biomolecules, i.e., molecular recognition, controls virtually all biological processes including the interactions between cells and biointerfaces, both natural and synthetic. Such binding often relies on the conformation of biomacromolecules, which can be highly heterogeneous and sensitive to environmental perturbations, and therefore difficult to characterize and control. An approach is demonstrated here that directly connects the binding kinetics and stability of the protein receptor integrin α_vβ₃ to the conformation of the ligand fibronectin (FN), which are believed to control cellular mechanosensing. Specifically, we investigated the influence of surface-adsorbed FN structure and dynamics on α_vβ₃ binding using high-throughput single-molecule three-color Förster resonance energy transfer (FRET) tracking methods. By controlling FN structure and dynamics through tuning surface chemistry, we found that as the conformational and translational dynamics of FN increased, the rate of binding, particularly to folded FN, and stability of the bound FN-α_vβ₃ complex decreased significantly. These findings highlight the importance of the conformational plasticity and accessibility of the arginine-glycine-aspartic acid (RGD) binding site in FN, which, in turn, mediates cell signaling in physiological and synthetic environments.

3D single-molecule tracking enables direct hybridization kinetics measurement in solution

Single-molecule measurements of DNA hybridization kinetics are mostly performed on a surface or inside a trap. Here we demonstrate a time-resolved, 3D single-molecule tracking (3D-SMT) method that allows us to follow a freely diffusing ssDNA molecule in solution for hundreds of milliseconds or even seconds and observe multiple annealing and melting events taking place on the same molecule. This is achieved by combining confocal-feedback 3D-SMT with time-domain fluorescence lifetime measurement, where fluorescence lifetime serves as the indicator of hybridization. With sub-diffraction-limit spatial resolution in molecular tracking and 15 ms temporal resolution in monitoring the change of reporter's lifetime, we have demonstrated a full characterization of annealing rate (kon = 5.13 × 106 M-1 s-1), melting rate (koff = 9.55 s-1), and association constant (Ka = 0.54 μM-1) of an 8 bp duplex model system diffusing at 4.8 μm2 s-1. As our method completely eliminates the photobleaching artifacts and diffusion interference, our kon and koff results well represent the real kinetics in solution. Our binding kinetics measurement can be carried out in a low signal-to-noise ratio condition (SNR ≈ 1.4) where ∼130 recorded photons are sufficient for a lifetime estimation. Using a population-level analysis, we can characterize hybridization kinetics over a wide range (0.5-125 s-1), even beyond the reciprocals of the lifetime monitoring temporal resolution and the average track duration.

Effects of Chain-Chain Associations on Hybridization in DNA Brushes

Hybridization of solution nucleic acids to DNA brushes is widely encountered in diagnostic and materials science applications. Typically, brush chain lengths of ten or more nucleotides are used to provide the needed sequence specificity and binding affinity. At these lengths, coincidental occurrence of complementary regions is expected to lead to associations between the nominally single-stranded brush chains due to intra- or interchain base pairing. This report investigates how these associations impact the brushes' hybridization activity toward complementary "target" sequences. Brushes were prepared from 20-mer chains with four-nucleotide-long "adhesive regions" through which neighboring chains could interact. The affinity and position of the adhesive region along the chain backbone were varied. DNA brushes were exposed to complementary solution targets, and the corresponding melting transitions were measured to estimate free energies of the brush-target hybridization. These results revealed that higher affinity adhesive regions more extensively suppressed brush hybridization relative to hybridization in solution. Associations near the middle of the chains were found to be more penalizing than those at the immobilized or the free end of the chains. Provided that the brush chains were close enough to associate, changes in brush density did not exert a significant effect on hybridization thermodynamics within the investigated coverage window. Comparison of the DNA brush results with those from commercial Affymetrix single-nucleotide-polymorphism (SNP) microarrays revealed agreement in the impact of chain associations on hybridization.

Toehold-Mediated Displacement of an Adenosine-Binding Aptamer from a DNA Duplex by its Ligand

DNA is increasingly used to engineer dynamic nanoscale circuits, structures, and motors, many of which rely on DNA strand-displacement reactions. The use of functional DNA sequences (e.g., aptamers, which bind to a wide range of ligands) in these reactions would potentially confer responsiveness on such devices, and integrate DNA computation with highly varied molecular stimuli. By using high-throughput single-molecule FRET methods, we compared the kinetics of a putative aptamer-ligand and aptamer-complement strand-displacement reaction. We found that the ligands actively disrupted the DNA duplex in the presence of a DNA toehold in a similar manner to complementary DNA, with kinetic details specific to the aptamer structure, thus suggesting that the DNA strand-displacement concept can be extended to functional DNA-ligand systems.

Surface vs. solution hybridization: effects of salt, temperature, and probe type

Hybridization thermodynamics on solid supports are compared with those in solution for two types of hybridization probe, DNA and uncharged morpholino oligonucleotides of identical sequences. Trends in hybridization affinity are discussed with respect to ionic strength, temperature, and surface behavior.

Label-free detection of DNA single-base mismatches using a simple reflectance-based optical technique

Rapid and quantitative detection of the binding of nucleic acids to surface-immobilized probes remains a challenge in many biomedical applications. We investigated the hybridization of a set of fully complementary and defected 12-base long DNA oligomers by using the Reflective Phantom Interface (RPI), a recently developed multiplexed label-free detection technique. Based on the simple measurement of reflected light intensity, this technology enables to quantify the hybridization directly as it occurs on the surface with a sensitivity of 10 pg mm(-2). We found a strong effect of single-base mismatches and of their location on hybridization kinetics and equilibrium binding. In line with previous studies, we found that DNA-DNA binding is weaker on a surface than in the bulk. Our data indicate that this effect is a consequence of weak nonspecific binding of the probes to the surface.

Dense Poly(ethylene glycol) Brushes Reduce Adsorption and Stabilize the Unfolded Conformation of Fibronectin

Polymer brushes, in which polymers are end-tethered densely to a grafting surface, are commonly proposed for use as stealth coatings for various biomaterials. However, although their use has received considerable attention, a mechanistic understanding of the impact of brush properties on protein adsorption and unfolding remains elusive. We investigated the effect of the grafting density of poly(ethylene glycol) (PEG) brushes on the interactions of the brush with fibronectin (FN) using high-throughput single-molecule tracking methods, which directly measure protein adsorption and unfolding within the brush. We observed that, as grafting density increased, the rate of FN adsorption decreased; however, surface-adsorbed FN unfolded more readily, and unfolded molecules were retained on the surface for longer residence times relative to those of folded molecules. These results, which are critical for the rational design of PEG brushes, suggest that there is a critical balance between protein adsorption and conformation that underlies the utility of such brushes in physiological environments.

Single-Molecule Fluorescence Imaging of Interfacial DNA Hybridization Kinetics at Selective Capture Surfaces

Accurate knowledge of the kinetics of complementary oligonucleotide hybridization is integral to the design and understanding of DNA-based biosensors. In this work, single-molecule fluorescence imaging is applied to measuring rates of hybridization between fluorescently labeled target ssDNA and unlabeled probe ssDNA immobilized on glass surfaces. In the absence of probe site labeling, the capture surface must be highly selective to avoid the influence of nonspecific adsorption on the interpretation of single-molecule imaging results. This is accomplished by increasing the probe molecule site densities by a factor of ∼100 compared to optically resolvable sites so that nonspecific interactions compete with a much greater number of capture sites and by immobilizing sulfonate groups to passivate the surface between probe strands. The resulting substrates exhibit very low nonspecific adsorption, and the selectivity for binding a complementary target sequence exceeds that of a scrambled sequence by nearly 3 orders of magnitude. The population of immobilized DNA probe sites is quantified by counting individual DNA duplexes at low target concentrations, and those results are used to calibrate fluorescence intensities on the same sample at much higher target concentrations to measure a full binding isotherm. Dissociation rates are determined from interfacial residence times of individual DNA duplexes. Equilibrium and rate constants of hybridization, K(a) = 38 (±1) μM(-1), k(on) = 1.64 (±0.06) × 10(6) M(-1) s(-1), and k(off) = 4.3 (±0.1) × 10(-2) s(-1), were found not to change with surface density of immobilized probe DNA, indicating that hybridization events at neighboring probe sites are independent. To test the influence of probe-strand immobilization on hybridization, the kinetics of the probe target reaction at the surface were compared with the same reaction in free solution, and the equilibrium constants and dissociation and association rates were found to be nearly equivalent. The selectivity of these capture surfaces should facilitate sensitive investigations of DNA hybridization at the limit of counting molecules. Because the immobilized probe DNA on these surfaces is unlabeled, photobleaching of a probe label is not an issue, allowing capture substrates to be used for long periods of time or even reused in multiple experiments.

Molecular and Thermodynamic Factors Explain the Passivation Properties of Poly(ethylene glycol)-Coated Substrate Surfaces against Fluorophore-Labeled DNA Oligonucleotides

Poly(ethylene glycol) (PEG) nanofilms are used to avert the nonspecific binding of biomolecules on substrate surfaces in biomedicine and bioanalysis including modern fluorescence-based DNA sensing and sequencing chips. A fundamental and coherent understanding of the interactions between fluorophore-tagged DNA, PEG-films, and substrates in terms of molecular and energetic factors is, however, missing. Here we explore a large parameter space to elucidate how PEG layers passivate metal oxide surfaces against Cy3-labeled DNA probes. The driving force for probe adsorption is found to be the affinity of the fluorophore to the substrate, while the high-quality PEG films prevent adsorption to bare ITO surfaces. The amount of nonrepelled, surface-bound DNA strongly depends on oligonucleotide size, PEG chain length, and incubation temperature. To explain these observations, we develop an experimentally validated theory to provide a microscopic picture of the PEG layer and show that adsorbed DNA molecules reside within the film by end-tethering the fluorophore to the ITO surface. To compensate for the local accumulation of negatively charged DNA, counterions condense on the adsorbed probes within the layer. The model furthermore explains that surface passivation is governed by the interdependence of molecular size, conformation, charge, ion condensation, and environmental conditions. We finally report for the first time on the detailed thermodynamic values that show how adsorption results from a balance between large opposing energetic factors. The insight of our study can be applied to rationally engineer PEG nanolayers for improved functional performance in DNA analysis schemes and may be expanded to other polymeric thin films.

Capturing Conformation-Dependent Molecule-Surface Interactions When Surface Chemistry Is Heterogeneous

Molecular building blocks, such as carbon nanotubes and DNA origami, can be fully integrated into electronic and optical devices if they can be assembled on solid surfaces using biomolecular interactions. However, the conformation and functionality of biomolecules depend strongly on the local chemical environment, which is highly heterogeneous near a surface. To help realize the potential of biomolecular self-assembly, we introduce here a technique to spatially map molecular conformations and adsorption, based on single-molecule fluorescence microscopy. On a deliberately patterned surface, with regions of varying hydrophobicity, we characterized the conformations of adsorbed helicogenic alanine-lysine copeptides using Förster resonance energy transfer. The peptides adopted helical conformations on hydrophilic regions of the surface more often than on hydrophobic regions, consistent with previous ensemble-averaged observations of α-helix surface stability. Interestingly, this dependence on surface chemistry was not due to surface-induced unfolding, as the apparent folding and unfolding dynamics were usually much slower than desorption. The most significant effect of surface chemistry was on the adsorption rate of molecules as a function of their initial conformational state. In particular, regions with higher adsorption rates attracted more molecules in compact, disordered coil states, and this difference in adsorption rates dominated the average conformation of the ensemble. The correlation between adsorption rate and average conformation was also observed on nominally uniform surfaces. Spatial variations in the functional state of adsorbed molecules would strongly affect the success rates of surface-based molecular assembly and can be fully understood using the approach developed in this work.

Optimized assembly and covalent coupling of single-molecule DNA origami nanoarrays

Artificial DNA nanostructures, such as DNA origami, have great potential as templates for the bottom-up fabrication of both biological and nonbiological nanodevices at a resolution unachievable by conventional top-down approaches. However, because origami are synthesized in solution, origami-templated devices cannot easily be studied or integrated into larger on-chip architectures. Electrostatic self-assembly of origami onto lithographically defined binding sites on Si/SiO2 substrates has been achieved, but conditions for optimal assembly have not been characterized, and the method requires high Mg2+ concentrations at which most devices aggregate. We present a quantitative study of parameters affecting origami placement, reproducibly achieving single-origami binding at 94±4% of sites, with 90% of these origami having an orientation within ±10° of their target orientation. Further, we introduce two techniques for converting electrostatic DNA-surface bonds to covalent bonds, allowing origami arrays to be used under a wide variety of Mg2+-free solution conditions.

Single-molecule FRET studies of HIV TAR-DNA hairpin unfolding dynamics

We directly measure the dynamics of the HIV trans-activation response (TAR)-DNA hairpin with multiple loops using single-molecule Förster resonance energy transfer (smFRET) methods. Multiple FRET states are identified that correspond to intermediate melting states of the hairpin. The stability of each intermediate state is calculated from the smFRET data. The results indicate that hairpin unfolding obeys a "fraying and peeling" mechanism, and evidence for the collapse of the ends of the hairpin during folding is observed. These results suggest a possible biological function for hairpin loops serving as additional fraying centers to increase unfolding rates in otherwise stable systems. The experimental and analytical approaches developed in this article provide useful tools for studying the mechanism of multistate DNA hairpin dynamics and of other general systems with multiple parallel pathways of chemical reactions.

Mechanisms of surface-mediated DNA hybridization

Single-molecule total internal reflection fluorescence microscopy was employed in conjunction with resonance energy transfer (RET) to observe the dynamic behavior of donor-labeled ssDNA at the interface between aqueous solution and a solid surface decorated with complementary acceptor-labeled ssDNA. At least 100,000 molecular trajectories were determined for both complementary strands and negative control ssDNA. RET was used to identify trajectory segments corresponding to the hybridized state. The vast majority of molecules from solution adsorbed nonspecifically to the surface, where a brief two-dimensional search was performed with a 7% chance of hybridization. Successful hybridization events occurred with a characteristic search time of ∼0.1 s, and unsuccessful searches resulted in desorption from the surface, ultimately repeating the adsorption and search process. Hybridization was reversible, and two distinct modes of melting (i.e., dehybridization) were observed, corresponding to long-lived (∼15 s) and short-lived (∼1.4 s) hybridized time intervals. A strand that melted back onto the surface could rehybridize after a brief search or desorb from the interface. These mechanistic observations provide guidance for technologies that involve DNA interactions in the near-surface region, suggesting a need to design surfaces that both enhance the complex multidimensional search process and stabilize the hybridized state.

A bottom-up approach to understanding protein layer formation at solid-liquid interfaces

A common goal across different fields (e.g. separations, biosensors, biomaterials, pharmaceuticals) is to understand how protein behavior at solid-liquid interfaces is affected by environmental conditions. Temperature, pH, ionic strength, and the chemical and physical properties of the solid surface, among many factors, can control microscopic protein dynamics (e.g. adsorption, desorption, diffusion, aggregation) that contribute to macroscopic properties like time-dependent total protein surface coverage and protein structure. These relationships are typically studied through a top-down approach in which macroscopic observations are explained using analytical models that are based upon reasonable, but not universally true, simplifying assumptions about microscopic protein dynamics. Conclusions connecting microscopic dynamics to environmental factors can be heavily biased by potentially incorrect assumptions. In contrast, more complicated models avoid several of the common assumptions but require many parameters that have overlapping effects on predictions of macroscopic, average protein properties. Consequently, these models are poorly suited for the top-down approach. Because the sophistication incorporated into these models may ultimately prove essential to understanding interfacial protein behavior, this article proposes a bottom-up approach in which direct observations of microscopic protein dynamics specify parameters in complicated models, which then generate macroscopic predictions to compare with experiment. In this framework, single-molecule tracking has proven capable of making direct measurements of microscopic protein dynamics, but must be complemented by modeling to combine and extrapolate many independent microscopic observations to the macro-scale. The bottom-up approach is expected to better connect environmental factors to macroscopic protein behavior, thereby guiding rational choices that promote desirable protein behaviors.

Structure and dynamics of single hydrophobic/ionic heteropolymers at the vapor-liquid interface of water

We focus on the conformational stability, structure, and dynamics of hydrophobic/charged homopolymers and heteropolymers at the vapor-liquid interface of water using extensive molecular dynamics simulations. Hydrophobic polymers collapse into globular structures in bulk water but unfold and sample a broad range of conformations at the vapor-liquid interface of water. We show that adding a pair of charges to a hydrophobic polymer at the interface can dramatically change its conformations, stabilizing hairpinlike structures, with molecular details depending on the location of the charged pair in the sequence. The translational dynamics of homopolymers and heteropolymers are also different, whereas the homopolymers skate on the interface with low drag, the tendency of charged groups to remain hydrated pulls the heteropolymers toward the liquid side of the interface, thus pinning them, increasing drag, and slowing the translational dynamics. The conformational dynamics of heteropolymers are also slower than that of the homopolymer and depend on the location of the charged groups in the sequence. Conformational dynamics are most restricted for the end-charged heteropolymer and speed up as the charge pair is moved toward the center of the sequence. We rationalize these trends using the fundamental understanding of the effects of the interface on primitive pair-level interactions between two hydrophobic groups and between oppositely charged ions in its vicinity.

BIOPHYSICAL PROPERTIES OF NUCLEIC ACIDS AT SURFACES RELEVANT TO MICROARRAY PERFORMANCE

Both clinical and analytical metrics produced by microarray-based assay technology have recognized problems in reproducibility, reliability and analytical sensitivity. These issues are often attributed to poor understanding and control of nucleic acid behaviors and properties at solid-liquid interfaces. Nucleic acid hybridization, central to DNA and RNA microarray formats, depends on the properties and behaviors of single strand (ss) nucleic acids (e.g., probe oligomeric DNA) bound to surfaces. ssDNA's persistence length, radius of gyration, electrostatics, conformations on different surfaces and under various assay conditions, its chain flexibility and curvature, charging effects in ionic solutions, and fluorescent labeling all influence its physical chemistry and hybridization under assay conditions. Nucleic acid (e.g., both RNA and DNA) target interactions with immobilized ssDNA strands are highly impacted by these biophysical states. Furthermore, the kinetics, thermodynamics, and enthalpic and entropic contributions to DNA hybridization reflect global probe/target structures and interaction dynamics. Here we review several biophysical issues relevant to oligomeric nucleic acid molecular behaviors at surfaces and their influences on duplex formation that influence microarray assay performance. Correlation of biophysical aspects of single and double-stranded nucleic acids with their complexes in bulk solution is common. Such analysis at surfaces is not commonly reported, despite its importance to microarray assays. We seek to provide further insight into nucleic acid-surface challenges facing microarray diagnostic formats that have hindered their clinical adoption and compromise their research quality and value as genomics tools.

Interfacial protein-protein associations

While traditional models of protein adsorption focus primarily on direct protein-surface interactions, recent findings suggest that protein-protein interactions may play a central role. Using high-throughput intermolecular resonance energy transfer (RET) tracking, we directly observed dynamic, protein-protein associations of bovine serum albumin on polyethylene glycol modified surfaces. The associations were heterogeneous and reversible, and associating molecules resided on the surface for longer times. The appearance of three distinct RET states suggested a spatially heterogeneous surface - with areas of high protein density (i.e., strongly interacting clusters) coexisting with mobile monomers. Distinct association states exhibited characteristic behavior, i.e., partial-RET (monomer-monomer) associations were shorter-lived than complete-RET (protein-cluster) associations. While the fractional surface area covered by regions with high protein density (i.e., clusters) increased with increasing concentration, the distribution of contact times between monomers and clusters was independent of solution concentration, suggesting that associations were a local phenomenon, and independent of the global surface coverage.

Single-molecule resolution of protein structure and interfacial dynamics on biomaterial surfaces

A method was developed to monitor dynamic changes in protein structure and interfacial behavior on surfaces by single-molecule Förster resonance energy transfer. This method entails the incorporation of unnatural amino acids to site-specifically label proteins with single-molecule Förster resonance energy transfer probes for high-throughput dynamic fluorescence tracking microscopy on surfaces. Structural changes in the enzyme organophosphorus hydrolase (OPH) were monitored upon adsorption to fused silica (FS) surfaces in the presence of BSA on a molecule-by-molecule basis. Analysis of >30,000 individual trajectories enabled the observation of heterogeneities in the kinetics of surface-induced OPH unfolding with unprecedented resolution. In particular, two distinct pathways were observed: a majority population (∼ 85%) unfolded with a characteristic time scale of 0.10 s, and the remainder unfolded more slowly with a time scale of 0.7 s. Importantly, even after unfolding, OPH readily desorbed from FS surfaces, challenging the common notion that surface-induced unfolding leads to irreversible protein binding. This suggests that protein fouling of surfaces is a highly dynamic process because of subtle differences in the adsorption/desorption rates of folded and unfolded species. Moreover, such observations imply that surfaces may act as a source of unfolded (i.e., aggregation-prone) protein back into solution. Continuing study of other proteins and surfaces will examine whether these conclusions are general or specific to OPH in contact with FS. Ultimately, this method, which is widely applicable to virtually any protein, provides the framework to develop surfaces and surface modifications with improved biocompatibility.