Session S04: Single-Molecule Characterization in Polymers and Soft Matter: Morphology and Mechanics

­­Molecular dynamics simulations of the self-assembly of polypeptoid nanocrystals

The formation of nanosheets, nanofibrils and nanotubes have been reported for amphiphilic diblock copolypeptoids. To tune the morphology through chemical variations, it is important to know the atomic details of the self-assembly. This work utilizes molecular dynamic simulations to study the morphologies of the solvated poly(N-decylglycine)-b-poly(N-2-(2-(2-methoxyethoxy)ethoxy)ethylglycine) [Ndc-Nte] in aqueous solutions. Computational models were established based on previously developed force fields and measured cryoTEM images. Ndc blocks were preassembled into nanocrystals, while Nte blocks were left to extend into the solvent. We explored different thicknesses of nanocrystals to determine thermodynamic origins for preferential nanofiber formation vs. extended nanosheets. To explain the origins of these thermodynamic differences, we explored factors which are seen to control nanoscale morphology in experiment, but which are not evident in imaging due to inherent disorder: (1) functionalization of the N-terminus; (2) the morphology of the disordered Nte blocks; and (3) the presence and possible aggregation of urea molecules at the nanocrystal/solvent interface.    

Single Molecule Measurements of the Morphology and Stiffness of Acid-Hydrolyzed Phytoglycogen Nanoparticles

Phytoglycogen (PG) is a soft and highly branched nanoparticle that is produced in the kernels of sweet corn. Its deformability, biocompatibility and digestibility make PG nanoparticles ideal for applications in personal care, nutraceuticals, and drug delivery. Chemical modifications can be used to tune the physical and chemical properties of the particles, adapting the nanoparticles for specific applications. We have explored the effect of acid hydrolysis, a process in which bonds between the glucose subunits can be broken through exposure to dilute acid at high temperature, and measured changes to the radius, deformability, and outer chain length of the PG particles. By performing atomic force microscopy (AFM) force spectroscopy, we have generated high resolution maps of the Young's modulus E of the hydrated acid-hydrolyzed PG nanoparticles, characterizing the dependence of the particles' size and deformability on the hydrolysis time. Intermediate hydrolysis times produced changes in both the inner and outer regions of PG, whereas long hydrolysis times produced significant overall decreases in E.   

Atomic-scale imaging of self-assembled polypeptoid crystals with varying molecular side chains

Crystal engineering requires a knowledge of the weak noncovalent interactions that produce a three-dimensional crystal structure. In spite of significant progress, it is not yet possible to design molecules that would yield a targeted crystal structure due to uncertainties in the relative importance of these interactions. Polypeptoids are sequence-defined polymers that have a similar structure to peptides, except the side chain is appended to the nitrogen rather than the alpha carbon in the backbone. When amphiphilic diblock copolypeptoids are dissolved in water, they self-assemble into well-ordered planar crystals. The resulting crystals are sensitive to radiation damage when imaged in a transmission electron microscope. In order to achieve atomic-scale images of these crystals, techniques from the structural biology community are adopted to produce images without damaging the crystals. In this work, a series of polypeptoids with a phenyl, amino, or methyl group in the side chain is created and the crystal structure is studied using atomic-scale TEM images.  The atomic-scale information obtained from TEM images enriches our knowledge of the competing factors that determine polypeptoid crystal structure and is a step towards enabling crystal engineering of these materials.  

    

Super Resolution Imaging Studies of Spatially Heterogeneous Thermosensitive Gels

The impact of spatial variations in crosslink densities within polymer gels are often masked by ensemble averaged measurements. This talk will present insights into the nature, origin and impact on mechanical properties of these network inhomogeneities from super resolution microscopy studies of model thermosensitive PNIPAM gels.   

Single-Molecule Force Experiments Reveal Effects of Sequence Charge on Polyelectrolyte Conformation

For sequence-defined polyelectrolytes, modulating their charge location and patterning can significantly alter their physical properties and functionality. This calls for a fundamental understanding of how charge sequence influences polyelectrolyte conformation. Single-molecule force experiments can provide a unique perspective on the polymer sequence-structure relationship through direct probing of polymer structures and interactions. Here, by designing and testing simple charged polypeptoid sequences, we investigate the effect of net charge and charge spacing on the conformational behaviors of a flexible polyelectrolyte. Surprisingly, we see that the peptoid net charge does not significantly affect polymer stiffness (persistence length) and extent (Flory exponent) at low ionic strength. Instead, charge spacing is the primary driver of peptoid conformation in solution. Sequences with shorter charge spacings transition to ideal chain behavior at higher ionic strengths. We find that the theta-point of all charged sequences can be normalized to when their charge spacings are equal to the Debye screening lengths. Overall, this study challenges conventional wisdom on polyelectrolyte conformation and demonstrates the use of single-molecule force experiment as a useful tool to study polymer sequence-conformation relationship.   

Molecular-scale mechanical properties of calcium-responsive proteins

Chemically responsive proteins are responsible for many biological processes, but the molecular-scale mechanics of ion-driven protein folding remain elusive. Repeats-in-Toxin (RTX) proteins respond to calcium by undergoing conformational changes from random coils to folded beta-roll structures upon binding to calcium ions. We have generated fusion proteins containing RTX domains that replicate the calcium-responsive behavior of naturally occurring RTX proteins. Using atomic force microscopy (AFM), we demonstrate single-molecule force spectroscopy (SMFS) of RTX domains. Proteins were genetically modified for SMFS to have terminal amino acid tags recognized by the enzyme Sortase A, for both enzyme-mediated polyprotein conjugation and specific molecular tethering between the AFM tip and substrate. We characterize the mechanical response of tethered RTX polyproteins in both their disordered calcium-free state and folded calcium-bound state, and we compare AFM force–extension curves to the worm-like chain model. Understanding the molecular-scale mechanical behavior of RTX proteins will enable the development of tunable biomaterials and other chemically responsive proteins.

    

Unraveling the mechanics of collagen

Collagen is the predominant structural protein in humans, representing over 25% of protein mass in our bodies. Its unique triple-helical structure has the ability to assemble into biomechanically important, higher-order structures, such as the extracellular matrix and connective tissues including tendon. How amino-acid sequence prescribes local mechanics of collagen is not known, yet is key for understanding mechanobiological signalling and remodelling of strained and damaged tissues.  We are investigating this link between sequence and mechanics, using atomic force microscopy (AFM) imaging of collagens and centrifuge force microscopy (CFM) for high-throughput force measurements. In this talk, I’ll present our first mapping of sequence to mechanics, and share insight into how this might influence the properties driving self-assembly of collagens into higher-order structures. I’ll also highlight the importance of environmental conditions on collagen’s mechanics and stability at the single-molecule level.

    

Effect of hydrogen bonding on single polymer growth

We investigated the effect of hydrogen bonding on the dynamics and kinetics of single polymer growth using magnetic tweezers. In the previous study, it was found that, during single polymer growth, nonequilibrium polymer entanglements were formed and unraveled due to the intra-chain van der Waals force, and the microstructure of the entanglements was found to affect the apparent chain propagation rate. To demonstrate how the intra-chain interactions affect the chain propagation rate, we introduced hydroxyl-functionalized monomers and studied the effect of the hydrogen bonding on the chain propagation rate. The results showed that the chain propagation rate first decrease and then increase with increasing hydroxyl-functionalized monomer ratio unexpectedly. This phenomenon can be explained by the competition between the intra-chain hydrogen bond and the inter-molecular hydrogen bond between polymer and free monomers in solution.

    

Single-molecule imaging in commercial stationary phase particles using highly inclined and laminated optical sheet microscopy

Chromatographic separations often rely on porous materials coated with covalently bound polymers to bind and separate analytes from mixtures based on specific forces. Two-dimensional single-molecule and super-resolution microscopy has previously quantified causes of peak broadening in separations, but prior studies have focused in model surfaces that have different physical and chemical properties than the three-dimensional materials used in real columns and membranes. In this work we resolve the three-dimensional, nanoscale locations of single-molecule analytes within unpacked commercial chiral stationary phase particles (CSPs) using highly inclined and laminated optical sheet (HILO) microscopy. We show improvements up to 32% in signal-to-background ratio and 118% in the number of single molecules detected within the CSPs when using HILO compared to epifluorescence. We also generate 3D super-resolution maps of the single-molecule sorption within the particles with nanoscale lateral and axial resolutions, clearly visualizing surface heterogeneities and distinct sorption sites. We observe that the number of localized molecules remains constant axially and between different particles, indicating that heterogeneity in a separation would not derive from such affinity differences at microscales, but instead kinetic differences at nanoscales. Further, the polymer-coated CSPs limit analyte diffusion within the particle, suggesting CSPs may be acting as superficially porous particles compared to results with uncoated porous silica stationary phases. While this work focuses on 5 µm CSPs, HILO can be applied to broad classes of real polymeric and soft materials to inform separations from the bottom-up.

    

Adsorption and Packing of Non-Spherical Nanoparticles at Liquid-Air Interfaces

The near 2D assembly of interfacial ellipsoidal (NE) and disk-like (ND) nanoparticles (NPs) was examined by variable-pressure SEM, and single-particle resolution was achieved at tunable areal density for NPs adsorbed on low-volatility liquids such as ionic liquids, glycerol, and tetra-ethylene glycol. Out-of-plane orientation of the silica-coated hematite NPs was noted for both NEs and NDs of aspect ratios across the range 3 to 6, with this orientation affected by molecular weight (MW) of grafted PEG; in-plane orientation increased at higher MW.  In systems of mixed orientation, in-plane NEs segregated into locally ordered domains while out-of-plane NEs were ejected into smaller disordered domains.  At jamming (areal density ~0.6), the in-plane NEs assembled into slightly curved but closed-packed stacks of ~40-60 NPs. Differently, in-plane NDs remain disordered but out-of-plane NDs preferentially assembled into columnar closed-packed stacks, with NDs oriented perpendicular to the liquid interface.  Overall, in-plane disorder was much higher for NDs than NEs. The same systems were probed by GISAXS and interfacial tensiometry, the latter establishing that vacuum is not a prerequisite for the interfacial assembly.

    

Author not Attending

In this talk, we intend to give an overview of the structure and dynamics of a single polymer chain in active baths, Gaussian or non-Gaussian. We consider a single flexible or semiflexible linear polymer chain subjected to two noises, thermal and active. The active noise has either Gaussian [1] or non-Gaussian [2] distribution but has a memory, accounting for the persistent motion of the active bath particles. This finite persistence makes the reconfiguration dynamics of the chain slow as compared to the purely thermal case and the chain swells. The active noise also results in superdiffusive or ballistic motion of the tagged monomer. In addition, we point out the relevance of polymer chains in active baths in biologically relevant contexts [3].   

Back to basics: a lattice framework for understanding coil-globule transitions in polymers

Coil-to-globule transitions in polymers have been observed in a wide range of systems and can be induced in a variety of ways (e.g., exposure to heat, addition of cosolvent). However, the precise physics and chemistries that determine whether coil-globule transitions occur is not fully understood, and to date, there is no universal theoretical or computational framework that can generically describe the transformation subject to multiple forms of stimuli. In this talk, I will describe a simple, lattice framework that allows us to capture and elucidate the physics of coil-globule transition induced either by heat or cononsolvency. By thorough investigation of numerous interaction types and energetic regimes, we provide a general roadmap that supports a broader understanding of stimuli-response in polymeric systems.