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1. Programming co-assembled peptide nanofiber morphology via anionic amino acid type: Insights from molecular dynamics simulations

   https://doi.org/10.1371/journal.pcbi.1011685  

   Dong, Xin Y.; Liu, Renjie; Seroski, Dillon T.; Hudalla, Gregory A.; Hall, Carol K. (December 2023, PLOS Computational Biology)

   Levy, Yaakov Koby (Ed.)

   Co-assembling peptides can be crafted into supramolecular biomaterials for use in biotechnological applications, such as cell culture scaffolds, drug delivery, biosensors, and tissue engineering. Peptide co-assembly refers to the spontaneous organization of two different peptides into a supramolecular architecture. Here we use molecular dynamics simulations to quantify the effect of anionic amino acid type on co-assembly dynamics and nanofiber structure in binary CATCH(+/-) peptide systems. CATCH peptide sequences follow a general pattern: CQCFCFCFCQC, where all C’s are either a positively charged or a negatively charged amino acid. Specifically, we investigate the effect of substituting aspartic acid residues for the glutamic acid residues in the established CATCH(6E-) molecule, while keeping CATCH(6K+) unchanged. Our results show that structures consisting of CATCH(6K+) and CATCH(6D-) form flatter β-sheets, have stronger interactions between charged residues on opposing β-sheet faces, and have slower co-assembly kinetics than structures consisting of CATCH(6K+) and CATCH(6E-). Knowledge of the effect of sidechain type on assembly dynamics and fibrillar structure can help guide the development of advanced biomaterials and grant insight into sequence-to-structure relationships. 

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2. Abnormal Association between Metal−Organic Cages and Counterions Regulated by the Hydration Shells

   https://doi.org/10.1002/chem.202104332  

   Yang, Yuqing; Raee, Ehsan; Liu, Tianbo (January 2022, Chemistry – A European Journal)

   Abstract A unique trend in the binding affinity between cationic metal−organic cages (MOCs) and external counteranions in aqueous media was observed. Similar to many macroions, two MOCs, sharing similar structures but carrying different number of charges, self‐assembled into hollow spherical single‐layered blackberry‐type structures through counterion‐mediated attraction. Dynamic and static light scattering and isothermal titration calorimetry measurements confirm the stronger interactions among less charged MOCs and counteranions than that of highly charged MOCs, leading to larger assembly sizes. DOSY NMR measurements suggest the significance of thick hydration shells of highly charged MOCs, inhibiting the MOC‐counterion binding and weakening the interaction between them. This study demonstrates that the greater role played by hydration shell on ion‐pair formation comparing with charge density of MOCs. 

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3. Peptide framework for screening the effects of amino acids on assembly

   https://doi.org/10.1126/sciadv.abj0305  

   Hamsici, Seren; White, Andrew D.; Acar, Handan (January 2022, Science Advances)

   Co-assembly of oppositely charged peptides (CoOP) links intermolecular interactions and material properties. 

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4. Mapping the phase behavior of coacervate-driven self-assembly in diblock copolyelectrolytes

   https://doi.org/10.1039/c9sm00741e  

   Ong, Gary M.; Sing, Charles E. (June 2019, Soft Matter)

   Oppositely-charged polymers can undergo an associative phase separation process known as complex coacervation, which is driven by the electrostatic attraction between the two polymer species. This driving force for phase separation can be harnessed to drive self-assembly, via pairs of block copolyelectrolytes with opposite charge and thus favorable coulombic interactions. There are few predictions of coacervate self-assembly phase behavior due to the wide variety of molecular and environmental parameters, along with fundamental theoretical challenges. In this paper, we use recent advances in coacervate theory to predict the solution-phase assembly of diblock polyelectrolyte pairs for a number of molecular design parameters (charged block fraction, polymer length). Phase diagrams show that self-assembly occurs at high polymer, low salt concentrations for a range of charge block fractions. We show that we qualitatively obtain limiting results seen in the experimental literature, including the emergence of a high polymer-fraction reentrant transition that gives rise to a self-compatibilized homopolymer coacervate behavior at the limit of high charge block fraction. In intermediate charge block fractions, we draw an analogy between the role of salt concentration in coacervation-driven assembly and the role of temperature in χ -driven assembly. We also explore salt partitioning between microphase separated domains in block copolyelectrolytes, with parallels to homopolyelectrolyte coacervation. 

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5. Self‐Propulsion by Directed Explosive Emulsification

   https://doi.org/10.1002/adma.202310435  

   Wu, Xuefei; Xue, Han; Bordia, Gautam; Fink, Zachary; Kim, Paul_Y; Streubel, Robert; Han, Jiale; Helms, Brett_A; Ashby, Paul_D; Omar, Ahmad_K; et al (February 2024, Advanced Materials)

   Abstract An active droplet system, programmed to repeatedly move autonomously at a specific velocity in a well‐defined direction, is demonstrated. Coulombic energy is stored in oversaturated interfacial assemblies of charged nanoparticle‐surfactants by an applied DC electric field and can be released on demand. Spontaneous emulsification is suppressed by an increase in the stiffness of the oversaturated assemblies. Rapidly removing the field releases the stored energy in an explosive event that propels the droplet, where thousands of charged microdroplets are ballistically ejected from the surface of the parent droplet. The ejection is made directional by a symmetry breaking of the interfacial assembly, and the combined interaction force of the microdroplet plume on one side of the droplet propels the droplet distances tens of times its size, making the droplet active. The propulsion is autonomous, repeatable, and agnostic to the chemical composition of the nanoparticles. The symmetry‐breaking in the nanoparticle assembly controls the microdroplet velocity and direction of propulsion. This mechanism of droplet propulsion will advance soft micro‐robotics, establishes a new type of active matter, and introduces new vehicles for compartmentalized delivery. 

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