Synthetic oligopeptides containing π -conjugated cores self-assemble novel materials with attractive electronic and photophysical properties. All-atom, explicit solvent molecular dynamics simulations of Asp-Phe-Ala-Gly-OPV3-Gly-Ala-Phe-Asp peptides were used to parameterise an implicit solvent model to simulate early-stage self-assembly. Under low-pH conditions, peptides assemble into β-sheet-like stacks with strongly favorable monomer association free energies of F ≈ −25k B T . Aggregation at high-pH produces disordered aggregates destabilised by Coulombic repulsion between negatively charged Asp termini ( F ≈ −5k B T ). In simulations of hundreds of monomers over 70 ns we observe the spontaneous formation of up to undecameric aggregates under low-pH conditions. Modeling assembly as a continuoustime Markov process, we infer transition rates between different aggregate sizes and microsecond relaxation times for early-stage assembly. Our data suggests a hierarchical model of assembly in which peptides coalesce into small clusters over tens of nanoseconds followed by structural ripening and diffusion limited aggregation on longer time scales. This work provides new molecular-level understanding of early-stage assembly, and a means to study the impact of peptide sequence and aromatic core chemistry upon the thermodynamics, assembly kinetics, and morphology of the supramolecular aggregates.
ARTICLE HISTORY
Low-noise, tunable wavelength-conversion through nondegenerate four-wave mixing Bragg scattering in SiN(x) waveguides is experimentally demonstrated. Finite element method simulations of waveguide dispersion are used with the split-step Fourier method to predict device performance. Two 1550 nm wavelength band pulsed pumps are used to achieve tunable conversion of a 980 nm signal over a range of 5 nm with a peak conversion efficiency of ≈5%. The demonstrated Bragg scattering process is suitable for frequency conversion of quantum states of light.
We
describe a set of precise single-ion conducting polymers that form
self-assembled percolated ionic aggregates in glassy polymer matrices
and have decoupled transport of metal cations. These precise single-ion
conductors (SICs), synthesized by a scalable ring-opening metathesis
polymerization, consist of a polyethylene backbone with a sulfonated
phenyl group pendant on every fifth carbon and are fully neutralized
by a counterion X+ (Li+, Na+, or
Cs+). Experimental X-ray scattering measurements and fully
atomistic molecular dynamics (MD) simulations are in good agreement.
The MD simulations show that the ionic groups nanophase separate from
the polymer backbone to form percolating ionic aggregates. Using graph
theory, we find that within the Li+- and Na+-neutralized polymers the percolated aggregates exhibit planar and
ribbon-like configurations at intermediate length scales, while the
percolated aggregates within the Cs+-neutralized polymers
are more isotropic. Electrical impedance spectroscopy measurements
show that the ionic conductivities exhibit Arrhenius behavior, with
conductivities of 10–7 to 10–6 S/cm at 180 °C. In the MD simulations, the cations move between
sulfonate groups in the percolated aggregates, larger ions travel
further, and overall cations travel further than the polymer backbones,
indicating a decoupled ion-transport mechanism. Thus, the percolated
ionic aggregates in these polymers can serve as pathways to facilitate
decoupled ion motion through a glassy polymer matrix.
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