Polydopamine is a biomimetic self-adherent polymer, which can be easily deposited on a wide variety of materials. Despite the rapidly increasing interest in polydopamine-based coatings, the ...polymerization mechanism and the key intermediate species formed during the deposition process are still controversial. Herein, we report a systematic investigation of polydopamine formation on halloysite nanotubes; the negative charge and high surface area of halloysite nanotubes favour the capture of intermediates that are involved in polydopamine formation and decelerate the kinetics of the process, to unravel the various polymerization steps. Data from X-ray photoelectron and solid-state nuclear magnetic resonance spectroscopies demonstrate that in the initial stage of polydopamine deposition, oxidative coupling reaction of the dopaminechrome molecules is the main reaction pathway that leads to formation of polycatecholamine oligomers as an intermediate and the post cyclization of the linear oligomers occurs subsequently. Furthermore, TRIS molecules are incorporated into the initially formed oligomers.
Chiral nanoparticle assemblies are an interesting class of materials whose chiroptical properties make them attractive for a variety of applications. Here, C18-(PEPAu M‑ox)2 (PEPAu M‑ox = ...AYSSGAPPMoxPPF) is shown to direct the assembly of single-helical gold nanoparticle superstructures that exhibit exceptionally strong chiroptical activity at the plasmon frequency with absolute g-factor values up to 0.04. Transmission electron microscopy (TEM) and cryogenic electron tomography (cryo-ET) results indicate that the single helices have a periodic pitch of approximately 100 nm and consist of oblong gold nanoparticles. The morphology and assembled structure of C18-(PEPAu M‑ox)2 are studied using TEM, atomic force microscopy (AFM), Fourier transform infrared (FTIR) spectroscopy, circular dichroism (CD) spectroscopy, X-ray diffraction (XRD), and solid-state nuclear magnetic resonance (ssNMR) spectroscopy. TEM and AFM reveal that C18-(PEPAu M‑ox)2 assembles into linear amyloid-like 1D helical ribbons having structural parameters that correlate to those of the single-helical gold nanoparticle superstructures. FTIR, CD, XRD, and ssNMR indicate the presence of cross-β and polyproline II secondary structures. A molecular assembly model is presented that takes into account all experimental observations and that supports the single-helical nanoparticle assembly architecture. This model provides the basis for the design of future nanoparticle assemblies having programmable structures and properties.
•Hydrogels made from polysaccharide polymers are widely used.•Their functional properties depend on their molecular structure and mobility.•Dynamic and responsive hydrogels are challenging for ...molecular characterization.•Modern solid-state NMR spectroscopy reveals structure and dynamics of hydrogels.•Information is obtained on the polysaccharides and also the associated water.
Hydrogels find application in many areas of technology and research due to their ability to combine responsiveness and robustness. A detailed understanding of their molecular structure and dynamics (which ultimately underpin their functional properties) is needed for their design to be optimized and these hydrogels to be exploited effectively. In this review, we shed light on the unique capabilities of solid-state NMR spectroscopy to reveal this information in molecular detail. We review recent literature on the advancements in solid-state NMR techniques in resolving the structure, degree of grafting, molecular organization, water-biopolymer interactions and internal dynamical behavior of hydrogels. Among various solid-state NMR techniques, 13C cross polarization (CP) magic angle spinning (MAS) NMR is examined for its ability to probe the hydrogel and its trapped solvent. Although widely applicable to many types of polymeric and supramolecular hydrogels, the current review focuses on polysaccharide-based hydrogels.
Lead halide perovskite nanocrystals are highly attractive for next‐generation optoelectronics because they are easy to synthesize and offer great compositional and morphological tunability. However, ...the replacement of lead by tin for sustainability reasons is hampered by the unstable nature of Sn2+ oxidation state and by an insufficient understanding of the chemical processes involved in the synthesis. Here, an optimized synthetic route is demonstrated to obtain stable, tunable, and monodisperse CsSnI3 nanocrystals, exhibiting well‐defined excitonic peaks. Similar to lead halide perovskites, these nanocrystals are prepared by combining a precursor mixture of SnI2, oleylamine, and oleic acid, with a Cs‐oleate precursor. Among the products, nanocrystals with 10 nm lateral size in the γ‐orthorhombic phase prove to be the most stable. To achieve such stability, an excess of precursor SnI2 as well as substoichiometric Sn:ligand ratios are key. Structural, compositional, and optical investigations complemented by first‐principle density functional theory calculations confirm that nanocrystal nucleation and growth follow the formation of (R‐NH3+)2SnI4 nanosheets, with R = C18H35. Under specific synthetic conditions, stable mixtures of 3D nanocrystals CsSnI3 and 2D nanosheets (Ruddlesden–Popper (R‐NH3+)2Csn−1SnnI3n+1 with n > 1) are obtained. These results set a path to exploiting the high potential of Sn halide perovskite nanocrystals for opto‐electronic applications.
The synthesis of stable, tunable, and monodisperse colloidal CsSnI3 nanocrystals or a mixture of 3D CsSnI3 nanocrystals and 2D Ruddlesden–Popper (R‐NH3+)2Csn−1SnnI3n+1 perovskite nanosheets is reported. The key factor in this chemical design is the nucleation and growth of the CsSnI3 nanocrystals from (R‐NH3+)2SnI4.
In Huntington’s disease, expansion of a polyglutamine (polyQ) domain in the huntingtin (htt) protein leads to misfolding and aggregation. There is much interest in the molecular features that ...distinguish monomeric, oligomeric, and fibrillar species that populate the aggregation pathway and likely differ in cytotoxicity. The mechanism and rate of aggregation are greatly affected by the domains flanking the polyQ segment within exon 1 of htt. A “protective” C-terminal proline-rich flanking domain inhibits aggregation by inducing polyproline II structure (PPII) within an extended portion of polyQ. The N-terminal flanking segment (httNT) adopts an α-helical structure as it drives aggregation, helps stabilize oligomers and fibrils, and is seemingly integral to their supramolecular assembly. Via solid-state nuclear magnetic resonance (ssNMR), we probe how, in the mature fibrils, the htt flanking domains impact the polyQ domain and in particular the localization of the β-structured amyloid core. Using residue-specific and uniformly labeled samples, we find that the amyloid core occupies most of the polyQ domain but ends just prior to the prolines. We probe the structural and dynamical features of the remarkably abrupt β-sheet to PPII transition and discuss the potential connections to certain htt-binding proteins. We also examine the httNT α-helix outside the polyQ amyloid core. Despite its presumed structural and demonstrated stabilizing roles in the fibrils, quantitative ssNMR measurements of residue-specific dynamics show that it undergoes distinct solvent-coupled motion. This dynamical feature seems reminiscent of molten-globule-like α-helix-rich features attributed to the nonfibrillar oligomeric species of various amyloidogenic proteins.
A number of recent advances in the field of magic-angle-spinning (MAS) solid-state NMR have enabled its application to a range of biological systems of ever increasing complexity. To retain ...biological relevance, these samples are increasingly studied in a hydrated state. At the same time, experimental feasibility requires the sample preparation process to attain a high sample concentration within the final MAS rotor. We discuss these considerations, and how they have led to a number of different approaches to MAS NMR sample preparation. We describe our experience of how custom-made (or commercially available) ultracentrifugal devices can facilitate a simple, fast and reliable sample preparation process. A number of groups have since adopted such tools, in some cases to prepare samples for sedimentation-style MAS NMR experiments. Here we argue for a more widespread adoption of their use for routine MAS NMR sample preparation.
The 17-residue N-terminus (httNT) directly flanking the polyQ sequence in huntingtin (htt) N-terminal fragments plays a crucial role in initiating and accelerating the aggregation process that is ...associated with Huntington’s disease pathogenesis. Here we report on magic-angle-spinning solid-state NMR studies of the amyloid-like aggregates of an htt N-terminal fragment. We find that the polyQ portion of this peptide exists in a rigid, dehydrated amyloid core that is structurally similar to simpler polyQ fibrils and may contain antiparallel β-sheets. In contrast, the httNT sequence in the aggregates is composed in part of a well-defined helix, which likely also exists in early oligomeric aggregates. Further NMR experiments demonstrate that the N-terminal helical segment displays increased dynamics and water exposure. Given its specific contribution to the initiation, rate, and mechanism of fibril formation, the helical nature of httNT and its apparent lack of effect on the polyQ fibril core structure seem surprising. The results provide new details about these disease-associated aggregates and also provide a clear example of an amino acid sequence that greatly enhances the rate of amyloid formation while itself not taking part in the amyloid structure. There is an interesting mechanistic analogy to recent reports pointing out the early-stage contributions of transient intermolecular helix−helix interactions in the aggregation behavior of various other amyloid fibrils.
•Solid-state NMR methods measure structure and dynamics in biological assemblies.•Dynamics affect NMR-detected resonances, interactions and relaxation properties.•Dynamics-based spectral editing ...isolates signals based on their relative dynamics.•Dynamic filtering enables simplification of complex spectra of large biomolecules.•Co-existing rigid and mobile molecules and domains are unambiguously distinguished.
Solid-state nuclear magnetic resonance (ssNMR) spectroscopy enables the structural characterization of a diverse array of biological assemblies that include amyloid fibrils, non-amyloid aggregates, membrane-associated proteins and viral capsids. Such biological samples feature functionally relevant molecular dynamics, which often affect different parts of the sample in different ways. Solid-state NMR experiments’ sensitivity to dynamics represents a double-edged sword. On the one hand, it offers a chance to measure dynamics in great detail. On the other hand, certain types of motion lead to signal loss and experimental inefficiencies that at first glance interfere with the application of ssNMR to overly dynamic proteins. Dynamics-based spectral editing (DYSE) ssNMR methods leverage motion-dependent signal losses to simplify spectra and enable the study of sub-structures with particular motional properties.
The Hsp40/Hsp70 chaperone families combine versatile folding capacity with high substrate specificity, which is mainly facilitated by Hsp40s. The structure and function of many Hsp40s remain poorly ...understood, particularly oligomeric Hsp40s that suppress protein aggregation. Here, we used a combination of biochemical and structural approaches to shed light on the domain interactions of the Hsp40 DnaJB8, and how they may influence recruitment of partner Hsp70s. We identify an interaction between the J-Domain (JD) and C-terminal domain (CTD) of DnaJB8 that sequesters the JD surface, preventing Hsp70 interaction. We propose a model for DnaJB8-Hsp70 recruitment, whereby the JD-CTD interaction of DnaJB8 acts as a reversible switch that can control the binding of Hsp70. These findings suggest that the evolutionarily conserved CTD of DnaJB8 is a regulatory element of chaperone activity in the proteostasis network.
Dynamic nuclear polarization (DNP) permits a ∼102−103 enhancement of the nuclear spin polarization and therefore increases sensitivity in nuclear magnetic resonance (NMR) experiments. Here, we ...demonstrate the efficient transfer of DNP-enhanced 1H polarization from an aqueous, radical-containing solvent matrix into peptide crystals via 1H−1H spin diffusion across the matrix−crystal interface. The samples consist of nanocrystals of the amyloid-forming peptide GNNQQNY7 - 13, derived from the yeast prion protein Sup35p, dispersed in a glycerol−water matrix containing a biradical polarizing agent, TOTAPOL. These crystals have an average width of 100−200 nm, and their known crystal structure suggests that the size of the biradical precludes its penetration into the crystal lattice; therefore, intimate contact of the molecules in the nanocrystal core with the polarizing agent is unlikely. This is supported by the observed differences between the time-dependent growth of the enhanced polarization in the solvent versus the nanocrystals. Nevertheless, DNP-enhanced magic-angle spinning (MAS) spectra recorded at 5 T and 90 K exhibit an average signal enhancement ε ≈ 120. This is slightly lower than the DNP enhancement of the solvent mixture surrounding the crystals (ε ≈ 160), and we show that it is consistent with spin diffusion across the solvent−matrix interface. In particular, we correlate the expected DNP enhancement to several properties of the sample, such as crystal size, the nuclear T 1, and the average 1H−1H spin diffusion constant. The enhanced 1H polarization was subsequently transferred to 13C and 15N via cross-polarization, and allowed rapid acquisition of two-dimensional 13C−13C correlation data.
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