Controlling Polymer Morphologies by Intramolecular and Intermolecular Dynamic Covalent Iron(III)/Catechol Complexation-From Polypeptide Single Chain Nanoparticles to Hydrogels

Effect of the Protein Chain Conformation on the Collapse into Nanoparticles

Protein single-chain nanoparticles can outperform synthetic nanoparticles in biomedical applications due to enhanced biocompatibility. Compared to synthetic (co)polymers, the chemical complexity of proteins challenges chain conformation control. Here, we investigate the impact of the precursor chain conformation of bovine serum albumin (BSA) on the nanoparticle structure after intramolecular cross-linking. We explore the urea concentration (denaturant), pH, salt, cross-linker length, and concentration. Small-angle neutron scattering and dynamic light scattering experiments reveal a shrinking chain conformation upon cross-linking. However, the ability to collapse depends on solvent conditions: more expanded chains collapse more, whereas proteins that are already compact barely change in size upon cross-linking. Static light scattering measurements demonstrate that binding is primarily intramolecular. The use of a shorter cross-linker does not lead to collapse of extended chains. Overall, BSA exhibits a similar behavior to that of polymer nanoparticles, which then allows to harness the precursor conformation for morphological control.

GelMA-catechol coated FeHAp nanorods functionalized nanofibrous reinforced bio-instructive and mechanically robust composite hydrogel scaffold for bone tissue engineering

Critical bone defects complicate tissue graft-based surgeries, raising healthcare expenditures and underscoring scaffold-based tissue-engineering strategies to support bone reconstruction. Our study highlighted that the phase-compatible combination of inorganic nanorods, nanofibers, and hydrogels is promising for developing biomimetic and cell-instructive scaffolds since the bone matrix is a porous organic/inorganic composite. In brief, methacrylated gelatin (GelMA) was reacted with dopamine to form catechol-modified GeLMA (GelMA-C). The GelMA-C was nanocoated onto an iron-doped hydroxyapatite (FeHAp) nanorod via metal-catechol network coordination. The modified nanorod (FeHAp@GelMA-C) was loaded onto GelMA-based nanofibers. The nanorods loaded pre-fibers were electrospun onto GelMA solution and photochemically crosslinked to fabricate a fiber-reinforced hydrogel. The structural, mechanical, physicochemical, biocompatibility, swelling properties, osteogenic potential, and bone remodelling potential (using rat femoral defect model) of modified nanorods, simple hydrogel, and nanorod-loaded fiber-reinforced hydrogel were studied. The results supported that the interface interaction between GelMA-C/nanorods, nanorods/nanofibers, nanorods/hydrogels, and nanofiber/hydrogels significantly improved the microstructural and mechanical properties of the scaffold. Compared to pristine hydrogel, the nanorod-loaded fiber-reinforced scaffold better supported cellular responses, osteogenic differentiation, matrix mineralization, and accelerated bone regeneration. The nanorod-loaded fiber-reinforced hydrogel proved more biomimetic and cell-instructive for guided bone reconstruction.

Dopamine-Conjugated Bovine Serum Albumin Nanoparticles Containing pH-Responsive Catechol-V(III) Coordination for In Vitro and In Vivo Drug Delivery

V(III) instead of commonly used Fe(III) provided a rich tris-catechol-metal coordination at pH 7.4, which is important for slow drug release at physiological pH. Bovine serum albumin (BSA) functionalized with catechol-containing dopamine (D) and cross-linked using tris-catechol-V(III) coordination yielded pH-responsive compact D-BSA NPs (253 nm). However, conversion to bis- and/or mono-catechol-V(III) complexes in an acidic medium resulted in degradation of NPs and rapid release of doxorubicin (DOX). It was shown that D-BSA NPs entered cancerous MCF-7 cells (66%) more efficiently than non-cancerous HEK293T (33%) in 3 h. Also, DOX-loaded NPs reduced cell viability of MCF-7 by 75% and induced apoptosis in a majority of cells after 24 h. Biodegradability and lack of hemolytic activity were shown in vitro, whereas a lack of toxicity was shown in histological sections of zebrafish. Furthermore, 30% of circulating tumor cells in vasculature in 24 h were killed by DOX-loaded NPs shown with the zebrafish CTC xenograft model.

Protein-Inspired Polymers with Metal-Site-Regulated Ordered Conformations

Metal ions play critical roles in facilitating peptide folding and inducing conformational transitions, thereby impacting on the biological activity of many proteins. However, the effect of metal sites on the hierarchical structures of biopolymers is still poorly understood. Herein, inspired by metalloproteins, we report an order-to-order conformational regulation in synthetic polymers mediated by a variety of metal ions. The copolymers are decorated with clinically available desferrioxamine (DFO) as an exogenous ligand template, which presents a geometric constraint toward peptide backbone via short-range hydrogen bonding interactions, thus dramatically altering the secondary conformations and self-assembly behaviors of polypeptides and allowing for a controllable β-sheet to α-helix transition modulated by metal-ligand interactions. These metallopolymers could form ferritin-inspired hierarchical structures with high stability and membrane activity for efficient brain delivery across the blood-brain barrier (BBB) and long-lasting magnetic resonance imaging (MRI) in vivo.

Neat Protein Single-Chain Nanoparticles from Partially Denatured BSA

The main challenge for the preparation of protein single-chain nanoparticles (SCNPs) is the natural complexity of these macromolecules. Herein, we report the suitable conditions to produce "neat" bovine serum albumin (BSA) single-chain nanoparticles (SCNPs) from partially denatured BSA, which involves denaturation in urea and intramolecular cross-linking below the overlap concentration. We use two disuccinimide ester linkers containing three and six methylene spacer groups: disuccinimidyl glutarate (DSG) and disuccinimidyl suberate (DSS), respectively. Remarkably, the degree of internal cross-linking can be followed simply and efficiently via ¹H NMR spectroscopy. The associated structural changes-as probed by small-angle neutron scattering (SANS)-reveal that the denatured protein has a random-like coil conformation, which progressively shrinks with the addition of DSG or DSS, thus allowing for size control of the BSA-SCNPs with radii of gyration down to 5.4 nm. The longer cross-linker exhibits slightly more efficiency in chain compaction with a somewhat stronger size reduction but similar reactivity at a given cross-linker concentration. This reliable method is applicable to a wide range of compact proteins since most proteins have appropriate reactive amino acids and denature in urea. Critically, this work paves the way to the synthesis of "neat", biodegradable protein SCNPs for a range of applications including nanomedicine.

Validity of Effective Potentials in Crowded Solutions of Linear and Ring Polymers with Reversible Bonds

We perform simulations to compute the effective potential between the centers-of-mass of two polymers with reversible bonds. We investigate the influence of the topology on the potential by employing linear and ring backbones for the precursor (unbonded) polymer, finding that it leads to qualitatively different effective potentials. In the linear and ring cases the potentials can be described by Gaussians and generalized exponentials, respectively. The interactions are more repulsive for the ring topology, in analogy with known results in the absence of bonding. We also investigate the effect of the specific sequence of the reactive groups along the backbone (periodic or with different degrees of randomness), establishing that it has a significant impact on the effective potentials. When the reactive sites of both polymers are chemically orthogonal so that only intramolecular bonds are possible, the interactions become more repulsive the closer to periodic the sequence is. The opposite effect is found if both polymers have the same types of reactive sites and intermolecular bonds can be formed. We test the validity of the effective potentials in solution, in a broad range of concentrations from high dilution to far above the overlap concentration. For this purpose, we compare simulations of the effective fluid and test particle route calculations with simulations of the real all-monomer system. Very good agreement is found for the reversible linear polymers, indicating that unlike in their nonbonding counterparts many-body effects are minor even far above the overlap concentration. The agreement for the reversible rings is less satisfactory, and at high concentration the real system does not show the clustering behavior predicted by the effective potential. Results similar to the former ones are found for the partial self-correlations in ring/linear mixtures. Finally, we investigate the possibility of creating, at high concentrations, a gel of two interpenetrated reversible networks. For this purpose we simulate a 50/50 two-component mixture of reversible polymers with orthogonal chemistry for the reactive sites, so that intermolecular bonds are only formed between polymers of the same component. As predicted by both the theoretical phase diagram and the simulations of the effective fluid, the two networks in the all-monomer mixture do not interpenetrate, and phase separation (demixing) is observed instead.