Daniel A. Savin

Dynamic-Covalent Macromolecular Stars with Boronic Ester Linkages

Macromolecular stars containing reversible boronic ester linkages were prepared by an arm-first approach by reacting well-defined boronic acid-containing block copolymers with multifunctional 1,2/1,3-diols. Homopolymers of 3-acrylamidophenylboronic acid (APBA) formed macroscopic dynamic-covalent networks when cross-linked with multifunctional diols. On the other hand, adding the diol cross-linkers to block copolymers of poly(N,N-dimethylacrylamide (PDMA))-b-poly(APBA) led to nanosized multiarm stars with boronic ester cores and PDMA coronas. The assembly of the stars under a variety of conditions was considered. The dynamic-covalent nature of the boronic ester cross-links allowed the stars to reconfigure their covalent structure in the presence of monofunctional diols that competed for bonding with the boronic acid component. Therefore, the stars could be induced to dissociate via competitive exchange reactions. The star formation-dissociation process was shown to be repeatable over multiple cycles.

Dynamic-covalent nanostructures prepared by Diels–Alder reactions of styrene-maleic anhydride-derived copolymers obtained by one-step cascade block copolymerization

Macromolecular star formation by Diels–Alder chemistry resulted in dynamic nanomaterials capable of reversibly demonstrating the properties of both linear and highly branched macromolecules. Well-defined block copolymers of maleic anhydride (MAn) and styrene [poly(styrene-alt-MAn)-b-polystyrene (P(S-alt-MAn)-b-PS)] were prepared via a one-pot cascade approach by reversible addition–fragmentation chain transfer (RAFT) polymerization. Subsequent ring opening of the anhydride groups in the P(S-alt-MAn) segments by amidation with furfurylamine led to the formation of block copolymers with pendant furan functionality. Diels–Alder reactions of the furan-functional block copolymer with a bismaleimide crosslinker resulted in core-crosslinked stars by an arm-first approach. Star-like structures were also prepared by first allowing the furan-functional block copolymers to pre-assemble into polymeric micelles in a solvent selective for the polystyrene block. Subsequent addition of a bismaleimide and heating to allow the Diels–Alder reaction resulted in core-crosslinked micelles with similar structures to the polymeric stars prepared by the arm-first approach. Regardless of the synthetic approach employed, the thermoreversibility of the Diels–Alder linkages within the cores rendered the stars/crosslinked micelles dynamic-covalent, as demonstrated by their ability to reversibly dissociate back to the individual arms on heating.

Effects of chronic nanoparticulate silver exposure to adult and juvenile sheepshead minnows (Cyprinodon variegatus).

The use of nanoparticulate silver (AgNP) is increasingly widespread and recently has been shown to have a plausible release route into aquatic environments. To date, relatively little research has examined the effects of AgNP on estuarine fish. The authors present data indicating that chronic exposure to low levels of AgNP induces significant adverse effects in both juvenile and adult sheepshead minnows (Cyprinodon variegarus; SHMs). Chronic exposure to low levels of AgNP produced significant increases in tissue burdens in both juvenile and adult SHMs, resulting in significant thickening of epithelia gill tissue and in dramatically altered gene expression profiles. The results do not appear to be attributable to the release of silver ions through particle dissolution. The alteration in gene expression was greatest in adult gonads, but no evidence of AgNP-related dysfunction was found at the tissue level. In contrast, the authors found a significant effect on gill morphology, but very little evidence of effect on gill transcription profiles.

Thiol–yne ‘click’ chemistry as a route to functional lipid mimetics

Thiol–alkyne ‘click’ chemistry is a modular, efficient mechanism to synthesize complex A2B 3-arm star polymers. This general motif is similar to a phospholipid where the A blocks correspond to lypophilic chains and the B block represents the polar head group. In this communication we employ thiol–yne chemistry to produce polypeptide-based A2B lipid mimetics. The utility of the thiol–yne reaction is demonstrated by using a divergent and a convergent approach in the synthesis. These polymers self-assemble in aqueous solution into spherical vesicles with a relatively narrow size distribution independent of block composition over the range studied. Using the thiol–yne convergent synthesis, we envision a modular approach to functionalize proteins or oligopeptides with lipophilic chains that can imbed seamlessly into a cell membrane.

Stimuli-Responsive Peptide-Based ABA-Triblock Copolymers: Unique Morphology Transitions With pH

We report the synthesis and solution characterization of poly(L-lysine)-b-poly(propylene oxide)-b-poly(L-lysine) (KPK) triblock copolymers with high lysine weight fractions (>75 wt%). In contrast to PK diblock copolymers in this composition range, KPK triblock copolymers exhibit morphology transitions as a function of pH. Using a combination of light-scattering and microscopy techniques, we demonstrate spherical micelle-vesicle and spherical micelle-disk micelle transitions for different K fractions. We interpret these morphology changes in terms of the energy penalty associated with folding the core P block to form a spherical micelle in relation to the interfacial curvature associated with different charged states of the K block.

Peptide-Based Lipid Mimetics With Tunable Core Properties via Thiol-Alkyne Chemistry

Thiol–alkyne chemistry is utilized to produce peptide-based A2B star polymers. This topography resembles a phospholipid where the polypeptide B block represents a functional, polar head group and the A blocks represent lipophilic units. We utilize a convergent, modular approach to produce lipid mimetics through conjugation of poly(L-glutamic acid) with three different lipophilic moieties (octadecane, cholesterol and polyhedral oligomeric silsesquioxane). All samples are shown to self-assemble in aqueous solution into pH-responsive vesicles. The self-assembly and pH-responsiveness were characterized using circular dichroism spectroscopy and light scattering. Detailed light scattering experiments determined that the aggregation number of the vesicles remains nearly constant as a function of pH, suggesting that the pH-responsiveness is a result of both the helix–coil transition as well as a change in chain packing at the vesicle interface.

Reversible gold “locked” synthetic vesicles derived from stimuli-responsive diblock copolymers

Polymersomes derived from a RAFT-generated, thermally responsive diblock copolymer, P(DMAEMA165-b-NIPAM435), were shell cross-linked by in situ gold nanoparticle formation. The cross-linking was subsequently reversed by the addition of the thiols capable of inducing a ligand exchange on the surface of the gold nanoparticle.

Structurally controlled “polysoaps” via RAFT copolymerization of AMPS and n-dodecyl acrylamide for environmental remediation

A series of micelle-forming, amphiphilic copolymers or “polysoaps” with potential as dispersants for oil spill remediation has been synthesized via statistical RAFT copolymerization of specific molar ratios of 2-acrylamido-2-methylpropane sulfonic acid (AMPS) and n-dodecyl acrylamide (DDAM). Control over composition, degree of polymerization, polydispersity, and reaction kinetics was attained by conducting the copolymerizations at 60 °C in dimethylformamide with AIBN and CEP as initiator and chain-transfer agent, respectively. Molecular weight, hydrodynamic dimensions, composition- and concentration-dependent associative properties, and hydrophobic domain characteristics in water for copolymers with 10, 20, 30, 40, 50, and 60 mol% feed of DDAM have been studied utilizing NMR, SEC/MALLS, DLS, SLS, surface tensiometry, and fluorescence spectroscopy. It was found that the lower hydrophobic content (10%, 20%, and 30%) polysoaps form multimeric associations as indicated by increasing hydrodynamic dimensions as concentration is increased. On the other hand, the higher hydrophobic content (40%, 50%, and 60%) polysoaps form unimolecular micelles with consistent sizes and with distinct hydrophobic cores over the entire concentration range probed. UV/Vis absorbance experiments provided additional insight into the association and sequestration properties of the polysoaps. The higher hydrophobic content polysoaps show increased capabilities for dissolution of pyrene as compared to SDS above its CMC. Finally, the cytotoxicity of these polysoaps was determined utilizing KB cell lines. The polysoaps of this study exhibited up to 60× less cytotoxicity than SDS as measured by IC50 values. It was also found that as the molecular weight of the polysoap increased, the cytotoxicity decreased. The results of this study point to potential of unimolecular micelles for oil spill remediation, allowing sequestration and subsequent hydrocarbon break-down by endogenous bacteria in the marine environment.

Sequential thiol click reactions: formation of ternary thiourethane/thiol-ene networks with enhanced thermal and mechanical properties.

We report the physical properties of thiol-ene networks modified with thiourethane or urethane linkages, either along the main chain or as a branched component in the network, respectively. Because of the robust and orthogonal nature of thiol-isocyanate and thiol-ene reactions, these networks can be formed in a two-step, one-pot synthesis. Resultant networks were characterized using dynamic mechanical analysis, mechanical testing and other complementary techniques. It was found that incorporating (thio)urethanes into the networks increased Tg, but also increased strain at break and toughness while decreasing cross-link density. The changes in physical properties are discussed in terms of a proposed dual network morphology. These facile modifications to thiol-ene networks demonstrate how molecular-level, nanoscale changes can have a profound influence on the macroscale properties through hierarchical development of network morphology.

Kinetics and Control of Self-Assembly of ABH1 Hydrophobin from the Edible White Button Mushroom

Hydrophobins are small fungal proteins that self-assemble at hydrophobic/hydrophilic interfaces to form stable, amyloid membranes that are resistant to denaturation. Their remarkable surface activity has driven intense research for their potential utility in biomedical and cosmetic applications. In this research, the self-assembly characteristics of the Class I hydrophobin ABH1 from Agaricus bisporus , the edible white button mushroom, were evaluated as a function of solution and interface properties, in an attempt to gain greater mechanistic understanding. The kinetics of self-assembly were examined using dynamic quartz crystal microbalance techniques in combination with AFM, ellipsometry, contact angle goniometry, light scattering, and circular dichroism spectroscopy. It was found that the strength of interfacial tension between two phases drives the speed of ABH1 assembly and that the nature and location of the molecular ordering was influenced by temperature. ABH1 demonstrates different characteristics and self-assembly properties than those reported for other Class I hydrophobins, including causing an instantaneous decrease in surface tension in aqueous solution and undergoing a direct transition to β-sheet conformation on self-assembly at elevated temperature.