Multifunctional Zwitterionic N-Oxide Polymers to Overcome Cascade Physiological Barriers for Efficient Anticancer Drug Delivery

For efficient anticancer drug delivery, cascade physiological barriers must be overcome, which requires the drug delivery vehicles to possess different or even opposite properties at different stages. Poly(tertiary amine-oxide) (PTAO) polymers with the zwitterionic feature have distinct antifouling properties in blood circulation, which can be reduced and protonated in hypoxic tumors to promote cellular internalization. Nevertheless, the effects of various PTAO structures have not been studied systemically and optimized. In this report, the side groups of PTAO are proposed to be optimized by modulating the structures. Poly(2-(N-oxide-hexamethyleneimino)ethyl methacrylate) (POC7A) with a cyclic seven-membered ring is screened as the optimized PTAO structure for in vivo applications. Moreover, the block copolymer POC7A-block-poly(ε-caprolactone) (POC7A-PCL) is prepared for the formation of micelles in aqueous solution for delivery of doxorubicin (DOX). The zwitterionic nature of POC7A shells with efficient bioreductive activity and protonation in the tumor microenvironment endows the micelles with excellent antifouling properties for long blood circulation, efficient tumor accumulation, deep penetration, and effective cellular internalization. Thus, the DOX-loaded micelles exhibit potent antitumor efficacy after intravenous administration. Zwitterionic POC7A can be used as antifouling shells of the anticancer drug delivery nanocarriers to overcome the cascade physiological barriers efficiently.

Functional coordination compounds for mechanoresponsive polymers

Small molecule probes that respond to a mechanical force ("mechanophores") have emerged as an important tool in the design of stimuli-responsive polymer materials. Although the majority of such mechanohphores are based on organic molecules, the utilization of metal complexes has also attracted attention as they offer a possibility to tune their spectroscopic properties and reactivity, and have the ability to reversibly form and break metal-ligand bonds through rational design of the ligand environment surrounding the metal. This review features representative examples of coordination compounds which were utilized as new, tunable tools to create various types of mechanoresponsive polymers.

Antifouling Properties of Amine-Oxide-Containing Zwitterionic Polymers

Biofouling due to nonspecific proteins or cells on the material surfaces is a major challenge in a range of applications such as biosensors, medical devices, and implants. Even though poly(ethylene glycol) (PEG) has become the most widely used stealth material in medical and pharmaceutical products, the number of reported cases of PEG-triggered rare allergic responses continues to increase in the past decades. Herein, a new type of antifouling material poly(amine oxide) (PAO) has been evaluated as an alternative to overcome nonspecific foulant adsorption and impart comparable biocompatibility. Alkyl-substituted PAO containing diethyl, dibutyl, and dihexyl substituents are prepared, and their solution properties are studied. Photoreactive copolymers containing benzophenone as the photo-cross-linker are prepared by reversible addition-fragmentation chain-transfer polymerization and fully characterized by gel permeation chromatography and dynamic light scattering. Then, these water-soluble polymers are anchored onto a silicon wafer with the aid of UV irradiation. By evaluating the fouling resistance properties of these modified surfaces against various types of foulants, protein adsorption and bacterial attachment assays show that the cross-linked PAO-modified surface can efficiently inhibit biofouling. Furthermore, human blood cell adhesion experiments demonstrate that our PAO polymer could be used as a novel surface modifier for biomedical devices.

Grafting of proteins onto polymeric surfaces: A synthesis and characterization challenge

This review aims at answering the following question: how can a researcher be sure to succeed in grafting a protein onto a polymer surface? Even if protein immobilization on solid supports has been used industrially for a long time, hence enabling natural enzymes to serve as a powerful tool, emergence of new supports such as polymeric surfaces for the development of so-called intelligent materials requires new approaches. In this review, we introduce the challenges in grafting protein on synthetic polymers, mainly because compared to hard surfaces, polymers may be sensitive to various aqueous media, depending on the pH or reductive molecules, or may exhibit state transitions with temperature. Then, the specificity of grafting on synthetic polymers due to difference of chemical functions availability or difference of physical properties are summarized. We present next the various available routes to covalently bond the protein onto the polymeric substrates considering the functional groups coming from the monomers used during polymerization reaction or post-modification of the surfaces. We also focus our review on a major concern of grafting protein, which is avoiding the potential loss of function of the immobilized protein. Meanwhile, this review considers the different methods of characterization used to determine the grafting efficiency but also the behavior of enzymes once grafted. We finally dedicate the last part of this review to industrial application and future prospective, considering the sustainable processes based on green chemistry.

Beyond protein tagging: Rewiring the genetic code of fluorescent proteins - A review

Fluorescent proteins (FP) are an integral part of modern biology due to its diverse biochemical and photophysical properties. The boundaries of FP have been extended through conventional mutagenesis and directed evolution approaches. Engineering of FP based on the standard genetic code consisting of 20 amino acids with limited functional groups restrict its diversification. Degeneracy of genetic code has helped in covering this substantial gap through genetic code engineering, wherein introduction of unnatural amino acid (UAA) analogues resulted in a collection of FP with varying properties. This review features the work carried till date in the area of FP incorporated with UAAs and explores strategies employed for incorporation, impact of UAAs in chromophore and surrounding residues and changes in inherent properties of FP. The long-standing association of FP as a tool for high throughput screening of orthogonal aaRS/tRNA pairs used in site specific incorporation of UAAs is expounded. Insertion of UAAs in FP has enabled their use in contemporary fields such as biophotovoltaics, bioremediation, biosensors, biomaterials and imaging of acidic vesicles. Thus, expansion of genetic code of FP is envisaged to rejig the existing spectra of colors and future research initiative in this direction is expected to glow brighter and brighter.

Reversible Soft Mechanochemical Control of Biaryl Conformations through Crosslinking in a 3D Macromolecular Network

Tuning the dihedral angle (DA) of axially chiral compounds can impact biological activity, catalyst efficiency, molecular motor performance, or chiroptical properties. Herein, we report gradual, controlled, and reversible changes in molecular conformation of a covalently linked binaphthyl moiety within a 3D polymeric network by application of a macroscopic stretching force. We managed direct observation of DA changes by measuring the circular dichroism signal of an optically pure BINOL-crosslinked elastomer network. Stretching the elastomer resulted in a widening of the DA between naphthyl rings when the BINOL was doubly grafted to the elastomer network; no effect was observed when a single naphthyl ring of the BINOL was grafted to the elastomer network. We have determined that ca. 170 % extension of the elastomers led to the transfer of a mechanical force to the BINOL moiety of 2.5 kcal mol^-1  Å^-1 (ca. 175 pN) in magnitude and results in the opening of the DA of BINOL up to 130°.

From Molecules to Polymers-Harnessing Inter- and Intramolecular Interactions to Create Mechanochromic Materials

The development of mechanophores as building blocks that serve as predefined weak linkages has enabled the creation of mechanoresponsive and mechanochromic polymer materials, which are interesting for a range of applications including the study of biological specimens or advanced security features. In typical mechanophores, covalent bonds are broken when polymers that contain these chemical motifs are exposed to mechanical forces, and changes of the optical properties upon bond scission can be harnessed as a signal that enables the detection of applied mechanical stresses and strains. Similar chromic effects upon mechanical deformation of polymers can also be achieved without relying on the scission of covalent bonds. The dissociation of motifs that feature directional noncovalent interactions, the disruption of aggregated molecules, and conformational changes in molecules or polymers constitute an attractive element for the design of mechanoresponsive and mechanochromic materials. In this article, it is reviewed how such alterations of molecules and polymers can be exploited for the development of mechanochromic materials that signal deformation without breaking covalent bonds. Recent illustrative examples are highlighted that showcase how the use of such mechanoresponsive motifs enables the visual mapping of stresses and damage in a reversible and highly sensitive manner.

Mechano-chromic protein-polymer hybrid hydrogel to visualize mechanical strain

In a proof-of-concept study, a mechano-chromic hydrogel was synthesized here, via chemoenzymatic click conjugation of fluorophore-labeled fibronectin into a synthetic hydrogel co-polymers (i.e., poly-N-isopropylacrylamide/polyethylene glycol). The optical FRET response could be tuned by macroscopic stretch.

Synthesis and Mechanochemical Activity of Peptide-Based Cu(I) Bis(N-heterocyclic carbene) Complexes

With the class of shock-absorbing proteins, nature created some of the most robust materials combining both mechanical strength and elasticity. Their excellent ability to dissipate energy to prevent surrounding cells from damage is an interesting property that regularly is exploited for applications in biomimetic materials. Similar to biomaterials, where mechanical stimuli are transmitted into a (bio)chemical response, mechanophoric catalysts transform mechanical energy into a chemical reaction. Force transmission is realized commonly by polymeric handles directing the applied force to the mechanophoric bond, which in turn leads to stress-induced activation of the catalyst. Therefore, shock-absorbing proteins able to take up and store mechanical energy elastically for subsequent force transduction to the labile bond seem to be perfect candidates to fulfill this task. Here, we report on the synthesis of two different latent mechanophoric copper(I) bis(N-heterocyclic carbene) complexes bearing either two carboxyl groups or two amino groups which allow conjugation reactions with either the N- or the C-terminus of amino acids or peptides. The chosen catalysts can be activated, for instance, by applying external mechanical force via ultrasound, removing one N-heterocyclic carbene (NHC) ligand. Post-modification of the mechanophoric catalysts via peptide coupling (Gly, Val) and first reactions showed that the mechanoresponsive behavior was still present after the coupling. Subsequent polycondensation of both catalysts lead to a polyamide including the Cu(I) moiety. Mechanochemical activation by ultrasound showed conversions in the copper(I)-catalyzed alkyne-azide “click” reaction (CuAAC) up to 9.9% proving the potential application for the time and spatial controlled CuAAC.

Modular Design of Programmable Mechanofluorescent DNA Hydrogels

Mechanosensing systems are ubiquitous in nature and control many functions from cell spreading to wound healing. Biologic systems typically rely on supramolecular transformations and secondary reporter systems to sense weak forces. By contrast, synthetic mechanosensitive materials often use covalent transformations of chromophores, serving both as force sensor and reporter, which hinders orthogonal engineering of their sensitivity, response and modularity. Here, we introduce FRET-based, rationally tunable DNA tension probes into macroscopic 3D all-DNA hydrogels to prepare mechanofluorescent materials with programmable sacrificial bonds and stress relaxation. This design addresses current limitations of mechanochromic system by offering spatiotemporal resolution, as well as quantitative and modular force sensing in soft hydrogels. The programmable force probe design further grants temporal control over the recovery of the mechanofluorescence during stress relaxation, enabling reversible and irreversible strain sensing. We show proof-of-concept applications to study strain fields in composites and to visualize freezing-induced strain patterns in homogeneous hydrogels.

Self-Reporting Fiber-Reinforced Composites That Mimic the Ability of Biological Materials to Sense and Report Damage

Sensing of damage, deformation, and mechanical forces is of vital importance in many applications of fiber-reinforced polymer composites, as it allows the structural health and integrity of composite components to be monitored and microdamage to be detected before it leads to catastrophic material failure. Bioinspired and biomimetic approaches to self-sensing and self-reporting materials are reviewed. Examples include bruising coatings and bleeding composites based on dye-filled microcapsules, hollow fibers, and vascular networks. Force-induced changes in color, fluorescence, or luminescence are achieved by mechanochromic epoxy resins, or by mechanophores and force-responsive proteins located at the interface of glass/carbon fibers and polymers. Composites can also feel strain, stress, and damage through embedded optical and electrical sensors, such as fiber Bragg grating sensors, or by resistance measurements of dispersed carbon fibers and carbon nanotubes. Bioinspired composites with the ability to show autonomously if and where they have been damaged lead to a multitude of opportunities for aerospace, automotive, civil engineering, and wind-turbine applications. They range from safety features for the detection of barely visible impact damage, to the real-time monitoring of deformation of load-bearing components.

Site-Specific Protein Labeling with Tetrazine Amino Acids

Genetic code expansion is commonly used to introduce bioorthogonal reactive functional groups onto proteins for labeling. In recent years, the inverse electron demand Diels-Alder reaction between tetrazines and strained trans-cyclooctenes has increased in popularity as a bioorthogonal ligation for protein labeling due to its fast reaction rate and high in vivo stability. We provide methods for the facile synthesis of a tetrazine containing amino acid, Tet-v2.0, and the site-specific incorporation of Tet-v2.0 into proteins via genetic code expansion. Furthermore, we demonstrate that proteins containing Tet-v2.0 can be quickly and efficiently reacted with strained alkene labels at low concentrations. This chemistry has enabled the labeling of protein surfaces with fluorophores, inhibitors, or common posttranslational modifications such as glycosylation or lipidation.

Self-reporting Polymeric Materials with Mechanochromic Properties

The mechanical transduction of force onto molecules is an essential feature of many biological processes that results in the senses of touch and hearing, gives important cues for cellular interactions and can lead to optically detectable signals, such as a change in colour, fluorescence or chemoluminescence. Polymeric materials that are able to visually indicate deformation, stress, strain or the occurrence of microdamage draw inspiration from these biological events. The field of self-reporting (or self-assessing) materials is reviewed. First, mechanochromic events in nature are discussed, such as the formation of bruises on skin, the bleeding of a wound, or marine glow caused by dinoflagellates. Then, materials based on force-responsive mechanophores, such as spiropyrans, cyclobutanes, cyclooctanes, Diels–Alder adducts, diarylbibenzofuranone and bis(adamantyl)-1,2-dioxetane are reviewed, followed by mechanochromic blends, chromophores stabilised by hydrogen bonds, and pressure sensors based on ionic interactions between fluorescent dyes and polyelectrolyte brushes. Mechanobiochemistry is introduced as an important tool to create self-reporting hybrid materials that combine polymers with the force-responsive properties of fluorescent proteins, protein FRET pairs, and other biomacromolecules. Finally, dye-filled microcapsules, microvascular networks, and hollow fibres are demonstrated to be important technologies to create damage-indicating coatings, self-reporting fibre-reinforced composites and self-healing materials.

At the Interface of Chemical and Biological Synthesis: An Expanded Genetic Code

The ability to site-specifically incorporate noncanonical amino acids (ncAAs) with novel structures into proteins in living cells affords a powerful tool to investigate and manipulate protein structure and function. More than 200 ncAAs with diverse biological, chemical, and physical properties have been genetically encoded in response to nonsense or frameshift codons in both prokaryotic and eukaryotic organisms with high fidelity and efficiency. In this review, recent advances in the technology and its application to problems in protein biochemistry, cellular biology, and medicine are highlighted.

Soft-Mechanochemistry: Mechanochemistry Inspired by Nature

Cells and bacteria use mechanotransduction processes to transform a mechanical force into a chemical/biochemical response. The area of chemistry where chemical reactions are induced by mechanical forces is called mechanochemistry. Over the last few years, chemists developed force-induced reactions affecting covalent bonds in molecules under tension which requires high energy input and/or high intensity forces. In contrast, in nature, mechanotransduction processes take place with forces of much weaker intensity and much less demanding energy. They are mainly based on protein conformational changes or changes in supramacromolecular architectures. Mechanochemistry based on such low-energy-demanding processes and which does not affect chemical bonds can be called soft-mechanochemistry. In this feature article, we first discuss some examples of soft-mechanochemistry processes encountered in nature, in particular, cryptic sites, allowing us to define more precisely the concepts underlying soft-mechanochemistry. A series of examples, developed over the past few years, of chemomechanoresponsive systems based on soft-mechanochemistry principles are given. We describe, in particular, cryptic site surfaces, enzymatically active films whose activity can be modulated by stretching and films where stretching induces changes in their fluorescence properties. Finally, we give our view of the future of soft-mechanochemistry.

A new biomimetic route to engineer enzymatically active mechano-responsive materials

Using modified β-galactosidase covalently linked to cross-linked polyelectrolyte multilayers (PEM), catalytically active materials have been designed. Their enzymatic activity can be modulated, partially in a reversible way, simply by stretching. This strategy, based on enzyme conformational changes, constitutes a new tool for the development of biocatalytic mechano-responsive materials.

Expansion of bioorthogonal chemistries towards site-specific polymer–protein conjugation

Bioorthogonal chemistries have been used to achieve polymer-protein conjugation with the retained critical properties.

Self-Assembly of Temperature-Responsive Protein-Polymer Bioconjugates

We report a simple temperature-responsive bioconjugate system comprising superfolder green fluorescent protein (sfGFP) decorated with poly[(oligo ethylene glycol) methyl ether methacrylate] (PEGMA) polymers. We used amber suppression to site-specifically incorporate the non-canonical azide-functional amino acid p-azidophenylalanine (pAzF) into sfGFP at different positions. The azide moiety on modified sfGFP was then coupled using copper-catalyzed "click" chemistry with the alkyne terminus of a PEGMA synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. The protein in the resulting bioconjugate was found to remain functionally active (i.e., fluorescent) after conjugation. Turbidity measurements revealed that the point of attachment of the polymer onto the protein scaffold has an impact on the thermoresponsive behavior of the resultant bioconjugate. Furthermore, small-angle X-ray scattering analysis showed the wrapping of the polymer around the protein in a temperature-dependent fashion. Our work demonstrates that standard genetic manipulation combined with an expanded genetic code provides an easy way to construct functional hybrid biomaterials where the location of the conjugation site on the protein plays an important role in determining material properties. We anticipate that our approach could be generalized for the synthesis of complex functional materials with precisely defined domain orientation, connectivity, and composition.

A poly(ADP-ribose) polymerase-1 activity assay based on the FRET between a cationic conjugated polymer and supercharged green fluorescent protein

A label-free strategy for PARP-1 activity assay and inhibitors assessment has been developed based on the FRET between a cationic conjugated polymer (CCP) and supercharged green fluorescent protein (scGFP).