Machine learning-aided engineering of hydrolases for PET depolymerization.

Plastic waste poses an ecological challenge1-3 and enzymatic degradation offers one, potentially green and scalable, route for polyesters waste recycling4. Poly(ethylene terephthalate) (PET) accounts for 12% of global solid waste5, and a circular carbon economy for PET is theoretically attainable through rapid enzymatic depolymerization followed by repolymerization or conversion/valorization into other products6-10. Application of PET hydrolases, however, has been hampered by their lack of robustness to pH and temperature ranges, slow reaction rates and inability to directly use untreated postconsumer plastics11. Here, we use a structure-based, machine learning algorithm to engineer a robust and active PET hydrolase. Our mutant and scaffold combination (FAST-PETase: functional, active, stable and tolerant PETase) contains five mutations compared to wild-type PETase (N233K/R224Q/S121E from prediction and D186H/R280A from scaffold) and shows superior PET-hydrolytic activity relative to both wild-type and engineered alternatives12 between 30 and 50 °C and a range of pH levels. We demonstrate that untreated, postconsumer-PET from 51 different thermoformed products can all be almost completely degraded by FAST-PETase in 1 week. FAST-PETase can also depolymerize untreated, amorphous portions of a commercial water bottle and an entire thermally pretreated water bottle at 50 ºC. Finally, we demonstrate a closed-loop PET recycling process by using FAST-PETase and resynthesizing PET from the recovered monomers. Collectively, our results demonstrate a viable route for enzymatic plastic recycling at the industrial scale.

Electron-Deficient Polycyclic π-System Fused with Multiple B←N Coordinate Bonds.

An extensive polycyclic π-system with 23 fused rings is synthesized via a highly efficient borylation reaction, in which four B-N covalent bonds and four B←N coordinate bonds are formed in one pot. B←N coordinate bonds not only lock the backbone into a near-coplanar conformation but also decrease the LUMO energy level to around -3.82 eV, demonstrating the dual utility of this strategy for the synthesis of extensive rigid polycyclic molecules and the development of n-type conjugated materials for organic electronics and organic photovoltaics.

Pauli Paramagnetism of Stable Analogues of Pernigraniline Salt Featuring Ladder-Type Constitution.

Polyaniline derivatives represent one of the most widely used classes of conductive polymers. The fundamentally important electronic properties of pernigraniline salts, the fully oxidized and acid-doped derivatives of polyanilines, however, are still not well-understood due to their poor stability and configurational uncertainty. To address these issues and to synthetically access stable analogues of pernigraniline salts, ladder-type constitution was imparted into a series of model oligomer analogues with rigid backbones constituted by up to 27 fused rings. The syntheses were achieved through iterative cross-coupling reactions followed by cyclization and oxidation. In contrast to their unstable non-ladder-type counterparts, these ladder-type pernigraniline-like molecules all adopt a well-defined all-trans configuration and demonstrate an excellent chemical stability after protonation, rendering it possible to reveal the intrinsic electronic and magnetic properties of molecules resembling pernigraniline. Protonated salts of these oligomers feature a significant diradicaloid open-shell resonance contribution. A dominant temperature-independent Pauli paramagnetism was observed in the solid state, an indication of the delocalization nature of the polarons in ladder-type analogues of pernigraniline salt.

Covalent and Noncovalent Approaches to Rigid Coplanar π-Conjugated Molecules and Macromolecules.

Molecular conformation and rigidity are essential factors in determining the properties of individual molecules, the associated supramolecular assemblies, and bulk materials. This correlation is particularly important for π-conjugated molecular and macromolecular systems. Within such an individual molecule, a coplanar conformation facilitates the delocalization of not only molecular orbitals but also charges, excitons, and spins, leading to synergistically ensembled properties of the entire conjugated system. A rigid backbone, meanwhile, imposes a high energy cost to disrupt such a favorable conformation, ensuring the robustness and persistence of coplanarity. From a supramolecular and material point of view, coplanarity and rigidity often promote strong intermolecular electronic coupling and reduce the energy barrier for the intermolecular transport of charges, excitons, and phonons, affording advanced materials properties in bulk. In this context, pursuing a rigid and coplanar molecular conformation often represents one of the primary objectives when designing and synthesizing conjugated molecules for electronic and optical applications. Two general bottom-up strategies-covalent annulation and noncovalent conformational control-are often employed to construct rigid coplanar π systems. These strategies have afforded various classes of such molecules and macromolecules, including so-called conjugated ladder polymers, graphene nanoribbons, polyacenes, and conformationally locked organic semiconductors. While pursuing these targets, however, one often confronts challenges associated with precise synthesis and limited solubility of the rigid coplanar systems, which could further impede their large-scale preparation, characterization, processing, and application. To address these issues, we developed and utilized a number of synthetic methods and molecular engineering approaches to construct and to process rigid coplanar conjugated molecules and macromolecules. Structure-property correlations of this unique class of organic materials were established, providing important chemical principles for molecular design and materials applications. In this Account, we first describe our efforts to synthesize rigid coplanar π systems fused by various types of bonds, including kinetically formed covalent bonds, thermodynamically formed covalent bonds, N→B coordinate bonds, and hydrogen bonds, in order of increasing dynamic character. The subsequent section discusses the characteristic properties of selected examples of these rigid coplanar π systems in comparison with control compounds that are not rigid and coplanar, particularly focusing on the optical, electronic, and electrochemical properties. For systems bridged with noncovalent interactions, active manipulation of the dynamic bonds can tune variable properties at the molecular or collective level. Intermolecular interactions, solid-state packing, and processing of several cases are then discussed to lay the foundation for future materials applications of rigid coplanar π conjugated compounds.

Extraordinary Redox Activities in Ladder-Type Conjugated Molecules Enabled by B ← N Coordination-Promoted Delocalization and Hyperconjugation.

The introduction of B ← N coordinate bond-isoelectronic to C-C single bond-into π-systems represents a promising strategy to impart exotic redox and electrochromic properties into conjugated organic molecules and macromolecules. To achieve both reductive and oxidative activities using this strategy, a cruciform ladder-type molecular constitution was designed to accommodate oxidation-active, reduction-active, and B ← N coordination units into a compact structure. Two such compounds (BN-F and BN-Ph) were synthesized via highly efficient N-directed borylation. These molecules demonstrated well-separated, two reductive and two oxidative electron-transfer processes, corresponding to five distinct yet stable oxidation states, including a rarely observed boron-containing radical cation. Spectroelectrochemical measurements revealed unique optical characteristics for each of these reduced/oxidized species, demonstrating multicolor electrochromism with excellent recyclability. Distinct color changes were observed between each redox state with clear isosbestic points on the absorption spectra. The underlying redox mechanism was elucidated by a combination of computational and experimental investigations. Single-crystal X-ray diffraction analysis on the neutral state, the oxidized radical cation, and the reduced dianion of BN-Ph revealed structural transformations into two distinct quinonoid constitutions during the oxidation and reduction processes, respectively. B ← N coordination played an important role in rendering the robust and reversible multistage redox properties, by extending the charge and spin delocalization, by modulating the π-electron density, and by a newly established hyperconjugation mechanism.

Locking the Coplanar Conformation of π-Conjugated Molecules and Macromolecules Using Dynamic Noncovalent Bonds.

Torsional conformation of the backbone of a π-conjugated molecule or macromolecule shapes its solubility, optoelectronic characteristics, rheological behaviors, and ultimately solid-state functions. In order to tailor these molecular, supramolecular, and materials properties, the desired coplanar conformation in π-conjugated systems can be locked by using dynamic noncovalent bonds. In this article, the syntheses, characterizations, and unique properties of conjugated molecules/polymers involving a variety of bridging noncovalent bonds are disussed in the context of coplanar backbone conformation. In addition, challenges in this specific field are identified and discussed for future breakthroughs in exploiting the promising potential of noncovalent-bond-bridged, π-conjugated organic materials.

Low Band Gap Coplanar Conjugated Molecules Featuring Dynamic Intramolecular Lewis Acid-Base Coordination.

Ladder-type conjugated molecules with a low band gap and low LUMO level were synthesized through an N-directed borylation reaction of pyrazine-derived donor-acceptor-donor precursors. The intramolecular boron-nitrogen coordination bonds played a key role in rendering the rigid and coplanar conformation of these molecules and their corresponding electronic structures. Experimental investigation and theoretical simulation revealed the dynamic nature of such coordination, which allowed for active manipulation of the optical properties of these molecules by using competing Lewis basic solvents.

Synthesis and Solution Processing of a Rigid Polymer Enabled by Active Manipulation of Intramolecular Hydrogen Bonds.

Global intramolecular hydrogen bonds were installed and manipulated in a rigid artificial synthetic polymer in order to actively control its conformation for synthesis and processing. The polymer solubility was switched on and off by chemically inhibiting and regenerating these preorganized intramolecular hydrogen bonds. Such active manipulation made it possible to synthesize this highly rigid polymer with elevated molecular weights. A well-solubilized, noncoplanar polymer precursor with thermally cleavable Boc groups was synthesized (Mn = 32.4 kg/mol). After processing this precursor into thin films, in situ thermal treatment regenerated the latent intramolecular hydrogen bonds and led to a rigid ladder-type conformation. Such manipulation of the intramolecular hydrogen bonds allowed for multilayer deposition of this polymer, laying the foundation for potential additive manufacturing using this strategy.

Molecular Coplanarity and Self-Assembly Promoted by Intramolecular Hydrogen Bonds.

Active conformational control is realized in a conjugated system using intramolecular hydrogen bonds to achieve tailored molecular, supramolecular, and solid-state properties. The hydrogen bonding functionalities are fused to the backbone and precisely preorganized to enforce a fully coplanar conformation of the π-system, leading to short π-π stacking distances, controllable molecular self-assembly, and solid-state growth of one-dimensional nano-/microfibers. This investigation demonstrates the efficiency and significance of an intramolecular noncovalent approach in promoting conformational control and self-assembly of organic molecules.