Evolution-guided engineering of trans-acyltransferase polyketide synthases.

Bacterial multimodular polyketide synthases (PKSs) are giant enzymes that generate a wide range of therapeutically important but synthetically challenging natural products. Diversification of polyketide structures can be achieved by engineering these enzymes. However, notwithstanding successes made with textbook cis-acyltransferase (cis-AT) PKSs, tailoring such large assembly lines remains challenging. Unlike textbook PKSs, trans-AT PKSs feature an extraordinary diversity of PKS modules and commonly evolve to form hybrid PKSs. In this study, we analyzed amino acid coevolution to identify a common module site that yields functional PKSs. We used this site to insert and delete diverse PKS parts and create 22 engineered trans-AT PKSs from various pathways and in two bacterial producers. The high success rates of our engineering approach highlight the broader applicability to generate complex designer polyketides.

Dilution-induced gel-sol-gel-sol transitions by competitive supramolecular pathways in water.

Fascinating properties are displayed by synthetic multicomponent supramolecular systems that comprise a manifold of competitive interactions, thereby mimicking natural processes. We present the integration of two reentrant phase transitions based on an unexpected dilution-induced assembly process using supramolecular polymers and surfactants. The co-assembly of the water-soluble benzene-1,3,5-tricarboxamide (BTA-EG4) and a surfactant at a specific ratio yielded small-sized aggregates. These interactions were modeled using the competition between self-sorting and co-assembly of both components. The small-sized aggregates were transformed into supramolecular polymer networks by a twofold dilution in water without changing their ratio. Kinetic experiments show the in situ growth of micrometer-long fibers in the dilution process. We were able to create systems that undergo fully reversible hydrogel-solution-hydrogel-solution transitions upon dilution by introducing another orthogonal interaction.

Structure of a Promiscuous Thioesterase Domain Responsible for Branching Acylation in Polyketide Biosynthesis.

Thioesterases (TEs) are fundamentally important enzymes present in all bacteria and eukaryotes, where they have conserved functions in fatty acid biosynthesis and secondary metabolism. This work provides the first structural insights into a functionally distinct group of TEs that perform diverse acylations in polyketide and peptide biosynthesis (TEB s). Structural analysis of the oocydin (OocS) TEB domain facilitated identification and engineering of the active site to modulate acyl-group acceptance. In this way, we achieved higher reactivity using a structure-based approach, building a foundation for biocatalytic development of TEB -mediated O-acylation, a modification known to improve the bioactivity of oocydin-type polyketides. Lastly, the promiscuity of the OocS TEB motivated us to investigate, and ultimately provide evidence for, the production of longer chain branched oocydins in the native host Serratia plymuthica 4Rx13. This work frames the OocS TEB and homologs as invaluable synthetic biology tools for polyketide drug development.

In situ Synthesis of Supramolecular Polymers: Finding the Right Conditions when Combining Covalent and Non-Covalent Synthesis.

The combination of covalent and non-covalent synthesis is omnipresent in nature and potentially enables access to new materials. Yet, the fundamental principles that govern such a synthesis are barely understood. Here, we demonstrate how even simple reaction mixtures behave surprisingly complex when covalent reactions are coupled to self-assembly processes. Specifically, we study the reaction behavior of a system in which the in situ formation of discotic benzene-1,3,5-tricarboxamide (BTA) monomers is linked to an intertwined non-covalent reaction network including self-assembly into helical BTA polymers. This system shows an unexpected phase-separation behavior in which an interplay of reactant/product concentrations, side-products and solvent purity determines the system composition. We envision that these insights can bring us one step closer to how to design the synthesis of systems in a combined covalent/non-covalent fashion.

Temperature-dependent modulation by biaryl-based monomers of the chain length and morphology of biphenyl-based supramolecular polymers.

Supramolecular copolymerizations offer attractive options to introduce structural and functional diversity in supramolecular polymer materials. Yet, general principles and structure-property relationships for rational comonomer design remain lacking. Here, we report on the supramolecular (co)aggregation of a phenylpyridine and bipyridine derivative of a recently reported biphenyl tetracarboxamide-based monomer. We show that both arylpyridines are poor monomers for supramolecular homopolymerizations. However, the two arylpyridines efficiently influence supramolecular polymers of a biphenyl-based polymer. The phenylpyridine derivatives primarily sequestrate biphenyl monomers, while the bipyridine intercalates into the polymers at high temperatures. Thereby, these two poorly homopolymerizing monomers allow for a fine control over the length of the biphenyl-based supramolecular polymers. As such, our results highlight the potential to control the structure and morphology of supramolecular polymers by tailoring the electronic properties of additives.

Oligodimethylsiloxane-Oligoproline Block Co-Oligomers: the Interplay between Aggregation and Phase Segregation in Bulk and Solution.

Discrete block co-oligomers (BCOs) assemble into highly ordered nanostructures, which adopt a variety of morphologies depending on their environment. Here, we present a series of discrete oligodimethylsiloxane-oligoproline (oDMS-oPro) BCOs with varying oligomer lengths and proline end-groups, and study the nanostructures formed in both bulk and solution. The conjugation of oligoprolines to apolar siloxanes permits a study of the aggregation behavior of oligoproline moieties in a variety of solvents, including a highly apolar solvent like methylcyclohexane. The apolar solvent is more reminiscent of the polarity of the siloxane bulk, which gives insights into the supramolecular interactions that govern both bulk and solution assembly processes of the oligoproline. This extensive structural characterization allows the bridging of the gap between solution and bulk assembly. The interplay between the aggregation of the oligoproline block and the phase segregation induced by the siloxane drives the assembly. This gives rise to disordered, micellar microstructures in apolar solution and crystallization-driven lamellar nanostructures in the bulk. While most di- and triblock co-oligomers adopt predictable morphological features, one of them, oDMS15-oPro6-NH2, exhibits pathway complexity leading to gel formation. The pathway selection in the complex interplay between aggregation and phase segregation gives rise to interesting material properties.

Supramolecular Depolymerization in the Mixture of Two Poor Solvents: Mechanistic Insights and Modulation of Supramolecular Polymerization of Ionic π-Systems.

Solvents are fundamentally essential for the synthesis and processing of soft materials. Supramolecular polymers (SPs), an emerging class of soft materials, are usually stable in single and mixtures of poor solvents. In contrast to these preconceived notions, here we report the depolymerization of SPs in the mixture of two poor solvents. This surprising behavior was observed for well-known cationic perylene diimides (cPDIs) in the mixtures of water and amphiphilic organic solvents such as isopropanol (IPA). cPDIs form stable SPs in water and IPA but readily depolymerize into monomers in 50-70 vol% IPA containing water. This is due to the selective solvation of the π-surface of cPDIs by alkyl chains of IPA and ionic side chains by water, as evidenced by molecular dynamic simulations. Moreover, by systematically changing the ratio between water and amphiphilic organic solvent, we could achieve an unprecedented supramolecular polymerization both by increasing and decreasing the solvent polarity.

Competition between chiral solvents and chiral monomers in the helical bias of supramolecular polymers.

Solute-solvent interactions are key for the assembly and proper functioning of biomacromolecules and play important roles in many fields of organic and polymer chemistry. Despite numerous reports describing the effects of (chiral) solvents on helical conformations of (supramolecular) polymers, the combination of chiral solvents and chiral monomers is unexplored. Here we report diastereomeric differences in the supramolecular polymerization of enantiomers of chiral triphenylene-2,6,10-tricarboxamides in chiral chlorinated solvents. Competition between the preferences induced by the stereocentres of the assembled monomers and those present in the solvent molecules results in unforeseen temperature-dependent solvation effects. By combining experiments and mathematical modelling, we show that the observed differences between enantiomers originate from the combined additive entropic effects of stereocentres present in the monomer and in the solvent. Remarkably, copolymerizations show that the chiral solvent can bias the copolymer helicity and thereby overrule the helical preference of the monomers. Our results highlight the importance of cumulative solvation effects in supramolecular polymerizations.

Double Porphyrin Cage Compounds.

The synthesis and characterization of double porphyrin cage compounds are described. They consist of two porphyrins that are each attached to a diphenylglycoluril-based clip molecule via four ethyleneoxy spacers, and are linked together by a single alkyl chain using "click"-chemistry. Following a newly developed multistep synthesis procedure we report three of these double porphyrin cages, linked by spacers of different lengths, i.e. 3, 5, and 11 carbon atoms. The structures of the double porphyrin cages were fully characterized by NMR, which revealed that they consist of mixtures of two diastereoisomers. Their zinc derivatives are capable of forming sandwich-like complexes with the ditopic ligand 1,4-diazabicyclo[2,2,2]octane (dabco).

Solute-Solvent Interactions in Modern Physical Organic Chemistry: Supramolecular Polymers as a Muse.

Interactions between solvents and solutes are a cornerstone of physical organic chemistry and have been the subject of investigations over the last century. In recent years, a renewed interest in fundamental aspects of solute-solvent interactions has been sparked in the field of supramolecular chemistry in general and that of supramolecular polymers in particular. Although solvent effects in supramolecular chemistry have been recognized for a long time, the unique opportunities that supramolecular polymers offer to gain insight into solute-solvent interactions have become clear relatively recently. The multiple interactions that hold the supramolecular polymeric structure together are similar in strength to those between solute and solvent. The cooperativity found in ordered supramolecular polymers leads to the possibility of amplifying these solute-solvent effects and will shed light on extremely subtle solvation phenomena. As a result, many exciting effects of solute-solvent interactions in modern physical organic chemistry can be studied using supramolecular polymers. Our aim is to put the recent progress into a historical context and provide avenues toward a more comprehensive understanding of solvents in multicomponent supramolecular systems.

Biasing the Screw-Sense of Supramolecular Coassemblies Featuring Multiple Helical States.

By enchaining a small fraction of chiral monomer units, the helical sense of a dynamic polymer constructed from achiral monomer units can be disproportionately biased. This phenomenon, known as the sergeants-and-soldiers (S&S) effect, has been found to be widely applicable to dynamic covalent and supramolecular polymers. However, it has not been exemplified with a supramolecular polymer that features multiple helical states. Herein, we demonstrate the S&S effect in the context of the temperature-controlled supramolecular copolymerization of chiral and achiral biphenyl tetracarboxamides in alkanes. The one-dimensional helical structures presented in this study are unique because they exhibit three distinct helical states, two of which are triggered by coassembling with monomeric water that is codissolved in the solvent. The self-assembly pathways are rationalized using a combination of mathematical fitting and simulations with a thermodynamic mass-balance model. We observe an unprecedented case of an "abnormal" S&S effect by changing the side chains of the achiral soldier. Although the molecular structure of these aggregates remains elusive, the coassembly of water is found to have a profound impact on the helical excess.

Supramolecular double-stranded Archimedean spirals and concentric toroids.

Connecting molecular-level phenomena to larger scales and, ultimately, to sophisticated molecular systems that resemble living systems remains a considerable challenge in supramolecular chemistry. To this end, molecular self-assembly at higher hierarchical levels has to be understood and controlled. Here, we report unusual self-assembled structures formed from a simple porphyrin derivative. Unexpectedly, this formed a one-dimensional (1D) supramolecular polymer that coiled to give an Archimedean spiral. Our analysis of the supramolecular polymerization by using mass-balance models suggested that the Archimedean spiral is formed at high concentrations of the monomer, whereas other aggregation types might form at low concentrations. Gratifyingly, we discovered that our porphyrin-based monomer formed supramolecular concentric toroids at low concentrations. Moreover, a mechanistic insight into the self-assembly process permitted a controlled synthesis of these concentric toroids. This study both illustrates the richness of self-assembled structures at higher levels of hierarchy and demonstrates a topological effect in noncovalent synthesis.

How Water in Aliphatic Solvents Directs the Interference of Chemical Reactivity in a Supramolecular System.

Water is typically considered to be insoluble in alkanes. Recently, however, monomerically dissolved water in alkanes has been shown to dramatically impact the structure of hydrogen-bonded supramolecular polymers. Here, we report that water in methylcyclohexane (MCH) also determines the outcome of combining a Michael reaction with a porphyrin-based supramolecular system. In dry conditions, the components of the reaction do not affect or destabilize the supramolecular polymer, whereas in ambient or wet conditions the polymers are rapidly destabilized. Although spectroscopic investigations show no effect of water on the molecular structure of the supramolecular polymer, light scattering and atomic force microscopy experiments show that water increases the flexibility of the supramolecular polymer and decreases the polymer length. Through a series of titrations, we show that a cooperative interaction, involving the coordination of the amine catalyst to the porphyrin and complexation of the substrates to the flexible polymers invokes the depolymerization of the aggregates. Water crucially stabilizes these cooperative interactions to cause complete depolymerization in humid conditions. Additionally, we show that the humidity-controlled interference in the polymer stability occurs with various substrates, indicating that water may play a ubiquitous role in supramolecular polymerizations in oils. By controlling the amount of water, the influence of a covalent chemical process on noncovalent aggregates can be mediated, which holds great potential to forge a connection between chemical reactivity and supramolecular material structure. Moreover, our findings highlight that understanding cooperative interactions in multicomponent noncovalent systems is crucial to design complex molecular systems.

Highly Efficient and Tunable Filtering of Electrons' Spin by Supramolecular Chirality of Nanofiber-Based Materials.

Organic semiconductors and organic-inorganic hybrids are promising materials for spintronic-based memory devices. Recently, an alternative route to organic spintronic based on chiral-induced spin selectivity (CISS) is suggested. In the CISS effect, the chirality of the molecular system itself acts as a spin filter, thus avoiding the use of magnets for spin injection. Here, spin filtering in excess of 85% in helical π-conjugated materials based on supramolecular nanofibers at room temperature is reported. The high spin-filtering efficiency can even be observed in nanofibers assembled from mixtures of chiral and achiral molecules through chiral amplification effect. Furthermore and most excitingly, it is shown that both "up" and "down" orientations of filtered spins can be obtained in a single enantiopure system via the temperature-dependent helicity (P and M) inversion of supramolecular nanofibers. The findings showcase that materials based on helical noncovalently assembled systems are modular platforms with an emerging structure-property relationship for spintronic applications.

Distinct Pathways in "Thermally Bisignate Supramolecular Polymerization": Spectroscopic and Computational Studies.

In general, supramolecular polymers are thermally labile in solution and easily depolymerized upon heating. This dynamic nature is beneficial in many aspects but limits certain applications. Recently, we developed "thermally bisignate supramolecular polymerization", through which a polymer is formed upon heating as well as cooling in a hydrocarbon solvent containing a small amount of alcohol. Here, we present a detailed mechanistic picture for this polymerization based on both spectroscopic and computational studies. For this particular type of polymerization, we mainly employed a copper porphyrin derivative ((S)PORCu) as a monomer with eight hydrogen-bonding (H-bonding) amide units in its chiral side chains. Because of a strong multivalent interaction, the resulting supramolecular polymer displayed an extraordinarily high thermal stability in a hydrocarbon medium such as methylcyclohexane (MCH)/chloroform (CHCl3) (98/2 v/v; denoted as MCH*). However, when a small volume (<2.0 vol %) of ethanol (EtOH) was added to this solution at ambient temperatures as a H-bond scavenger, the supramolecular polymer dissociated into its monomers. Here, it should be noted that, both upon cooling (clustering of EtOH) and heating (lower-critical-solution-temperature behavior, LCST), the monomer was liberated from the H-bond scavenger and underwent supramolecular polymerization. In this Article, we conducted detailed spectroscopic studies, analyzed the results using theoretical models, and eventually succeeded in supporting the pathways explaining why the monomer deactivated by the H-bond scavenger turns active upon both heating and cooling. We also investigated the thermally bisignate nature of the supramolecular polymerization of other monomers such as triphenylamine ((S)TPA) and pyrene ((S)Py) derivatives together with free-base ((R)POR2H) and zinc porphyrin ((S)PORZn) derivatives and rationalized the large potential for this multicomponent supramolecular polymerization.

Selenoamides modulate dipole-dipole interactions in hydrogen bonded supramolecular polymers of 1,3,5-substituted benzenes.

We report the synthesis and self-assembly behavior of a chiral C3-symmetrical benzene-tricarboselenoamide. The introduction of the selenoamide moiety enhances the dipolar character of the supramolecular interaction and confers a remarkable thermal stability to the supramolecular polymers obtained.

Competing Interactions in Hierarchical Porphyrin Self-Assembly Introduce Robustness in Pathway Complexity.

Pathway complexity in supramolecular polymerization has recently sparked interest as a method to generate complex material behavior. The response of these systems relies on the existence of a metastable, kinetically trapped state. In this work, we show that strong switch-like behavior in supramolecular polymers can also be achieved through the introduction of competing aggregation pathways. This behavior is illustrated with the supramolecular polymerization of a porphyrin-based monomer at various concentrations, solvent compositions, and temperatures. It is found that the monomers aggregate via an isodesmic mechanism in weakly coupled J-type aggregates at intermediate solvent quality and temperature, followed by nucleated H-aggregates at lower solvent qualities and temperatures. At further increased thermodynamic driving forces, such as high concentration and low temperature, the H-aggregates can form hierarchical superhelices. Our mathematical models show that, contrary to a single-pathway polymerization, the existence of the isodesmic aggregation pathway buffers the free monomer pool and renders the nucleation of the H-aggregates insensitive to concentration changes in the limit of high concentrations. We also show that, at a given temperature or solvent quality, the thermodynamically stable aggregate morphology can be selected by controlling the remaining free external parameter. As a result, the judicious application of pathway complexity allows us to synthesize a diverse set of materials from only a single monomer. We envision that the engineering of competing pathways can increase the robustness in a wide variety of supramolecular polymer materials and lead to increasingly versatile applications.

Potential enthalpic energy of water in oils exploited to control supramolecular structure.

Water directs the self-assembly of both natural1,2 and synthetic3-9 molecules to form precise yet dynamic structures. Nevertheless, our molecular understanding of the role of water in such systems is incomplete, which represents a fundamental constraint in the development of supramolecular materials for use in biomaterials, nanoelectronics and catalysis 10 . In particular, despite the widespread use of alkanes as solvents in supramolecular chemistry11,12, the role of water in the formation of aggregates in oils is not clear, probably because water is only sparingly miscible in these solvents-typical alkanes contain less than 0.01 per cent water by weight at room temperature 13 . A notable and unused feature of this water is that it is essentially monomeric 14 . It has been determined previously 15 that the free energy cost of forming a cavity in alkanes that is large enough for a water molecule is only just compensated by its interaction with the interior of the cavity; this cost is therefore too high to accommodate clusters of water. As such, water molecules in alkanes possess potential enthalpic energy in the form of unrealized hydrogen bonds. Here we report that this energy is a thermodynamic driving force for water molecules to interact with co-dissolved hydrogen-bond-based aggregates in oils. By using a combination of spectroscopic, calorimetric, light-scattering and theoretical techniques, we demonstrate that this interaction can be exploited to modulate the structure of one-dimensional supramolecular polymers.

Non-Covalent Microgel Particles Containing Functional Payloads: Coacervation of PEG-Based Triblocks via Microfluidics.

Well-defined microgel particles were prepared by combining coacervate-driven cross-linking of ionic triblock copolymers with the ability to control particle size and encapsulate functional cargos inherent in microfluidic devices. In this approach, the efficient assembly of PEO-based triblock copolymers with oppositely charged end-blocks allows for bioinspired cross-linking under mild conditions in dispersed aqueous droplets. This strategy enables the integration of charged cargos into the coacervate domains (e.g., the loading of anionic model compounds through electrostatic association with cationic end-blocks). Distinct release profiles can be realized by systematically varying the chemical nature of the payload and the microgel dimensions. This mild and noncovalent assembly method represents a promising new approach to tunable microgels as scaffolds for colloidal biomaterials in therapeutics and regenerative medicine.

Ultra-responsive soft matter from strain-stiffening hydrogels.

The stiffness of hydrogels is crucial for their application. Nature's hydrogels become stiffer as they are strained. This stiffness is not constant but increases when the gel is strained. This stiffening is used, for instance, by cells that actively strain their environment to modulate their function. When optimized, such strain-stiffening materials become extremely sensitive and very responsive to stress. Strain stiffening, however, is unexplored in synthetic gels since the structural design parameters are unknown. Here we uncover how readily tuneable parameters such as concentration, temperature and polymer length impact the stiffening behaviour. Our work also reveals the marginal point, a well-described but never observed, critical point in the gelation process. Around this point, we observe a transition from a low-viscous liquid to an elastic gel upon applying minute stresses. Our experimental work in combination with network theory yields universal design principles for future strain-stiffening materials.