The role of the secondary structure of helical poly(phenylacetylene)s in the formation of nanoparticles from polymer-metal complexes (HPMCs)

Size Control of Chiral Nanospheres Obtained via Nanoprecipitation of Helical Poly(phenylacetylene)s in the Absence of Surfactants

Nanostructuration of dynamic helical polymers such as poly(phenylacetylene)s (PPAs) depends on the secondary structure adopted by the polymer and the functional group used to connect the chiral pendant to the PPA backbone. Thus, while PPAs with dynamic and flexible scaffolds (para- and meta-substituted, ω1<165°) generate by nanoprecipitation low polydisperse nanospheres with controllable size at different acetone/water mixtures, those with a quasi-static behavior and the presence of an extended, almost planar structure (ortho-substituted, ω1>165°), aggregate into a mixture of spherical and oval nanostructures whose size is not controlled. Photostability studies show that poly(phenylacetylene) particles are more stable to light irradiation than when dissolved macromolecularly. Moreover, the photostability of the particle depends on the secondary structure of the PPA and its screw sense excess. This fact, in combination with the encapsulation ability of these polymer particles, allows the creation of light stimuli-responsive nanocarriers, whose cargo can be delivered by light irradiation.

Stimuli-responsive synthetic helical polymers

Synthetic dynamic helical polymers (supramolecular and covalent) and foldamers share the helix as a structural motif. Although the materials are different, these systems also share many structural properties, such as helix induction or conformational communication mechanisms. The introduction of stimuli responsive building blocks or monomer repeating units in these materials triggers conformational or structural changes, due to the presence/absence of the external stimulus, which are transmitted to the helix resulting in different effects, such as assymetry amplification, helix inversion or even changes in the helical scaffold (elongation, J/H helical aggregates). In this review, we show through selected examples how different stimuli (e.g., temperature, solvents, cations, anions, redox, chiral additives, pH or light) can alter the helical structures of dynamic helical polymers (covalent and supramolecular) and foldamers acting on the conformational composition or molecular structure of their components, which is also transmitted to the macromolecular helical structure.

Thermoresponsive Helical Dendronized Poly(phenylacetylene)s: Remarkable Stabilization of Their Helicity via Photo-Dimerization of the Dendritic Pendants

Dynamic helical polymers can change their helicity according to external stimuli due to the low helix-inversion barriers, while helicity stabilization for polymers is important for applications in chiral recognition or chiral separations. Here, we present a convenient methodology to stabilize dynamic helical conformations of polymers through intramolecular cross-linking. Thermoresponsive dendronized poly(phenylacetylene)s (PPAs) carrying 3-fold dendritic oligoethylene glycol pendants containing cinnamate moieties were synthesized. These polymers exhibit typical features of dynamic helical structures in different solvents, that is, racemic contracted conformations in less polar organic solvents and predominantly one-handed stretched helical conformations in highly polar solvents. This dynamic helicity can be enhanced through selective solvation by increasing the polarity of the organic solvents or simply via their thermally mediated dehydration in water. However, through photocycloaddition of the cinnamate moieties between the neighboring pendants via UV irradiation, these dendronized PPAs adopt stable helical conformations either below or above their phase transition temperatures in water, and their helical conformations can even be retained in less polar organic solvents. Spectroscopic and atomic force microscopy measurements demonstrate that photocycloaddition between the cinnamate moieties occurs on the individual molecular level, and this is found to be helpful in restraining the photodegradation of the PPA backbones. Molecular dynamics simulations reveal that the spatial orientation of the pendants along the rigid polyene backbone is crucial for the photodimerization of cinnamates within one helix pitch.

Screw sense excess and reversals of helical polymers in solution

The helix reversal is a structural motif found in helical polymers in the solid state, but whose existence is elusive in solution. Herein, we have shown how the photochemical electrocyclization (PEC) of poly(phenylacetylene)s (PPAs) can be used to determine not only the presence of helix reversals in polymer solution, but also to estimate the screw sense excess. To perform these studies, we used a library of well folded PPAs and different copolymers series made by enantiomeric comonomers that show chiral conflict effect. The results obtained indicate that the PEC of a PPA will depend on the helical scaffold adopted by the PPA backbone and on its folding degree. Then, from these studies it is possible to determine the screw sense excess of a PPA, highly important in applications such as chiral stationary phases in HPLC or asymmetric synthesis.

Chiroptical and colorimetric switches based on helical polymer-metal nanocomposites prepared via redox metal translocation of helical polymer metal complexes

A helical copoly(phenylacetylene) that follows a dynamic chiral accord effect has been designed to further synthesize dynamic chiral nanocomposites. Its two pendants are benzamides of (L)-methionine methyl ester [(L)-1, 20%] and (L)-alanine methyl ester [(L)-2, 80%], the former being responsible for binding the copolymer to metallic nanoparticles (MNPs, M = Au, Ag) via the thioether. The two chiral comonomers have analogous dynamic behavior, and therefore, the copolymer-poly-[(L)-1_0.2-co-(L)-2_0.8]-adopts a preferred helical sense that can be amplified or inverted by stimuli acting simultaneously on both pendants. The formation of nanocomposites can be followed by different sequential chiroptical responses of the copolymer once the helical polymer metal-complexes are formed-M to P helix inversion by the formation of poly-[(L)-1_0.2-co-(L)-2_0.8]/Au³⁺ or poly-[(L)-1_0.2-co-(L)-2_0.8]/Ag⁺-and further reduction with NaBH₄ to generate the corresponding nanocomposites-P to M helix inversion by the formation of poly-[(L)-1_0.2-co-(L)-2_0.8]-AuNPs (6 nm) and poly-[(L)-1_0.2-co-(L)-2_0.8]-AgNPs (5 nm). These nanocomposites exhibit the properties of both components, helix inversion in the PPA and a colorimetric response in the MNPs triggered by metal ions.

Photostability and Dynamic Helical Behavior in Chiral Poly(phenylacetylene)s with a Preferred Screw‐Sense

Helical polymers such as poly(phenylacetylene)s (PPAs) are interesting materials due to the possibility of tuning their helical scaffold (sense and elongation) once they have been prepared and by the presence of external stimuli. The main limitation in the application of PPAs is their poor photostability. These polymers degrade under visible light exposure through a photochemical electrocyclization process. In this work, it was demonstrated, through a selected example, how the photochemical degradation in PPAs is directly related to their dynamic helical behavior. Thus, while PPAs with dynamic helical structures show poor photostability under UV/Vis light exposure, poly‐(R)‐1, bearing an enantiopure sulfoxide group as pendant group and designed to have a quasi‐static helical behavior, shows a large photostability due to the restricted conformational composition at the polyene backbone, needed to orient the conjugated double bonds prior to the photochemical electrocyclization process and the subsequent degradation of the material.

The Role of Polymer-AuNP Interaction in the Stimuli-Response Properties of PPA-AuNP Nanocomposites

The helical sense control of dynamic helical polymers such as poly(phenylacetylene)s (PPAs) is greatly affected when they are conjugated to AuNPs through a strong thiol-Au connection, which restricts conformational changes at the polymer. Thus, the classical thiol-MNP bonds must be replaced by weaker ones, such as supramolecular amide-Au interactions. A straightforward preparation of the PPA-Au nanocomposite by reduction of a preformed PPA-Au³⁺ complex cannot be used due to a redox reaction between the two components of the complex which degrades the polymer. To avoid the interaction between the PPA and the Au³⁺ ions before the reduction takes place, the metal ions are added to the polymer solution capped as a TOAB complex, which keeps the PPA stable due to the lack of PPA-Au³⁺ interactions. Ulterior reduction of the Au³⁺ ions by NaBH₄ affords the desired nanocomposite, where the AuNPs are stabilized by supramolecular anilide-AuNPs interactions. By using this approach, 3.7 nm gold nanoparticles are generated and aligned along the polymer chain with a regular distance between particles of 6 nm that corresponds to two helical pitches. These nanocomposites show stimuli-responsive properties and are also able to form macroscopically chiral nanospheres with tunable size.

Photochemical Electrocyclization of Poly(phenylacetylene)s: Unwinding Helices to Elucidate their 3D Structure in Solution

Photochemical electrocyclization of poly(phenylacetylene)s (PPAs) is used for the structural elucidation of a polyene backbone. This method not only allows classification of PPAs in cis-cisoidal (ω₁ <90°) or cis-transoidal structures (ω₁ >90°), but also approximating ω₁ . A PPA solution is illuminated with visible light and monitoring the photochemical electrocyclization of the PPA helix by measuring the ECD spectra at different times. PPAs with a cis-cisoidal structure show a reduction of the ECD signal of at least 50 % before 30 min of irradiation, while cis-transoidal helices need much longer time because the transoidal bond must be isomerized. The different cis-cisoidal and cis-transoidal helices require different times to decrease their ECD signal by 50 % (t_1/2 ), depending on the degree of compression or stretching of the helix, establishing a relationship between the secondary structure adopted by PPA (ω₁ ) and the time required to lose the ECD vinylic signal by light irradiation.

Raman Optical Activity (ROA) as a New Tool to Elucidate the Helical Structure of Poly(phenylacetylene)s

Poly(phenylacetylene)s are a family of helical polymers constituted by conjugated double bonds. Raman spectra of these polymers show a structural fingerprint of the polyene backbone which, in combination with its helical orientation, makes them good candidates to be studied by Raman optical activity (ROA). Four different well-known poly(phenylacetylene)s adopting different scaffolds and ten different helical senses have been prepared. Raman and ROA spectra were recorded and allowed to establish ROA-spectrum/helical-sense relationships: a left/right-handed orientation of the polyene backbone (M_helix /P_helix ) produces a triplet of positive/negative ROA bands. Raman and ROA spectra of each polymer exhibited the same profile, and the sign of the ROA spectrum was opposite to the lowest-energy electronic circular dichroism (ECD) band, indicating a resonance effect. Resonance ROA appears then as an indicator of the helical sense of poly(phenylacetylene)s, especially for those with an extra Cotton band in the ECD spectrum, where a wrong helical sense is assigned based on ECD, while ROA alerts of this misassignment.

Chiral Overpass Induction in Dynamic Helical Polymers Bearing Pendant Groups with Two Chiral Centers

The dynamic behavior of helical polymers bearing pendant groups with two chiral centers was studied. Controlled conformational changes at the chiral units placed either closer to or further away from the main chain promote different helical structures. Although the first residue is usually responsible for determining a specific helicity (P or M), we now found that the second chiral center is also able to induce a preferred helical sense when it is located closer in space to the main chain, thereby cancelling the order from the first chiral moiety. This result was achieved through proper coordination with a metal cation. As proof of concept, poly(phenylacetylene)s (PPAs) that bear one and two chiral amino acid units of different sizes and configuration combinations (l/d-alanine and l/d-phenylalanine) as pendants were evaluated. In total, ten polymers were studied. This constitutes the first report of axial control from a remote stereocenter in polymers bearing complex chiral pendants.

Chiral gold-PPA nanocomposites with tunable helical sense and morphology

A novel type of stimuli-responsive dynamic helical polymer-metal nanoparticle nanocomposite formed by a helical poly(phenylacetylene) (PPA) combined with gold nanoparticles (AuNPs) is described. Thus, several PPA copolymers containing the ethynyl-4-benzamide of (S)-phenylglycine methyl ester (M1) to dictate the helical structure/sense of the copolymer, and the ethynyl-4-benzamide of the 11-((2-(2-(2-aminoethoxy)ethoxy)ethyl)amino)undecane-1-thiol (M2) to link the copolymer to the AuNPs are prepared. Different morphologies of these nanocomposites were obtained by considering the thiol ratio and the self-assembly properties of the PPA, which generates from dispersed AuNPs to fibre-like structures. All these nanocomposites show a dynamic chiral behaviour, it being possible to manipulate their helical sense by the action of external stimuli. Moreover, it is possible to control the aggregation of these nanocomposites into macroscopically chiral nanospheres with low polydispersity by using Ba2+ as a crosslinking agent.

Synthesis of poly(phenylacetylene)s containing chiral phenylethyl carbamate residues as coated-type CSPs with high solvent tolerability

Two novel helical poly(phenylacetylene) derivatives containing chiral phenylethyl carbamate residues in the end of each side chain (PPA-S and PPA-R) were synthesized by polymerization of the corresponding phenylacetylene monomers using Rh(nbd)BPh₄ as a catalyst in DMF. The enantioseparation properties of the polymers were evaluated as coated-type chiral stationary phases (CSPs) for high-performance liquid chromatography (HPLC). Under the same chromatographic conditions, PPA-S and PPA-R showed different enantioseparation properties, indicating that the different interactions between the analytes and the polymers, which result from the different chiral phenylethyl carbamate groups in the end of each side chains. Racemates 1, 7, and 8 could be better resolved on PPA-S, while racemate 6 was separated on PPA-R more efficiently. In addition, the coated-type CSPs showed good solvent tolerability and could work without any damage by introducing the polar solvents, such as CHCl₃ and THF, in eluent. Moreover, some racemates could be better resolved on these coated-type CSPs with the addition of THF to the eluent.

Nonspherical chiral helical polymer particles with programmable morphology prepared by electrospraying

Chirality and chiral materials demonstrate ever-growing importance. As a new type of chiral material, chiral polymer particles hold huge potential in both scientific research and practical applications. Meanwhile, nonspherical polymer particles (NPPs) have witnessed substantial progress in recent years because of their unique structures and especially the properties distinguishable from the corresponding spherical particles. We hypothesize that combining chirality with NPPs will open up an unprecedented category of advanced materials. The present contribution reports the first protocol for preparing electrosprayed nonspherical chiral particles constructed from chiral helical polymers, using helical substituted polyacetylenes as the model. SEM images demonstrate the successful fabrication of nonspherical chiral particles with tunable morphologies (bowl-, golf- and apple-like particles). Circular dichroism (CD) measurement proves the remarkable optical activity of the particles, which is observed in the predominantly one-handed helical polymer chains. The present work establishes a novel, versatile, and powerful platform for preparing nonspherical chiral polymer particles with controllable morphology.

Decoding the ECD Spectra of Poly(phenylacetylene)s: Structural Significance

The role of the main dihedral angles in the electronic circular dichroism (ECD) spectra of poly(phenylacetylene)s (PPAs) was estimated by using time-dependent density functional theory (TD-DFT) for oligo(phenylacetylene)s (n = 12). These studies reveal that in cis-transoidal arrangements, the first Cotton effect is dominated by excitations involving molecular orbitals (MOs) mainly related to the polyene backbone. Hence, for this scaffold, the ± sign of the first Cotton effect reflects the P/M helical sense of the internal helix of the polymer. However, in cis-cisoidal arrangements, contribution of MOs in the polyene and the aryl rings of the PPA backbone produce the first Cotton effect band. As a result, two different ECD signatures with three or four alternating Cotton effects can be produced depending on the sign of the ω₁ and ω₃ dihedral angles which determine the helical sense of the polyene (ω₁) and the relative orientation of the aryl ring toward the polyene (ω₃), respectively. Thus, on the one hand, if ω₁ and ω₃ rotate in opposite directions, a CD with three alternating Cotton effects is observed, where the sign of first Cotton correlates with the P/M helical sense of the polyene. On the other hand, if ω₁ and ω₃ rotate in the same direction, a CD signature with four alternating Cotton effects is produced where the information relative to the P/M helical sense of the polyene is contained in the second Cotton effect.

Macromolecular helicity control of poly(phenyl isocyanate)s with a single stimuli-responsive chiral switch

The introduction of a single chiral substituent with a switchable conformer equilibrium at the poly(phenyl isocyanate) terminus allowed the control of the helical sense of the polymer backbone using various external stimuli. Helical sense enhancement was achieved through a domino effect, where the helical sense was assigned by VCD spectroscopy and DFT calculations.

Chiral-to-Chiral Communication in Polymers: A Unique Approach To Control Both Helical Sense and Chirality at the Periphery

A novel approach to the classical Sergeants and Soldiers effect, using chiral Sergeants and chiral Soldiers, allows control over both helical and external chirality in helical polymers. In the systems reported here, it is possible to induce the same helical sense ( M or P) from either of the two enantiomers of a chiral pendant group ["chiral Soldier", major component; i.e., ( R)- or ( S)-1] when it faces a single enantiomer of an appropriate "chiral Sergeant" [minor component; i.e., ( S)-2]. For instance, the copolymer series poly[( R)-1 _r- co-( S)-2_{(1- r)}], poly[( S)-1 _r- co-( S)-2_{(1- r)}], and poly[( rac)-1 _r- co-( S)-2_{(1- r)}] adopt the same P helix even though the major component shows the opposite absolute configuration. This chiral-to-chiral communication effect is transmitted by the stabilization of different conformations in each enantiomeric form of the Soldier. As a result, this groundbreaking approximation to the Sergeants and Soldiers effect allows the preparation of a single-handed helix-which depends only on the Sergeant's configuration-with different chiralities on the helix periphery. Thus, a P helix can be decorated with the R isomer, S isomer, or even a racemic mixture of the chiral Soldier. A change in the absolute configuration of the Sergeant affords the opposite M helix, which can also be decorated with the R isomer, S isomer, or racemic mixture of the chiral Soldier.

Functional identification of glutamate cysteine ligase and glutathione synthetase in the marine yeast Rhodosporidium diobovatum

Glutathione (GSH) fulfills a variety of metabolic functions, participates in oxidative stress response, and defends against toxic actions of heavy metals and xenobiotics. In this study, GSH was detected in Rhodosporidium diobovatum by high-performance liquid chromatography (HPLC). Then, two novel enzymes from R. diobovatum were characterized that convert glutamate, cysteine, and glycine into GSH. Based on reverse transcription PCR, we obtained the glutathione synthetase gene (GSH2), 1866 bp, coding for a 56.6-kDa protein, and the glutamate cysteine ligase gene (GSH1), 2469 bp, coding for a 90.5-kDa protein. The role of GSH1 and GSH2 for the biosynthesis of GSH in the marine yeast R. diobovatum was determined by deletions using the CRISPR-Cas9 nuclease system and enzymatic activity. These results also showed that GSH1 and GSH2 were involved in the production of GSH and are thus being potentially useful to engineer GSH pathways. Alternatively, pET-GSH constructed using vitro recombination could be used to detect the function of genes related to GSH biosynthesis. Finally, the fermentation parameters determined in the present study provide a reference for industrial GSH production in R. diobovatum.