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.

Chiral information harvesting in helical poly(acetylene) derivatives using oligo(p-phenyleneethynylene)s as spacers

A chiral harvesting transmission mechanism is described in poly(acetylene)s bearing oligo(p-phenyleneethynylene)s (OPEs) used as rigid achiral spacers and derivatized with chiral pendant groups. The chiral moieties induce a positive or negative tilting degree in the stacking of OPE units along the polymer structure, which is further harvested by the polyene backbone adopting either a P or M helix.

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.

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.

Stimuli-Directed Colorimetric Interconversion of Helical Polymers Accompanied by a Tunable Self-Assembly Process

Interconversion between extended and bent structures at the pendant groups of a chiral polyene framework [poly(phenylacetylene) with (R)-(2-methoxy-2-phenylacetyl)glycine residues linked to 4-vinylanilines] allows the reversible colorimetric transformation from stretched to compressed helical cis-transoid polyenic structures through manipulation of the flexible spacer. This transformation generates either organogels (stretched helical form) or nanoparticles (compressed helical form) under the control of polar/low polar stimuli respectively and opens the way to the development of new sensors and stimuli-sensitive materials based on these concepts.

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.

Sequential Induction of Chirality in Helical Polymers: From the Stereocenter to the Achiral Solvent

Several steps of chiral induction have been detected in poly(phenylacetylene)s among their different hierarchical levels of chirality by vibrational circular dichroism, namely, (i) from the stereogenic centers to the innermost polyacetylene helical covalent backbone (helix_int), (ii) from this to the external helix (helix_ext) formed by the side phenyl pendants that form a complementary helix or counter-helix, and (iii) from this pendant helix to the helical solvation sphere (helix_solv.), the last one being observed along this work. The pendant to polyene backbone chiral induction determines the helical structure adopted by the polymer and therefore the solvation helix. This helical structure is promoted by two mechanisms: steric effects and hydrogen bonding. An important finding concerns the demonstration by VCD of how an achiral solvent becomes chirally organized owing to the template effect of the covalent polymer helices, an effect that is silent to other structural techniques such as ECD or AFM and that hence significantly broadens the scope of these previous analyses.

Fast NMR Screening of Macromolecular Complexes by a Paramagnetic Spin Relaxation Filter

The paramagnetic spin relaxation filter is described for the rapid NMR screening of intermolecular interactions between ligands and macromolecular anionic receptors with large transverse relaxation enhancements (R 2p). The addition of micromolar concentrations of Gd3+ to the mixture produces the immediate broadening/suppression of the NMR signals of interacting species while leaving unaffected those of noncompetitive binders (one-dimensional and two-dimensional experiments). The method is highly sensitive, unveiling interactions that are too weak to generate changes in chemical shifts or relaxation times. It is operationally very simple and hence, it is amenable to ready implementation by nonspecialists. Examples of application such as detecting the formation of interpolymer complexes, cyclodextrin host-guest interactions, and the screening of DNA ligands are included that demonstrate the reliability and broad applicability of the method.

Predicting the Helical Sense of Poly(phenylacetylene)s from their Electron Circular Dichroism Spectra

The calculated ECD spectrum (time-dependent density functional theory TD-DFT) for small oligomers of polyphenylacetylenes (PPAs) show a very good match with the experimental spectra of the PPA polymers, particularly with the first Cotton band associated to the helical sense of the internal polyenic backbone. This has been proven with a series of PPAs representative of cis-cisoidal, cis-transoidal, compressed and stretched polyene backbones, with identical or opposite internal/external rotational senses and allows the prediction of the helical sense of the internal helix of a PPA directly from its CD spectra.

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

The great importance of the secondary structure (compressed/stretched) of helical poly(phenylacetylene)s (PPAs) in the formation of nanostructures (nanospheres and nanotoroids) by complexation with metal ions of diverse valences is demonstrated. PPAs bearing the same chelating units [anilide of (R)-methoxyphenylacetic acid] but displaying different helical scaffolds show great differences in their nanostructuration due to the different secondary structures of their helices despite the analogous ways in which their mono- and divalent metal ions form complexes. This key 3-D structural feature has not been taken into account previously when studying the nanostructuration of helical polymer-metal complexes (HPMCs).

Chiral nanostructure in polymers under different deposition conditions observed using atomic force microscopy of monolayers: poly(phenylacetylene)s as a case study

Dynamic poly(phenylacetylene)s (PPAs) adopt helical structures with different elongation or helical senses depending on the types of pendants. Hence, a good knowledge of the parameters that define their structures becomes a key factor in the understanding of their properties and functions. Herein, the techniques used for the study of the secondary structure of PPAs using atomic-force microscopy (AFM) are presented, with special attention directed towards the methods used for the preparation of monolayers, and their consequences in the quality of the AFM images. Thus, monolayers formed by drop casting, spin coating followed by crystallization or annealing, Langmuir-Blodgett and Langmuir-Schaefer methods, onto highly oriented pyrolytic graphite (HOPG) or mica, are described, together with the AFM images and the resulting helical structure obtained for different PPAs. Furthermore, some conclusions are drawn both on the adequacy of the different techniques for the formation of monolayers and on the solid supports utilized to elucidate the secondary structure of different PPAs.

Simultaneous Adjustment of Size and Helical Sense of Chiral Nanospheres and Nanotubes Derived from an Axially Racemic Poly(phenylacetylene)

Nanospheres and nanotubes with full control of their size and helical sense are obtained in chloroform from the axially racemic chiral poly(phenylacetylene) poly-(R)-1 using either Ag⁺ as both chiral inducer and cross-linking agent or Na⁺ as chiral inducer and Ag⁺ as cross-linking agent. The size is tuned by the polymer/ion ratio while the helical sense is modulated by the polymer/cosolvent (i.e., MeCN) ratio. In this way, the helicity and the size of the nanoparticles can be easily interconverted by very simple experimental changes.

A general route to chiral nanostructures from helical polymers: P/M switch via dynamic metal coordination

Macroscopically enantiomeric chiral nanospheres made from P or M helical polymer metal complexes can be obtained via dynamic coordination chemistry.

Multipodal dynamic coordination involving cation–π interactions to control the structure of helical polymers

Dynamic coordination, by means of multipodal metal complexes and cation–π interactions, controls the structure of helical polymers.

Biodegradable PEG–dendritic block copolymers: synthesis and biofunctionality assessment as vectors of siRNA

New hybrid-biodegradable PEG–dendritic block copolymers as versatile delivery vectors for biomedical applications. Here, their biofunctionality as siRNA vectors is presented.

Helical sense selective domains and enantiomeric superhelices generated by Langmuir-Schaefer deposition of an axially racemic chiral helical polymer

The chiral polymer poly-(R)-1 behaves in solution, despite its chiral pendants, as a dynamic axially racemic (i.e., 1 : 1) mixture of left- and right-handed helices, but its deposition on graphite by a Langmuir-Schaefer (LS) technique leads to a helical sense-selective packing that forms separate enantiomeric domains of left- and right-handed helical chains observed by high resolution atomic force microscopy (AFM). The polymer structure within these domains is very uniform, seldom altered by the presence of reversals, grouped always in contiguous pairs maintaining a single helical sense along the polymer chain. The LS deposition technique has been shown to be crucial to obtain good quality monolayers from poly-(R)-1 and other poly(phenylacetylene)s (PPAs: poly-2, poly-3 and poly-4) with short pendants, where spin coating, drop casting and Langmuir-Blodgett (LB) failed, and suggests that this technique could be the method of choice for the preparation of 2D monolayers for high resolution AFM studies of PPAs with short pendants. Key helical parameters (i.e., sense, pitch, packing angle) are easily measured in this way.

Chiral Nanostructures from Helical Copolymer-Metal Complexes: Tunable Cation-π Interactions and Sergeants and Soldiers Effect

Poly(phenylacetylene) (PPA) copolymers containing (R)- or (S)-MPA as minor chiral pendant can be forced to selectively adopt the right- o left-handed helix, in the presence of small amounts of Na(+) or Ag(+) ("Sergeants and Soldiers Effect") by addition of a donor cosolvent. The helical sense depends exclusively on the chiral monomer/donor cosolvent ratio, and this allows a perfect on/off tuning of the helicity of the copolymer. When the amount of the donor cosolvent is low, the metal ion complex is stabilized by a cation-π interaction, which is selectively cleaved when the amount of cosolvent is higher. Macroscopically chiral nanospheres and nanotubes composed by helical copolymers with P or M helical sense are also described. Our results demonstrate that it is possible to obtain the two enantiomeric helical structures (P and M helicities) and the corresponding nanospheres and nanotubes from a single helical copolymer, by controlled activation/deactivation of the Sergeant and Soldiers Effect with a donor cosolvent.

Systemically administered brain-targeted nanoparticles transport peptides across the blood-brain barrier and provide neuroprotection

Although growth factors and anti-apoptotic peptides have been shown to be neuroprotective in stroke models, translation of these experimental findings to clinic is hampered by limited penetration of peptides to the brain. Here, we show that a large peptide like the basic fibroblast growth factor (bFGF) and a small peptide inhibitor of caspase-3 (z-DEVD-FMK) can effectively be transported to the brain after systemic administration by incorporating these peptides to brain-targeted nanoparticles (NPs). Chitosan NPs were loaded with peptides and then functionalized by conjugating with antibodies directed against the transferrin receptor-1 on brain endothelia to induce receptor-mediated transcytosis across the blood-brain barrier (BBB). Pre-ischemic systemic administration of bFGF- or z-DEVD-FMK-loaded NPs significantly decreased the infarct volume after 2-hour middle cerebral artery occlusion and 22-hour reperfusion in mice. Co-administration of bFGF- or z-DEVD-FMK-loaded NPs reduced the infarct volume further and provided a 3-hour therapeutic window. bFGF-loaded NPs were histologically detected in the brain parenchyma and also restored ischemia-induced Akt dephosphorylation. The neuroprotection was not observed when receptor-mediated transcytosis was inhibited with imatinib or when bFGF-loaded NPs were not conjugated with the targeting antibody, which enables them to cross the BBB. Nanoparticles targeted to brain are promising drug carriers to transport large as well as small BBB-impermeable therapeutics for neuroprotection against stroke.

The leading role of cation–π interactions in polymer chemistry: the control of the helical sense in solution

Cation–π interactions determine the helical sense adopted by a polyphenylacetylene bearing (R)-α-methoxy-α-phenylacetamide as a pendant group (poly-1).

Reversible assembly of enantiomeric helical polymers: from fibers to gels

A novel class of stereocomplexes is described by the interaction of helically complementary poly(phenylacetylene)s (PPAs) carrying an α-methoxy-α-trifluoromethylphenylacetamide pendant group. The formation of the stereocomplex requires the presence of cis amide bonds on the external crest of the polymer to provide efficient cooperative supramolecular hydrogen bonding between matching enantiomeric helical structures. The interlocking of the chains gives rise to supramolecular fiber-like aggregates that, at higher concentrations, result in gels. The modification of the cis-trans amide conformation at the pendant groups allows the controlled formation and cleavage of the stereocomplex due to a dramatic change between the intermolecular and intramolecular hydrogen bond interactions.

Predicting PSR filters by transverse relaxation enhancements

The paramagnetic spin relaxation (PSR) filter allows the suppression of the NMR resonances of individual components in mixtures according to their Gd(3+)-complexing ability. The difficulty in predicting this property hampers, however, the widespread application of this filter. Herein we describe that the PSR filter is dominated by the transverse relaxation enhancement (R(2p)) experienced by nuclei in the presence of Gd(3+), so that R(2p) represents a reliable predictive tool of suppression in the 1D and 2D PSR filter of complex mixtures. The robustness of R(2p) as a predictive tool in PSR filters has been demonstrated at different magnetic fields and for the (1)H, (13)C, COSY, and HMQC filtering of commercial multicomponent compositions, including beverages and drugs.

GATG dendrimers and PEGylated block copolymers: from synthesis to bioapplications

Dendrimers are synthetic macromolecules composed of repetitive layers of branching units that emerge from a central core. They are characterized by a tunable size and precise number of peripheral groups which determine their physicochemical properties and function. Their high multivalency, functional surface, and globular architecture with diameters in the nanometer scale makes them ideal candidates for a wide range of applications. Gallic acid-triethylene glycol (GATG) dendrimers have attracted our attention as a promising platform in the biomedical field because of their high tunability and versatility. The presence of terminal azides in GATG dendrimers and poly(ethylene glycol) (PEG)-dendritic block copolymers allows their efficient functionalization with a variety of ligands of biomedical relevance including anionic and cationic groups, carbohydrates, peptides, or imaging agents. The resulting functionalized dendrimers have found application in drug and gene delivery, as antiviral agents and for the treatment of neurodegenerative diseases, in diagnosis and as tools to study multivalent carbohydrate recognition and dendrimer dynamics. Herein, we present an account on the preparation and recent applications of GATG dendrimers in these fields.

An aquaporin 4 antisense oligonucleotide loaded, brain targeted nanoparticulate system design

Aquaporins (AQPs), members of the water-channel protein family, are highly expressed in brain tissue especially in astrocytic end-feet. They are important players for water hemostasis during development of cytotoxic as well as vasogenic edema. Increased expression of AQPs is important in pathophysiology of neurological diseases such as neuroinflammation and ischemia. Unfortunately, there are a few pharmacological inhibitors of AQP4 with several side effects limiting their translation as a drug for use in clinical conditions. Another therapeutic approach is using antisense oligonucleotides (ASOs) to block AQP4 activity. These are short, synthetic, modified nucleic acids that bind RNA to modulate its function. However, they cannot pass the blood brain barrier (BBB). To overcome this obstacle we designed a nanoparticulate system made up of chitosan nanoparticles surface modified with PEG and conjugated with monoclonal anti transferrin receptor-1 antibody via streptavidin-biotin binding. The nanocarrier system could be targeted to the transferrin receptor-1 at the brain endothelial capillaries through monoclonal antibodies. It is hypothesized that the nanoparticles could pass the BBB via receptor mediated transcytosis and reach brain parenchyma. Particle size, zeta potential, loading capacity and release profiles of nanoparticles were investigated. It was observed that all types of chitosau (CS) nanoparticles had positive zeta potential values and nanoparticle particle size distribution varied between 100 and 800 nm. The association efficiency of ASOs into the nanoparticles was between 80-97% and the release profiles of the nanoparticles exhibited an initial burst effect followed by a controlled release. The results showed that the designed chitosan based nanocarriers could be a promising carrier system to transport nucleic acid based drugs to brain parenchyma.

The ON/OFF switching by metal ions of the “Sergeants and Soldiers” chiral amplification effect on helical poly(phenylacetylene)s

Chiral amplification can be switched ON/OFF to both helical senses by the presence/absence of metal ions in copolymers of poly(phenylacetylene)s.