Meta-Linked Poly(phenylene ethynylene) Conjugated Polyelectrolyte Featuring a Chiral Side Group: Helical Folding and Guest Binding

A water soluble, meta-linked poly(phenylene ethynylene) featuring chiral and optically active side groups based on l-alanine (mPPE-Ala) has been studied by using absorption, fluorescence, and circular dichroism spectroscopy. Studies of mPPE-Ala in methanol/water solvent mixtures show that the polymer folds into a helical conformation, and the extent of helical folding increases with the volume % water in the solvent. The presence of the helical conformation is signaled by the appearance of a broad, excimer-like visible fluorescence band, combined with a strong bisignate circular dichroism signal in the region of the π,π* absorption of the polymer backbone. The circular dichroism signal exhibits negative chirality, suggesting that the left-handed (M-form) of the helix is in enantiomeric excess. Binding of the metallointercalator [Ru(bpy)2(dppz)]2+ (where bpy = 2,2-bipyridine and dppz = dipyrido[3,2-a:2‘,3‘−c]phenazine) with the helical polymer is accompanied by the appearance of the orange-red photoluminescence from the metal complex. This effect is directly analogous to that observed when [Ru(bpy)2(dppz)]2+ binds to DNA via intercalation, suggesting that the metal complex binds to mPPE-Ala by intercalating between the π-stacked phenylene ethynylene residues. Cationic cyanine dyes also bind to the periphery of the helical polymer in a manner that is interpreted as “groove binding”. A circular dichroism signal is observed that is believed to arise from exciton coupling within the chiral cyanine dye chromophore aggregate that is formed as the dye molecules are oriented by the helical mPPE-Ala “template”

Amplified Fluorescence Quenching of a Conjugated Polyelectrolyte Mediated by Ca²⁺

The fluorescence of conjugated polyelectrolytes (CPEs) is quenched with very high efficiency by small molecule quenchers. This effect has been referred to as amplified quenching. In the present communication, we demonstrate that aggregation of a poly(phenylene ethynylene)-type CPE (PPE-CO2-) induced by Ca2+ has a pronounced effect on the amplified quenching of the polymer by the dication methyl viologen (MV2+). In particular, absorption and fluorescence spectroscopy of PPE-CO2- in methanol solution indicate that addition of a low concentration of Ca2+ induces aggregation of the polymer chains. The range of MV2+ concentrations within which linear Stern−Volmer quenching behavior is observed systematically decreases with increasing Ca2+ concentration to a point where superlinear quenching is observed immediately upon addition of MV2+. This finding is unequivocal evidence that the superlinear Stern−Volmer quenching behavior typically observed in CPE-quencher systems arises due to quencher-induced aggregation of the CPE chains

Polymer Chain Length Dependence of Amplified Fluorescence Quenching in Conjugated Polyelectrolytes

This paper reports the synthesis and photophysical study of a series of anionic, carboxylate-substituted poly(phenylene ethynylene)-based conjugated polyelectrolytes (CPEs) with variable chain lengths. These CPEs are of interest as they allow the study of the effect of chain length on amplified fluorescence quenching. The CPEs were synthesized via organic soluble ester precursor polymers. The degree of polymerization of the precursor polymers was controlled by addition of a monofunctional “end-cap” to the polymerization reaction. The CPEs were obtained postpolymerization by base-promoted hydrolysis of the ester protecting groups. Stern−Volmer fluorescence quenching of the CPEs in methanol with monovalent electron-transfer quenchers (MV+ and HV+) show that the Stern−Volmer quenching constant (KSV) increases with polymer chain length reaching a maximum of ca. 2 × 105 M−1 at a degree of polymerization of 49. The results indicate that a maximum quenching amplification factor of 53 is attained under conditions where monovalent quencher ions interact with nonaggregated (single) polymer chains

Effects of Polymer Aggregation and Quencher Size on Amplified Fluorescence Quenching of Conjugated Polyelectrolytes

The fluorescence of conjugated polyelectrolytes (CPEs) is efficiently quenched by low concentrations of quenchers with opposite charges. We have reported the close correlation between this amplified quenching phenomenon and CPE chain aggregation. In this paper, we further demonstrate the profound correlation between the fluorescence quenching efficiency, CPE chain aggregation, and quencher molecular size. Aggregation of a poly(phenylene ethynylene)-type CPE (PPE-CO2-) is induced by the addition of either water or Ca2+ to methanol solution, as indicated by absorption, fluorescence, dynamic light scattering, and fluorescence microscope measurements. For quencher ions with a small molecular size, such as methyl viologen (MV2+), either the loose (induced by the addition of Ca2+) or the compact (induced by the addition of water) CPE chain aggregates are beneficial to the fluorescence quenching. For quencher ions with large molecular size, such as tris(4,7-diphenyl-1,10-phenanthroline)ruthenium (Ru(dpp)32+), however, the loose chain aggregates are found to be favorable for quenching, while the quenching efficiency is lower for the compact polymer aggregates present in aqueous solution

Layer-by-Layer Nanoparticles with a pH-Sheddable Layer for <i>in Vivo</i> Targeting of Tumor Hypoxia

Inspired by the simplicity and versatility of layer-by-layer (LbL) assembly, we applied multilayered polyelectrolyte assemblies on nanoparticles to create viable systemic delivery systems. Focusing on tumor-specific delivery, LbL nanoparticles that exhibit a pH-sensitive outer stealth layer are demonstrated to target and be retained in hypoxic tumor regions. The neutral layers shed in response to acidity to reveal a charged nanoparticle surface that is readily taken up by tumor cells. The first in vivo demonstration of this mechanism of targeting is presented, as well as an initial examination of the mechanism of uptake of the nanoparticles. We further demonstrate that this concept for tumor targeting is potentially valid for a broad range of cancers, with applicability for therapies that target hypoxic tumor tissue

Designing FeO@graphite@C Nanocomposites Based on Humins as Efficient Catalysts for Reverse Water-Gas Shift

Acid-catalyzed conversion of biomass into bio-based platform chemicals such as levulinic acid and 5-hydroxymethylfurfural is an important route in biorefineries, which has attracted much attention in recent years. Such a route however unavoidably yields massive recalcitrant byproducts called humins, which are now broadly considered as waste and are limited to combustion, causing unfavorable energy and environmental processes. Therefore, the development of a value-added utilization approach for such humin byproducts is crucial for making the biorefineries economical and environmentally viable. In this work, we present a starting point for valorization of humins via the preparation of carbon-based iron oxide nanocomposites of FeO@graphite@C by using the humins as carbon resources and material templates via a facile synthesis strategy. The as-prepared catalyst is capable of promoting the reverse water-gas shift reaction and reaching a high CO2 conversion ratio with excellent CO selectivity (> 99%) at 500–700 °C, enabling an efficient utilization of waste CO2. The unique graphite-capsuled FeO structure of FeO@graphite@C was found to be the origin of its excellent catalytic activity toward CO2 reduction into CO, which shifts electrons from the graphite layer to FeO, reconstructing the Fe electron structure. This strengthened the electrophilic attack ability toward CO2 and weakened the bond with the derived CO* species of the Fe active sites, associated with the excellent CO2 conversion and CO selectivity

Spectral Broadening in Nanocrystalline TiO₂ Solar Cells Based on Poly(<i>p</i>-phenylene ethynylene) and Polythiophene Sensitizers

Spectral Broadening in Nanocrystalline TiO2 Solar Cells Based on Poly(p-phenylene ethynylene) and Polythiophene Sensitizer

Enhanced Stability of Polymeric Micelles Based on Postfunctionalized Poly(ethylene glycol)-<i>b</i>-poly(γ-propargyl l-glutamate): The Substituent Effect

One of the major obstacles that delay the clinical translation of polymeric micelle drug delivery systems is whether these self-assembled micelles can retain their integrity in blood following intravenous (IV) injection. The objective of this study was to evaluate the impact of core functionalization on the thermodynamic and kinetic stability of polymeric micelles. The combination of ring-opening polymerization of <i>N</i>-carboxyanhydride (NCA) with highly efficient “click” coupling has enabled easy and quick access to a family of poly­(ethylene glycol)-block-poly­(γ-R-glutamate)­s with exactly the same block lengths, for which the substituent “R” is tuned. The structures of these copolymers were carefully characterized by ¹H NMR, FT-IR, and GPC. When pyrene is used as the fluorescence probe, the critical micelle concentrations (CMCs) of these polymers were found to be in the range of 10^–7–10^–6 M, which indicates good thermodynamic stability for the self-assembled micelles. The incorporation of polar side groups in the micelle core leads to high CMC values; however, micelles prepared from these copolymers are kinetically more stable in the presence of serum and upon SDS disturbance. It was also observed that these polymers could effectively encapsulate paclitaxel (PTX) as a model anticancer drug, and the micelles possessing better kinetic stability showed better suppression of the initial “burst” release and exhibited more sustained release of PTX. These PTX-loaded micelles exerted comparable cytotoxicity against HeLa cells as the clinically approved Cremophor PTX formulation, while the block copolymers showed much lower toxicity compared to the cremophor–ethanol mixture. The present work demonstrated that the <b>PEG-<i>b</i>-PPLG</b> can be a uniform block copolymer platform toward development of polymeric micelle delivery systems for different drugs through the facile modification of the PPLG block

Interaction of Anionic Phenylene Ethynylene Polymers with Lipids: From Membrane Embedding to Liposome Fusion

Here we report spectroscopic studies on the interaction of negatively charged, amphiphilic polyphenylene ethynylene (PPE) polymers with liposomes prepared either from negative, positive or zwitterionic lipids. Emission spectra of PPEs of 7 and 49 average repeat units bearing carboxylate terminated side chains showed that the polymer embeds within positively charged lipids where it exists as free chains. No interaction was observed between PPEs and negatively charged lipids. Here the polymer remained aggregated giving rise to broad emission spectra characteristic of the aggregate species. In zwitterionic lipids, we observed that the majority of the polymer remained aggregated yet a small fraction readily embedded within the membrane. Titration experiments revealed that saturation of zwitterionic lipids with polymer typically occurred at a polymer repeat unit to lipid mole ratio close to 0.05. No further membrane embedding was observed above that point. For liposomes prepared from positively charged lipids, saturation was observed at a PPE repeat unit to lipid mole ratio of ∼0.1 and liposome precipitation was observed above this point. FRET studies showed that precipitation was preceded by lipid mixing and liposome fusion induced by the PPEs. This behavior was prominent for the longer polymer and negligible for the shorter polymer at a repeat unit to lipid mole ratio of 0.05. We postulate that fusion is the consequence of membrane destabilization whereby the longer polymer gives rise to more extensive membrane deformation than the shorter polymer

MOESM1 of Bioactivity-explorer: a web application for interactive visualization and exploration of bioactivity data

Additional file 1. Case study: Exploring bioactivity data of Cyclin-dependent kinase 5