Yanli Tang

Membrane Perturbation Activity of Cationic Phenylene Ethynylene Oligomers and Polymers: Selectivity against Model Bacterial and Mammalian Membranes

Poly(phenylene ethyneylene) (PPE)-based cationic conjugated polyelectrolytes (CPEs) and cationic phenylene ethynylene oligomers (OPEs) exhibit broad-spectrum antimicrobial activity, and their main target is believed to be the cell membrane. To understand better how these antimicrobial molecules interact with membranes, a series of PPE-based CPEs and OPEs with different side chains were studied. Large unilamellar vesicles with lipid compositions mimicking those of mammalian or bacterial membranes were used as model membranes. Among the CPEs and OPEs tested, the anionic CPE, PPE-SO(3)(2-) and the smallest cationic OPE-1 are inactive against all vesicles. Other cationic CPEs and OPEs show significant membrane perturbation ability against bacterial membrane mimics but are inactive against a mammalian cell membrane mimic with the exception of PPE-DABCO and two end-only-functionalized OPEs, which also disrupted a mammalian cell membrane mimic. The results suggest that the phospholipid composition of vesicles dominates the interaction of CPE and OPE with lipid membranes.

Light-Induced Antibacterial Activity of Symmetrical and Asymmetrical Oligophenylene Ethynylenes

The light-induced antibacterial activity of symmetric and asymmetric oligophenylene ethynylenes (OPEs) was investigated against Gram-positive (Staphylococcus aureus and Staphylococcus epidermidis) and Gram-negative (Escherichia coli) bacteria. To understand the light-induced biocidal effect better, the transient absorption and triplet lifetime of OPEs were studied in methanol and water. A higher triplet lifetime was observed for OPE samples in water than in methanol. The magnitudes of the changes in optical density (ΔOD) of the S-OPE-n(H) series of symmetric oligomers are much higher than that of the asymmetric OPE-n series in water and are generally correlated with the singlet oxygen yield. It was found that the antibacterial activity against both Gram-positive and Gram-negative bacteria is size-, concentration-, and time-dependent. The light-induced antibacterial activity may result from the coordinated interactions of membrane disruption and interfacial or intracellular singlet oxygen generation, and the dominant factor is most likely the latter. The results obtained in this study will aid in the design of more efficient biocides in the future.

Synthesis, Self-Assembly, and Photophysical Behavior of Oligo Phenylene Ethynylenes: From Molecular to Supramolecular Properties

A pair of cationic phenylene ethynylene oligomers (OPEs) have been synthesized, and their optical properties have been studied in solution with and without added scaffold materials, including carboxymethylcellulose, carboxymethylamylose, and Laponite. The OPEs are strongly fluorescent in methanol solution, but in water, the fluorescence yield is suppressed. The addition of scaffolds to aqueous solutions of OPEs leads to a red shift in the absorption and in most cases a significant increase in the fluorescence quantum yield. The effects most likely arise because of template-induced formation of linear J-dimers or possibly because of planarization, which give rise to an effective increase in the conjugation length of OPEs.

Direct Visualization of Bactericidal Action of Cationic Conjugated Polyelectrolytes and Oligomers

The bactericidal mechanisms of poly(phenylene ethynylene) (PPE)-based cationic conjugated polyelectrolytes (CPE) and oligo-phenylene ethynylenes (OPE) were investigated using electron/optical microscopy and small-angle X-ray scattering (SAXS). The ultrastructural analysis shows that polymeric PPE-Th can significantly remodel the bacterial outer membrane and/or the peptidoglycan layer, followed by the possible collapse of the bacterial cytoplasm membrane. In contrast, oligomeric end-only OPE (EO-OPE) possesses potent bacteriolysis activity, which efficiently disintegrates the bacterial cytoplasm membrane and induces the release of bacterial cell content. Using single giant vesicles and SAXS, we demonstrated that the membrane perturbation mechanism of EO-OPE against model bacterial membranes results from a 3D membrane phase transition or perturbation.

Cationic phenylene ethynylene polymers and oligomers exhibit efficient antiviral activity.

The antiviral activities of poly(phenylene ethynylene) (PPE)-based cationic conjugated polyelectrolytes (CPE) and oligo-phenylene ethynylenes (OPE) were investigated using two model viruses, the T4 and MS2 bacteriophages. Under UV/visible light irradiation, significant antiviral activity was observed for all of the CPEs and OPEs; without irradiation, most of these compounds exhibited high inactivation activity against the MS2 phage and moderate inactivation ability against the T4 phage. Transmission electron microscopy (TEM) and SDS polyacrylamide gel electrophoresis (SDS-PAGE) reveal that the CPEs and OPEs exert their antiviral activity by partial disassembly of the phage particle structure in the dark and photochemical damage of the phage capsid protein under UV/visible light irradiation.

Label-free and real-time sequence specific DNA detection based on supramolecular self-assembly.

A new label-free, optical method was developed to detect sequence-specific DNA based on supramolecular self-assembly. A cationic phenylene ethynylene oligomer with two pairs of positively charged side chains (OPE-2) can form a J-dimer or J-aggregate with negatively charged DNA by a combination of electrostatic and hydrophobic interactions. At microM concentrations of dsDNA (number of bases in ssDNA ranges from 8 to 32), the optimum supramolecular self-assembly occurs between OPE-2 and dsDNA and is characterized by a new absorption peak emerging at 418 nm and an increase in fluorescence intensity (about 4.5-fold for dsDNA(1)). In contrast, the self-assembled complexes between OPE-2 and ssDNA are less readily formed under the same conditions. Interestingly, the induced circular dichroism (CD) signal for OPE-2/ssDNA is quite strong, likely owing to the self-assembly onto ssDNA simultaneously templating helix formation. In contrast, the induced CD signal for OPE-2/dsDNA is weak, likely because the dsDNA is in a double helix conformation, and OPE-2 associated with the dsDNA should be outside of the helix. In fact, there is a steady decrease in the induced CD signal for ssDNA with the addition of equimolar complementary ssDNA over time that allows the monitoring of DNA hybridization in real time. Introduction of mismatched bases into the target DNA sequence prevents the full hybridization between ssDNA and the target DNA. For these cases, the decrease in the induced CD signals occurs more slowly and to a lesser extent, as some of the unhybridized portions may remain in helical association with OPE-2. In view of these observed signal changes, sequence specific DNA and single nucleotide mismatch can be detected in a very simple and sensitive manner without any modification of the DNA.

New high-throughput screening protease assay based upon supramolecular self-assembly.

We previously demonstrated that the supramolecular self-assembly of cyanines could be useful for developing fluorescent enzymatic assays. We took that concept a step further by synthesizing a covalent adduct of the tetrapeptide Asp-Glu-Val-Asp (DEVD) and a cyanine (DEVD-cyanine). The DEVD-cyanine due to its canonical sequence was recognized and hydrolyzed by the proteases, Caspase-3 and -7 in 96- or 384-microwell plate reactions. The catalytically liberated cyanine self-assembled upon scaffolds of carboxymethylamylose (CMA), carboxymethylcellulose (CMC), or a mixture of CMA and CMC resulting in a J aggregate exhibiting bright fluorescence at a 470 nm emission wavelength (optimum signal/background using excitation wavelengths of 415-440 nm). The fluorescence intensity increased with enzyme and substrate concentrations or reaction time and exhibited classical saturation profiles of a rectangular hyperbola. Saturation of the reaction was at 30 U/mL (1 microg/mL) Caspase-3 and 250 microM DEVD-cyanine. The reaction kinetics was linear between 1 and 20 min and saturated at 60 min. The affinity constant (Km) for DEVD-cyanine was approximately 23 microM, similar to those of previously reported values for other DEVD substrates of Caspase-3. Maximal fluorescence emission was observed by using a mixture of CMA and CMC scaffolds at 65 and 35 microM, respectively. The reaction kinetics of Caspase-7 executed in a 384-well plate was similar to the reaction kinetics of Caspase-3 conducted in a 96-well plate. We believe that this is the first demonstration of a cyanine liberated from a covalent adduct due to protease action, leading to supramolecular self-assembly and the detection of protease activity.

Rapid Evaluation of the Antibacterial Activity of Arylene–Ethynylene Compounds

A series of oligo(arylene-ethynylene) (1-3 repeat units) compounds functionalized with quaternary ammonium groups was screened for their antibacterial activity in the dark and with activation by long-wavelength (365 nm) UV irradiation. Several of these compounds have effective bactericidal activity (>99.9% killing) at concentrations between 0.01 and 10 μg/mL. Our approach uses flow cytometry to rapidly screen and evaluate the susceptibility of bacterial populations. The rapidity, high information content, and accuracy of this approach make it an extremely valuable method for the study of antibacterial compounds.

A Retrospective: 10 Years of Oligo(phenylene-ethynylene) Electrolytes: Demystifying Nanomaterials

In this retrospective, we first reviewed the synthesis of the oligo(phenylene-ethynylene) electrolytes (OPEs) we created in the past 10 years. Since the general antimicrobial activity of these OPEs had been reported in our previous account in Langmuir, we are focusing only on the unusual spectroscopic and photophysical properties of these OPEs and their complexes with anionic scaffolds and detergents in this Feature Article. We applied classical all-atom MD simulations to study the hydrogen bonding environment in the water surrounding the OPEs with and without detergents present. Our finding is that OPEs could form a unit cluster or unit aggregate with a few oppositely charged detergent molecules, indicating that the photostability and photoreactivity of these OPEs might be considerably altered with important consequences to their activity as antimicrobials and fluorescence-based sensors. Thus, in the following sections, we showed that OPE complexes with detergents exhibit enhanced light-activated biocidal activity compared to either OPE or detergent individually. We also found that similar complexes between certain OPEs and biolipids could be used to construct sensors for the enzyme activity. Finally, the OPEs could covalently bind to microsphere surfaces to make a bactericidal surface, which is simpler and more ordered than the surface grafted from microspheres with polyelectrolytes. In the Conclusions and Prospects section, we briefly summarize the properties of OPEs developed so far and future areas for investigation.