Harry C. Pappas

Conjugated Polyelectrolytes with Imidazolium Solubilizing Groups. Properties and Application to Photodynamic Inactivation of Bacteria

This article reports an investigation of the photophysical properties and the light- and dark-biocidal activity of two poly(phenyleneethynylene) (PPE)-based conjugated polyelectrolytes (CPEs) bearing cationic imidazolium solubilizing groups. The two polymers feature the same PPE-type backbone, but they differ in the frequency of imidazoliums on the chains: PIM-4 features two imidazolium units on every phenylene repeat, whereas PIM-2 contains two imidazolium units on every other phenylene unit. Both polymers are very soluble in water and polar organic solvents, but their propensity to aggregate in water differs with the density of the imidazolium units. The polymers are highly fluorescent, and they exhibit the amplified quenching effect when exposed to a low concentration of anionic electron-acceptor anthraquinone disulfonate. The CPEs are also quenched by a relatively low concentration of pyrophosphate by an aggregation-induced quenching mechanism. The biocidal activity of the cationic imidazolium CPEs was studied against both Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus bacteria in the dark and under blue-light illumination. Both polymers are effective biocides, exhibiting greater than 3 log kill with 30-60 min of light exposure at concentrations of ≤10 μg mL(-1).

Assessing the Sporicidal Activity of Oligo-p-phenylene Ethynylenes and Their Role as Bacillus Germinants.

A wide range of oligo-p-phenylene ethynylenes has been shown to exhibit good biocidal activity against both Gram-negative and Gram-positive bacteria. While cell death may occur in the dark, these biocidal compounds are far more effective in the light as a result of their ability to sensitize the production of cell-damaging reactive oxygen species. In these studies, the interactions of a specific cationic oligo-p-phenylene ethynylene with spore-forming Bacillus atrophaeus and Bacillus anthracis Sterne have been investigated. Flow cytometry assays are used to rapidly monitor cell death as well as spore germination. This compound effectively killed Bacillus anthracis Sterne vegetative cells (over 4 log reduction), presumably by severe perturbations of the bacterial cell wall and cytoplasmic membrane, while also acting as an effective spore germinant in the dark. While 2 log reduction of B. anthracis Sterne spores was observed, it is hypothesized that further killing could be achieved through enhanced germination.

Cationic oligo-p-phenylene ethynylenes form complexes with surfactants for long-term light-activated biocidal applications

Cationic oligo-p-phenylene ethynylenes are highly effective light-activated biocides that deal broad-spectrum damage to a variety of pathogens, including bacteria. A potential problem arising in the long-term usage of these compounds is photochemical breakdown, which nullifies their biocidal activity. Recent work has shown that these molecules complex with oppositely-charged surfactants, and that the resulting complexes are protected from photodegradation. In this manuscript, we determine the biocidal activity of an oligomer and a complex formed between it and sodium dodecyl sulfate. The complexes are able to withstand prolonged periods of irradiation, continuing to effectively kill both Gram-negative and Gram-positive bacteria, while the oligomer by itself loses its biocidal effectiveness quickly in the presence of light. In addition, damage and stress responses induced by these biocides in both E. coli and S. aureus are discussed. This work shows that complexation with surfactants is a viable method for long-term light-activated biocidal applications.

Activating the Antimicrobial Activity of an Anionic Singlet-Oxygen Sensitizer through Surfactant Complexation

Cationic oligo-p-phenylene ethynylenes have shown much promise as broad-spectrum light-activated antimicrobial compounds against both Gram-positive and Gram-negative bacteria. The anionic varieties, however, have weak biocidal activity. In this study, a complex is formed between a weakly biocidal anionic oligomer and a cationic surfactant, and the effects on their biocidal activity against Gram-negative E. coli and Gram-positive S. aureus are explored. The enhancement in biocidal activity that is observed when the complex is irradiated suggests that interfacial surfactant gives the complex a net-positive charge, allowing it to associate strongly with the bacterial membrane. The results of this study demonstrate a method for the enhancement of biocidal activity of singlet-oxygen sensitizers and corroborate the use of surfactants as trans-membrane drug-delivery agents.

Antifungal Properties of Cationic Phenylene Ethynylenes and Their Impact on β-Glucan Exposure

ABSTRACT Candida species are the cause of many bloodstream infections through contamination of indwelling medical devices. These infections account for a 40% mortality rate, posing a significant risk to immunocompromised patients. Traditional treatments against Candida infections include amphotericin B and various azole treatments. Unfortunately, these treatments are associated with high toxicity, and resistant strains have become more prevalent. As a new frontier, light-activated phenylene ethynylenes have shown promising biocidal activity against Gram-positive and -negative bacterial pathogens, as well as the environmental yeast Saccharomyces cerevisiae. In this study, we monitored the viability of Candida species after treatment with a cationic conjugated polymer [poly(p-phenylene ethynylene); PPE] or oligomer [“end-only” oligo(p-phenylene ethynylene); EO-OPE] by flow cytometry in order to explore the antifungal properties of these compounds. The oligomer was found to disrupt Candida albicans yeast membrane integrity independent of light activation, while PPE is able to do so only in the presence of light, allowing for some control as to the manner in which cytotoxic effects are induced. The contrast in killing efficacy between the two compounds is likely related to their size difference and their intrinsic abilities to penetrate the fungal cell wall. Unlike EO-OPE-DABCO (where DABCO is quaternized diazabicyclo[2,2,2]octane), PPE-DABCO displayed a strong propensity to associate with soluble β-glucan, which is expected to inhibit its ability to access and perturb the inner cell membrane of Candida yeast. Furthermore, treatment with PPE-DABCO unmasked Candida albicans β-glucan and increased phagocytosis by Dectin-1-expressing HEK-293 cells. In summary, cationic phenylene ethynylenes show promising biocidal activity against pathogenic Candida yeast cells while also exhibiting immunostimulatory effects.

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.

Intended and Unintended Consequences and Applications of Unnatural Interfaces: Oligo p-Phenylene Ethynylene Electrolytes, Biological Cells and Biomacromolecules

This short review focuses on an extended study of synthetic oligomericphenyleneethynylene (OPE) electrolytes that have been investigated for both their antimicrobial activity and their fluorescence sensing properties. In both cases, interfaces between these synthetic electrolytes and naturally occurring materials such as proteins, lipids, nucleic acids and cells are critical to their function or activity. The review contains a general overview with a focus at the end on three recent examples including biological and chemical sensing, induction of spore germination and fluorescent detection of amyloid protein aggregates.