Selective Interaction of a Cationic Polyfluorene with Model Lipid Membranes: Anionic versus Zwitterionic Lipids

Heterogeneity in Lateral Distribution of Polycations at the Surface of Lipid Membrane: From the Experimental Data to the Theoretical Model

Natural and synthetic polycations of different kinds attract substantial attention due to an increasing number of their applications in the biomedical industry and in pharmacology. The key characteristic determining the effectiveness of the majority of these applications is the number of macromolecules adsorbed on the surface of biological cells or their lipid models. Their study is complicated by a possible heterogeneity of polymer layer adsorbed on the membrane. Experimental methods reflecting the structure of the layer include the electrokinetic measurements in liposome suspension and the boundary potential of planar bilayer lipid membranes (BLM) and lipid monolayers with a mixed composition of lipids and the ionic media. In the review, we systematically analyze the methods of experimental registration and theoretical description of the laterally heterogeneous structures in the polymer layer published in the literature and in our previous studies. In particular, we consider a model based on classical theory of the electrical double layer, used to analyze the available data of the electrokinetic measurements in liposome suspension with polylysines of varying molecular mass. This model suggests a few parameters related to the heterogeneity of the polymer layer and allows determining the conditions for its appearance at the membrane surface. A further development of this theoretical approach is discussed.

Formation of Multicolor Nanogels Based on Cationic Polyfluorenes and Poly(methyl vinyl ether-alt-maleic monoethyl ester): Potential Use as pH-Responsive Fluorescent Drug Carriers

In this study, we employed the copolymer poly(methyl vinyl ether-alt-maleic monoethyl ester) (PMVEMA-Es) and three fluorene-based cationic conjugated polyelectrolytes to develop fluorescent nanoparticles with emission in the blue, green and red spectral regions. The size, Zeta Potential, polydispersity, morphology, time-stability and fluorescent properties of these nanoparticles were characterized, as well as the nature of the interaction between both PMVEMA-Es and fluorescent polyelectrolytes. Because PMVEMA-Es contains a carboxylic acid group in its structure, the effects of pH and ionic strength on the nanoparticles were also evaluated, finding that the size is responsive to pH and ionic strength, largely swelling at physiological pH and returning to their initial size at acidic pHs. Thus, the developed fluorescent nanoparticles can be categorized as pH-sensitive fluorescent nanogels, since they possess the properties of both pH-responsive hydrogels and nanoparticulate systems. Doxorubicin (DOX) was used as a model drug to show the capacity of the blue-emitting nanogels to hold drugs in acidic media and release them at physiological pH, from changes in the fluorescence properties of both nanoparticles and DOX. In addition, preliminary studies by super-resolution confocal microscopy were performed, regarding their potential use as image probes.

Reusable Fluorescent Nanobiosensor Integrated in a Multiwell Plate for Screening and Quantification of Antidiabetic Drugs

A highly stable and reusable fluorescent multisample nanobiosensor for the detection of α-glucosidase inhibitors has been developed by coupling fluorescent liposomal nanoparticles based on conjugated polymers (L-CPNs) to the enzyme α-glucosidase, one of the main target enzymes in the treatment of type 2 diabetes. The mechanism of sensing is based on the fluorescence "turn-on" of L-CPNs by p-nitrophenol (PNP), the end product of the enzymatic hydrolysis of p-nitrophenyl-α-d-glucopyranoside. L-CPNs, composed of lipid vesicles coated with a blue-emitting cationic polyfluorene, were designed and characterized to obtain a good response to PNP. Two nanobiosensor configurations were developed in this study. In the first step, a single-sample nanobiosensor composed of L-CPNs and α-glucosidase entrapped in a sol-gel glass was developed in order to characterize and optimize the device. In the second part, the nanobiosensor was integrated and adapted to a multiwell microplate and the possibility of reusing it and performing multiple measurements simultaneously with samples containing different α-glucosidase inhibitors was investigated. Using super-resolution confocal microscopy, L-CPNs could be visualized within the sol-gel matrix, and the quenching of their fluorescence, induced by the substrate, was directly observed in situ. The device was also shown to be useful not only as a platform for screening of antidiabetic drugs but also for quantifying their presence. The latter application was successfully tested with the currently available drug, acarbose.

The Interaction of Temozolomide with Blood Components Suggests the Potential Use of Human Serum Albumin as a Biomimetic Carrier for the Drug

The interaction of temozolomide (TMZ) (the main chemotherapeutic agent for brain tumors) with blood components has not been studied at the molecular level to date, even though such information is essential in the design of dosage forms for optimal therapy. This work explores the binding of TMZ to human serum albumin (HSA) and alpha-1-acid glycoprotein (AGP), as well as to blood cell-mimicking membrane systems. Absorption and fluorescence experiments with model membranes indicate that TMZ does not penetrate into the lipid bilayer, but binds to the membrane surface with very low affinity. Fluorescence experiments performed with the plasma proteins suggest that in human plasma, most of the bound TMZ is attached to HSA rather than to AGP. This interaction is moderate and likely mediated by hydrogen-bonding and hydrophobic forces, which increase the hydrolytic stability of the drug. These experiments are supported by docking and molecular dynamics simulations, which reveal that TMZ is mainly inserted in the subdomain IIA of HSA, establishing π-stacking interactions with the tryptophan residue. Considering the overexpression of albumin receptors in tumor cells, our results propose that part of the administered TMZ may reach its target bound to plasma albumin and suggest that HSA-based nanocarriers are suitable candidates for designing biomimetic delivery systems that selectively transport TMZ to tumor cells.

Polyfluorene-Based Multicolor Fluorescent Nanoparticles Activated by Temperature for Bioimaging and Drug Delivery

Multifunctional nanoparticles have been attracting growing attention in recent years because of their capability to integrate materials with different features in one entity, which leads them to be considered as the next generation of nanomedicine. In this work, we have taken advantage of the interesting properties of conjugated polyelectrolytes to develop multicolor fluorescent nanoparticles with integrating imaging and therapeutic functionalities. With this end, thermosensitive liposomes were coated with three recently synthesized polyfluorenes: copoly-((9,9-bis(6'-N,N,N-trimethylammonium)hexyl)-2,7-(fluorene)-alt-1,4-(phenylene)) bromide (HTMA-PFP), copoly-((9,9-bis(6'-N,N,N-trimethylammonium)hexyl)-2,7-(fluorene)-alt-4,7-(2- (phenyl)benzo(d) (1,2,3) triazole)) bromide (HTMA-PFBT) and copoly-((9,9-bis(6'-N,N,N- trimethylammonium)hexyl)-2,7-(fluorene)-alt-1,4-(naphtho(2,3c)-1,2,5-thiadiazole)) bromide (HTMA-PFNT), in order to obtain blue, green and red fluorescent drug carriers, respectively. The stability, size and morphology of the nanoparticles, as well as their thermotropic behavior and photophysical properties, have been characterized by Dynamic Light Scattering (DLS), Zeta Potential, transmission electron microscope (TEM) analysis and fluorescence spectroscopy. In addition, the suitability of the nanostructures to carry and release their contents when triggered by hyperthermia has been explored by using carboxyfluorescein as a hydrophilic drug model. Finally, preliminary experiments with mammalian cells demonstrate the capability of the nanoparticles to mark and visualize cells with different colors, evidencing their potential use for imaging and therapeutic applications.

Synthesis and Characterization of a Novel Green Cationic Polyfluorene and Its Potential Use as a Fluorescent Membrane Probe

In the present work, we have synthesized a novel green-emitter conjugated polyelectrolyte Copoly-{[9,9-bis(6'-N,N,N-trimethylammonium)hexyl]-2,7-(fluorene)-alt-4,7-(2-(phenyl) benzo[d] [1,2,3] triazole)} bromide (HTMA-PFBT) by microwave-assisted Suzuki coupling reaction. Its fluorescent properties have been studied in aqueous media and in presence of model membranes of different composition, in order to explore its ability to be used as a green fluorescent membrane probe. The polyelectrolyte was bound with high affinity to the membrane surface, where it exhibited high fluorescence efficiency and stability. HTMA-PFBT showed lower affinity to zwitterionic membranes as compared to anionic ones, as well as a more external location, near the membrane-aqueous interface. Fluorescence microscopy studies confirmed the interaction of HTMA-PFBT with the model membranes, labelling the lipid bilayer without perturbing its morphology and showing a clear preference towards anionic systems. In addition, the polyelectrolyte was able to label the membrane of bacteria and living mammalian cells, separately. Finally, we explored if the polyelectrolyte can function also as a sensitive probe able of detecting lipid-phase transitions. All these results suggest the potential use of HTMA-PFBT as a green membrane marker for bioimaging and selective recognition of bacteria cell over mammalian ones and as a tool to monitor changes in physical state of lipid membranes.

Fluorescent Biosensor for Phosphate Determination Based on Immobilized Polyfluorene-Liposomal Nanoparticles Coupled with Alkaline Phosphatase

This work describes the development of a novel fluorescent biosensor based on the inhibition of alkaline phosphatase (ALP). The biosensor is composed of the enzyme ALP and the conjugated cationic polyfluorene HTMA-PFP. The working principle of the biosensor is based on the fluorescence quenching of this polyelectrolyte by p-nitrophenol (PNP), a product of the hydrolysis reaction of p-nitrophenyl phosphate (PNPP) catalyzed by ALP. Because HTMA-PFP forms unstable aggregates in buffer, with low fluorescence efficiency, previous stabilization of the polyelectrolyte was required before the development of the biosensor. HTMA-PFP was stabilized through its interaction with lipid vesicles to obtain stable blue-emitting nanoparticles (NPs). Fluorescent NPs were characterized, and the ability to be quenched by PNP was evaluated. These nanoparticles were coupled to ALP and entrapped in a sol-gel matrix to produce a biosensor that can serve as a screening platform to identify ALP inhibitors. The components of the biosensor were examined before and after sol-gel entrapment, and the biosensor was optimized to allow the determination of phosphate ion in aqueous medium.

Charge-Mediated Localization of Conjugated Polythiophenes in Zwitterionic Model Cell Membranes

The selective engineering of conjugated polyelectrolyte (CPE)-phospholipid interfaces is poised to play a key role in the design of advanced biomedical and biotechnological devices. Herein, we report a strategic study to investigate the relationship between the charge of the CPE side group and their association with zwitterionic phospholipid bilayers. The interaction of dipalmitoylphosphatidylcholine (DPPC) phospholipid vesicles with a series of poly(thiophene)s bearing zwitterionic, cationic, or anionic terminal groups (P3Zwit, P3TMAHT and P3Anionic, respectively) has been probed. Although all CPEs showed an affinity for the zwitterionic vesicles, the calculated partition coefficients determined using photoluminescence spectroscopy suggested preferential incorporation within the lipid bilayer in the order P3Zwit > P3Anionic ≫ P3TMAHT. The polarity probe Prodan was used to further qualify the position of the CPE inside the vesicle bilayers via Förster resonance energy transfer (FRET) studies. The varying proximity of the CPEs to Prodan was reflected in the Stern-Volmer quenching constants and decreased in the order P3Anionic > P3TMAHT ≫ P3Zwit. Dynamic light scattering measurements showed an increase in the hydrodynamic diameter of the DPPC vesicles upon addition of each poly(thiophene), but to the greatest extent for P3Anionic. Small-angle neutron scattering studies also revealed that P3Anionic specifically increased the thickness of the headgroup region of the phospholipid bilayer. Epifluorescence and atomic force microscopy imaging showed that P3TMAHT formed amorphous agglomerates on the vesicle surface, P3Zwit was buried throughout the bilayer, and P3Anionic formed a shell of protruding chains around the surface, which promoted vesicle fusion. The global data indicate three distinctive modes of interaction for the poly(thiophene)s within DPPC vesicles, whereby the nature of the association is ultimately controlled by the pendant charge group on each CPE chain. Our results suggest that charge-mediated self-assembly may provide a simple and effective route to design luminescent CPE probes capable of specific localization within phospholipid membranes.

New Red-Emitting Conjugated Polyelectrolyte: Stabilization by Interaction with Biomolecules and Potential Use as Drug Carriers and Bioimaging Probes

The design and development of fluorescent conjugated polyelectrolytes (CPEs) emitting in the red region of the visible spectrum is at present of great interest for bioimaging studies. However, despite the wide variety of CPEs available, stable bright red-emitters remain scarce due to their low solubility and instability in aqueous media, consequently limiting their applications. In this work, we have synthesized and characterized a new red-emitting cationic conjugated polyelectrolyte copoly-{[9,9-bis(6'-N,N,N-trimethylammonium)hexyl]-2,7-(fluorene)-alt-1,4-(naphtho[2,3c]-1,2,5-thiadiazole)} bromide (HTMA-PFNT), based on the incorporation of naphtha[2,3c][1,2,5] thiadiazole on fluorene backbone to increase the bathochromic emission, extending the conjugation length in the polymer backbone. Water stabilization was achieved by binding the polyelectrolyte to two different biological systems which are currently used as nanocarriers: human serum albumin (HSA) and lipid vesicles. Using both systems, stable nanostructures of different composition were obtained and their properties were characterized. The properties of the protein-based nanoparticles are consistent with polyelectrolyte aggregates covered with HSA molecules, while the liposome system is composed of lipid vesicles coated with polyelectrolyte chains partially inserted in the bilayer. Both protein and vesicle structural integrity were not affected after their interaction with HTMA-PFNT, as well as the carrier properties, allowing for the binding and transport of ligands. In addition, the nanoparticles displayed the ability of labeling the cell membrane of living cells. All these results extend the potential applications of these novel multifunctional nanoparticles as therapeutic carriers and bioimaging probes.

Selective recognition and imaging of bacterial model membranes over mammalian ones by using cationic conjugated polyelectrolytes

This work describes the use of cationic polyfluorenes as fluorescent markers to selectively recognize bacterial membranes.

Sequential detection of multiple phase transitions in model biological membranes using a red-emitting conjugated polyelectrolyte

Charge-mediated assembly of an anionic poly(thiophene) leads to a highly sensitive probe of membrane order.

From molecular modelling to photophysics of neutral oligo- and polyfluorenes incorporated into phospholipid bilayers

The combination of various experimental techniques with theoretical simulations has allowed elucidation of the mode of incorporation of fluorene based derivatives into phospholipid bilayers. Molecular dynamics (MD) simulations on a fully hydrated 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) bilayer, with benzene (B), biphenyl (BP), fluorene (F) and tri-(9,9-di-n-octylfluorenyl-2,7-diyl), TF, have provided insights into the topography of these molecules when they are present in the phospholipid bilayer, and suggest marked differences between the behavior of the small molecules and the oligomer. Further information on the interaction of neutral fluorenes within the phospholipid bilayer was obtained by an infrared (IR) spectroscopic study of films of DMPC and of the phospholipid with PFO deuterated specifically on its alkyl chains (DMPC-PFO-d34). This was complemented by measurements of the effect of F, TF and two neutral polymers: polyfluorene poly(9,9-di-n-octylfluorenyl-2,7-diyl), PFO, and poly(9,9-di-n-dodecylfluorenyl-2,7-diyl), PFD, on the phospholipid phase transition temperature using differential scanning calorimetry (DSC). Changes in liposome size upon addition of F and PFO were followed by dynamic light scattering. In addition, the spectroscopic properties of F, TF, PFO and PFD solubilised in DMPC liposomes (absorption, steady-state and time-resolved fluorescence) were compared with those of the same probes in typical organic solvents (chloroform, cyclohexane and ethanol). Combining the insight from MD simulations with the results at the molecular level from the various experimental techniques suggests that while the small molecules have a tendency to be located in the phospholipid head group region, the polymers are incorporated within the lipid bilayers, with the backbone predominantly orthogonal to the phospholipid alkyl chains and with interdigitation of them and the PFO alkyl chains.