Complex Formation between Cationic Polymethacrylates and Oligonucleotides

Helical Conjugated Polyelectrolyte Aggregation Induced by Biotin-Avidin Interaction

Fluorescence correlation spectroscopy (FCS) is applied to demonstrate avidin-induced cross-linking in a system consisting of a helical anionic conjugated polyelectrolyte (P1) and a biotin-tetramethylrhodamine (TMR) conjugate (2). In a previous study, we used fluorescence spectroscopy to demonstrate that 2 binds to P1 via intercalation of the TMR chromophore into the P1 helix. Addition of avidin to the P1/2 complex induces little change in the fluorescence of the system; however, FCS reveals a remarkable increase in the diffusion time of the P1/2 complex in the presence of avidin. This change is attributed to supramolecular polymer aggregates produced by cross-link formation between the biotin unit of intercalated 2 and avidin. Atomic force microscopy imaging provides evidence supporting the existence of these aggregates. The highly sensitive FCS method is used to develop a novel sensor for the biotin-avidin interaction, with a detection limit of <100 pM for avidin.

Fluorescence correlation spectroscopy in biology, chemistry, and medicine

This review describes the method of fluorescence correlation spectroscopy (FCS) and its applications. FCS is used for investigating processes associated with changes in the mobility of molecules and complexes and allows researchers to study aggregation of particles, binding of fluorescent molecules with supramolecular complexes, lipid vesicles, etc. The size of objects under study varies from a few angstroms for dye molecules to hundreds of nanometers for nanoparticles. The described applications of FCS comprise various fields from simple chemical systems of solution/micelle to sophisticated regulations on the level of living cells. Both the methodical bases and the theoretical principles of FCS are simple and available. The present review is concentrated preferentially on FCS applications for studies on artificial and natural membranes. At present, in contrast to the related approach of dynamic light scattering, FCS is poorly known in Russia, although it is widely employed in laboratories of other countries. The goal of this review is to promote the development of FCS in Russia so that this technique could occupy the position it deserves in modern Russian science.

Dependence of fluorescence quenching of a poly(p-phenyleneethynylene) polyelectrolyte on the electrostatic and hydrophobic properties of the quencher

This study investigates the fluorescence quenching of a poly(p-phenyleneethynylene) (1) based polyelectrolyte by positively charged and neutral macromolecules. This work shows that the change in the fluorescence yield of 1 depends on a number of factors, including electrostatic, hydrophobic, and energy transfer interactions with the quencher and also changes in the solution conditions such as concentration and ionic strength. The fluorescence quenching is attributed to the formation of aggregates that form upon addition of different quenchers to a solution of 1 and/or the solution conditions. The extent of 1's aggregation is shown to depend on the type of interaction between the polymer and the quencher, the concentration of the polymer, and the ionic strength of the solution.

Solvation and Aggregation of Polyphenylethynylene Based Anionic Polyelectrolytes in Dilute Solutions

The absorption and fluorescence properties of a polyphenylethynylene based conjugated polyelectrolyte with sulfonate solubilizing groups (PP2) are shown to change dramatically with solution conditions because of the equilibrium between unaggregated and aggregated forms of the polymer. The fluorescence of PP2 is strongly quenched on addition of counterions such as Na+, K+, Li+, and TBA+, an effect which arises from the creation of salt stabilized aggregates. The formation of aggregates has been further corroborated by concentration and temperature studies in water and comparisons to dimethylsulfoxide solvent, in which the polymer does not aggregate. In aqueous solutions, the addition of the cationic surfactant, octadecyltrimethyl ammonium, causes the polymer aggregates to dissociate and creates polymer/surfactant aggregates that have spectral properties like that of the unaggregated polymer.

Can we better understand the intracellular behavior of DNA nanoparticles by fluorescence correlation spectroscopy?

The use of non-viral gene carriers to deliver small nucleic acids like antisense oligonucleotides (ODNs) and small interfering RNA (siRNA) remains an attractive but challenging goal in antisense therapy. Indeed, different barriers need to be overcome in the delivery process before a therapeutic effect can be obtained. One promising technique which we have been evaluating to improve our understanding of the intracellular behavior of nucleic acids/carrier complexes is Fluorescence Correlation Spectroscopy (FCS). In particular, we have used FCS for studying the protection of the nucleic acids against enzymatic degradation, and the association and dissociation of the nucleic acids with their carrier, both in buffer and in living cells. In this report, we will review our experience and findings on the use of FCS for that purpose and discuss the strengths and weaknesses of this interesting technique.

Tetrahydrofuran Activates Fluorescence Resonant Energy Transfer from a Cationic Conjugated Polyelectrolyte to Fluorescein-Labeled DNA in Aqueous Media

A cationic water-soluble conjugated polyelectrolyte, poly[9,9-bis(6''-(N,N,N-trimethylammonium)hexyl)fluorene-co-alt-2,5-bis(6'-(N,N,N-trimethylammonium)hexyloxyphenylene) tetrabromide], was synthesized. Fluorescence resonant energy transfer (FRET) experiments between the polymer and fluorescein-labeled single-stranded DNA (ssDNA-Fl) were conducted in aqueous buffer and THF/buffer mixtures. Weak fluorescence emission in aqueous buffer was observed upon excitation of the polymer, whereas addition of THF turned on the fluorescence. Fluorescence self-quenching of ssDNA-Fl in the ssDNA-Fl/polymer complexes as well as electron transfer from the polymer to fluorescein may account for the low fluorescence emission in buffer. The improved sensitization of fluorescence by the polymer observed in THF/buffer could be attributed to the weaker binding between the polymer and ssDNA-Fl and a decrease in dielectric constant of the solvent mixture, which disfavors electron transfer. THF-assisted signal sensitization was also observed for the polymer and fluorescein-labeled double-stranded DNA (dsDNA-Fl). These results indicate that the use of cosolvent provides a strategy to improve the detection sensitivity for biosensors based on the optical amplification provided by conjugated polymers.

Lipopolyamine-mediated single nanoparticle formation of calf thymus DNA analyzed by fluorescence correlation spectroscopy

PURPOSE

The aim of this study is to analyze linear calf thymus DNA (ct DNA) nanoparticle formation with N4,N9-dioleoylspermine and N1-cholesteryl spermine carbamate.

METHODS

Fluorescence correlation spectroscopy (FCS) was used to determine the quality of ct DNA condensed by lipopolyamines. ct DNA was prelabeled with PicoGreen (PG) to allow fluorescence intensity fluctuation measurement and analysis.

RESULTS

N4,N9-dioleoylspermine efficiently condensed ct DNA into point-like molecules with diffusion coefficient (D) = 1.8 x 10(-12) m2/s and particle number (PN) = 0.7 [at ammonium/phosphate (N/P) charge ratio = 1.0-1.5]. The determined PN values are close to the theoretical value of 0.6, providing evidence that the DNA conformation has been fully transformed, and thus a single nanoparticle has been detected. N1-cholesteryl spermine carbamate showed (slightly) poorer DNA condensation efficiency, even at higher N/P ratios (N/P = 1.5-2.5) with D = 1.3 x 10(-12) m2/s and PN value of 5.2. N4,N9-dioleoylspermine is a more efficient DNA-condensing agent than N1-cholesteryl spermine carbamate.

CONCLUSIONS

FCS measurement using PG as the probe is a novel analytical method to detect single nanoparticles of condensed DNA in nonviral gene therapy formulation studies.

pH-Tunable rheological properties of a telechelic cationic polyelectrolyte reversible hydrogel

Steady shear properties and linear and nonlinear viscoelastic behaviors of a poly(methyl methacrylate)-poly(dimethyl amino ethyl methacrylate)-poly(methyl methacrylate) polymer, (PMMA-PDMAEMA-PMMA), telechelic polymers in salt-free aqueous solution have been investigated as a function of concentration and pH. Above a critical concentration, a transient physical network is formed through an association mechanism between hydrophobic end groups, leading to a gel-like behavior. The gel-like polymer solutions were shown to exhibit a peculiar flow behavior, associated with time fluctuation of the transient first normal stress difference, attributed to orientation effects of the stiff charged polymer chains. The viscoelastic behavior was shown to be governed by two pH dependent time scales: a short time scale controlled by the lifetime of the hydrophobic associative junctions and a long time scale corresponding to the network relaxation time. All rheological results show strong evidence that Coulomb interactions, which control both macromolecular chain rigidity and inter-chain interactions, lead to specific pH-tunable properties of great potential interest.

Optimization of the Molecular Orbital Energies of Conjugated Polymers for Optical Amplification of Fluorescent Sensors

Cationic water-soluble poly(fluorene-co-phenylene)s with electron withdrawing or donating substituents on the conjugated backbone were designed and synthesized. Fluorescence resonance energy transfer (FRET) experiments between these conjugated polymers and dye-labeled single-stranded DNA (ssDNA-C*) reveal the importance of matching donor and acceptor orbital energy levels to improve the sensitization of C* emission. Quenching of polymer fluorescence with ssDNA-C* and differences in C* emission suggest involvement of photoinduced charge transfer (PCT) as an energy wasting mechanism. The HOMO and LUMO energy levels of the conjugated polymers and C serve as a preliminary basis to understand the competition between FRET and PCT. Dilution of C in polymer/ssDNA-C complexes by addition of ssDNA yields insight into C*...C self-quenching. Under optimized conditions, where there is no probe self-quenching and minimum PCT, efficient signal amplification is demonstrated despite poor spectral overlap between polymer and C.

Towards a better understanding of the dissociation behavior of liposome-oligonucleotide complexes in the cytosol of cells

To obtain real breakthroughs in antisense therapy, it is necessary to understand the cellular behavior of antisense delivery systems. Fluorescence fluctuation spectroscopy (FFS), which measures in time fluorescence fluctuations in the excitation volume of a microscope and which can thus be applied on a cellular scale, shows potential for this purpose. In this study dual color FFS was explored to characterize the complexation (association and dissociation) between Cy5-labeled oligonucleotides (Cy5-ONs) and FITC-labeled cationic liposomes (FITC-liposomes) in respectively buffer, cell lysate and the cytosol of Vero cells. In Hepes buffer the association of the Cy5-ONs to the FITC-liposomes could be clearly observed from the high peaks of Cy5- and FITC-fluorescence, which appeared simultaneously in the excitation volume. This was explained by the fact that in the complexed state many Cy5-ONs and FITC-liposomes are bound to each other and thus move together through the excitation volume thereby resulting in high fluorescence 'FITC/Cy5-peaks'. FFS measurements on FITC-liposome/Cy5-ONs complexes in cell lysate revealed that a minor part of the Cy5-ONs was released from the complexes. The major part of the Cy5-ONs remained in the complexes, which also seemed to aggregate in cell lysate. In agreement with the measurements in cell lysate, after microinjection of FITC-liposome/Cy5-ONs complexes in the cytosol of Vero cells a part of the Cy5-ONs was released (as Cy-ONs were detected by FFS in the nuclei) while the other part remained bound (as Cy5-peaks were frequently observed in the cytosol). As will be explained, the Cy5-peaks could be due both to Cy5-ONs clustered with cytosol components and Cy5-ONs still complexed to FITC-liposomes with quenched FITC-fluorescence.

Fluorescence fluctuation analysis for the study of interactions between oligonucleotides and polycationic polymers

The interactions between a cationic polymer, poly(2-dimethylamino)ethyl methacrylate (pDMAEMA), and negatively charged rhodamine-labeled 25-mer phosphodiester oligonucleotides (Rh-ONs) were studied by fluorescence fluctuation spectroscopy and other techniques. The composition of the pDMAEMA/Rh-ON complexes was investigated as a function of the charge ratio (+/-) by increasing the pDMAEMA concentration and keeping the Rh-ON concentration constant. We applied two different methods for analyzing the fluorescence fluctuation profiles of the pDMAEMA/Rh-ON complexes, which depended on their composition. First, we analyzed the data with the photon counting histogram (PCH) technique, which determines the molecular brightness and the concentration of fluorophores (Chen et al, 1999). A particular challenge for the data analysis is the occurrence of sudden fluorescence bursts in the fluorescence fluctuation profiles, which are linked to the appearance of multimolecular complexes (i. e. when several Rh-ONs were present in one complex). A quantitative interpretation of the analysis for the complexes remains challenging and is connected to the rarity of the fluorescent bursts, which do not provide sufficient data statistics. To specifically address the problem of the fluorescent bursts we employed a method described by Van Craenenbroeck et al. (1999). This method, applicable only when data were integrated over much longer time bins, allowed us to estimate the number of fluorescence bursts which could be considered as a relative measure of the amount of multimolecular complexes present. When monomolecular complexes were formed, i. e. at high values of the charge ratio, highly intense fluorescence peaks were not present and the interpretation of the PCH analysis was more straightforward. The molecular brightness of the species (epsilon), as revealed from PCH analysis, was greater than epsilon for the free Rh-ONs, indicating that the Rh-ONs were attached to pDMAEMA chains.