Vladimir G. Sergeyev

Cooperativity or phase transition? Unfolding transition of DNA cationic surfactant complex

We recently reported that single duplex DNA, with the size above the order of several tens kilobase pairs, undergoes a large discrete transition from an elongated coil into a collapsed globule with the addition of a cationic surfactant. In the present article, we describe the manner of the unfolding transition of compact long DNA, or globule DNA, complexed with cationic surfactants, cetyltrimethylammonium bromide (CTAB) and distearyldimethylammonium bromide (D18DAB), as is induced by the addition of sodium bromide. The conformational dynamics of individual single duplex T4DNA molecules was directly observed with the use of fluorescence microscopy. We found that on the level of individual DNAs, the salt-induced unfolding transition of the globules is largely discrete, or first-order phase transition for the both complexes with CTAB and D18DAB. On the other hand, for the ensemble average of the DNAs, the transition is discrete with CTAB but is continuous (sigmoidal) with D18DAB. The discreteness for the coi...

Polyaniline: Synthesis, properties, and application

The methods of synthesis and the properties of polyaniline—a representative of the family of conducting polymers—are reviewed briefly. It is shown that variation in the conditions of aniline polymerization makes it possible to synthesize polymer materials with the desired structures and properties and, thus, to provide for the use of polyaniline in various fields of science and engineering. Special attention is given to the matrix synthesis of polyaniline as the main approach to obtain electroactive and conducting composite materials. The use of polyaniline and the related composite materials in polymer electronics is analyzed briefly.

DNA hydrogel as a template for synthesis of ultrasmall gold nanoparticles for catalytic applications.

DNA cross-linked hydrogel was used as a matrix for synthesis of gold nanoparticles. DNA possesses a strong affinity to transition metals such as gold, which allows for the concentration of Au precursor inside a hydrogel. Further reduction of HAuCl4 inside DNA hydrogel yields well dispersed, non-aggregated spherical Au nanoparticles of 2-3 nm size. The average size of these Au nanoparticles synthesized in DNA hydrogel is the smallest reported so far for in-gel metal nanoparticles synthesis. DNA hybrid hydrogel containing gold nanoparticles showed high catalytic activity in the hydrogenation reaction of nitrophenol to aminophenol. The proposed soft hybrid material is promising as environmentally friendly and sustainable material for catalytic applications.

Potentiometric monitoring DNA hybridization

The usual procedure to monitor the ion exchange of small ions utilizes a potentiometer with a selective membrane as part of the working electrode. As the next step, we have applied polyaniline electrodes to the monitoring the activity macromolecular ions during DNA hybridization. Single-strand oligonucleotide (ssODN) probes were immobilized using a nucleophilic substitution reaction of the thiolated ssODN molecules with polyaniline. The anionic phosphate groups of the probe molecules also interacted with the cationic-doped polyaniline surface. Three useful findings were observed with the potentiometric experiments. First, the binding of the complimentary target molecules with the immobilized probes revealed a substantial potential change. Further, potential change was observed neither with the non-complimentary targets nor with the samples with a mutation in the sequence. The last two experiments were important for the future evaluation of the impact of medium and potential interfering compounds: anionic groups and hydrogen bonding groups in the non-complimentary samples did not cause any interactions.

DNA Compaction by Divalent Cations: Structural Specificity Revealed by the Potentiality of Designed Quaternary Diammonium Salts

DNA interaction with quaternary diammonium dications, R(CH3)2N+(CH2)nN+(CH3)2R, having various intercharge distances, lengths, and branching, and the chemical nature of the hydrophobic substituents were investigated by fluorescent microscopy and circular dichroism (CD) spectroscopy to reveal their structural specificity for binding to DNA. The conformational behavior of DNA was found to be highly sensitive to the structure of the dications with separated charges. The distance between two ammonium groups greatly influences the compaction activity of the dications. To explain this situation, we proposed a model that demonstrates that the charge density of the dication and the geometric fit between DNA phosphates and the ammonium groups in the dications play an important role in providing efficient DNA collapse. Elongation of the alkyl substituents (R) in the diammonium salts from ethyl to hexyl did not generate any significant alterations in the compaction activities, whereas the branching of substituents caused a drastic decrease in their compaction ability. Based on the results of CD spectroscopy, it was found that the ability of the dications to provoke a DNA transition from the B‐form to A‐form was also specific: it depended on their intercharge distances and was independent of the length of alkyl substituents.

Conformational Behavior of Giant DNA through Binding with Ag+ and Metallization

The conformational behavior of a long single-chain double-stranded DNA in solutions of free silver ions and silver nanoparticles generated via the reduction of AgNO3 by NaBH4 was monitored by fluorescence and electron microscopies and UV spectroscopy. The interaction of monovalent silver ions with DNA induces shrinking of a DNA-coiled polymer chain as a result of a decrease in the DNA persistence length through the complexation of Ag+ with DNA bases. In contrast, the reduction of silver ions by NaBH4 in DNA solutions triggers DNA compaction: a DNA transition from elongated coil state into a compact state. This transition is continuous, unlike the all-or-none discrete DNA compaction that is commonly seen with multications. It is suggested that the collapse of DNA is accompanied by growth aggregation of silver nanoparticles generated on the DNA template.

FOLDING OF LONG DNA CHAINS IN THE PRESENCE OF DISTEARYLDIMETHYLAMMONIUM BROMIDE AND UNFOLDING INDUCED BY NEUTRAL LIPOSOMES

The interaction between large duplex T4DNA (166 kbp) and a synthetic dialkyl cationic lipid, distearyldimethylammonium bromide (D 18 DAB), was studied using fluorescence microscopy for single-chain observation. The dependence of the higher-order structure of single T4DNAs on the surfactant concentration was evaluated, starting at extremely low values (1.0×10 -8 M). We found that individual T4DNA chains undergo a marked discrete transition between elongated coil and compact globule states, and that there is a very wide region of coexistence (about two orders of magnitude of the surfactant concentration) for the coiled and globular T4DNAs. We propose a simple theoretical model for assessing the relationship between the binding equilibrium and the coil–globule transition. In addition, we found that liposomes composed of neutral phospholipids induce unfolding of DNA compacted by a cationic surfactant.

Dispersible composites of exfoliated graphite and polyaniline with improved electrochemical behaviour for solid-state chemical sensor applications

We report here the in situ polymerization of aniline in the presence of exfoliated graphite of two different grades (graphene and graphite) resulting in composite materials which are readily dispersible in N-methylpyrrolidone. Compared to polyaniline (PANI) prepared without graphene/graphite which becomes electrically non-conducting at pH > 3, the PANI–graphene/graphite composites showed significantly improved pH stability and electrochemical behaviour in aqueous electrolyte solutions at pH ≤ 8, without any further need of surface functionalization of the graphene/graphite flakes to stabilize the conducting form of polyaniline (PANI). The improved electroactivity is ascribed to the synergistic effect of graphene/graphite and PANI, and the network formation of the electrically conducting exfoliated graphites in the PANI matrix, which was electrochemically confirmed by simple cyclic voltammetric measurements at pH = 9.5 in the presence of the Ru(NH3)62+/3+ and Fe(CN)63−/4− redox couples. Due to the dispersibility of the composites, thin films possessing stability in water can easily be prepared by solution casting for different types of solid-state chemical sensor and ion-selective electrode applications operating at neutral pH. By using sodium ascorbate as a model substance, we show that its amperometric detection at pH = 7.3 with the PANI–graphite films results in a current amplification of 1.3–10.2 times in the concentration range of 10−4–10−2 M, compared to conventional PANI, which clearly demonstrates the advantage of incorporating exfoliated graphites in the PANI films. The materials reported in this paper were systematically characterized with cyclic voltammetry, FTIR, Raman and X-ray photoelectron spectroscopy, scanning electron microscopy, X-ray diffraction and electrical conductivity measurements.

Nanoreactor-Assisted Polymerization Toward Stable Dispersions of Conductive Composite Particles

We demonstrate the functioning of a macromolecular nanoreactor which guides a reaction in a confined volume and leads toward improved functional properties of a product material. In our approach, the polymerization of aniline (ANi) is conducted within the interfacial volume of spherical polyelectrolyte brushes (SPB) which are densely affixed to colloidal particles. The SPB provide optimal conditions for matrix polymerization by the efficient confinement of ANi monomers within the finite volume of polyelectrolyte brushes and controlled delivery of the oxidizing reagent to the reaction volume. The excellent kinetic stability of the resulting core-shell particles together with the high macroscopic conductivity of the respective composite open up perspectives for novel materials (a conductive ink).

DNA-Assisted Solubilization of Carbon Nanotubes and Construction of DNA-MWCNT Cross-Linked Hybrid Hydrogels

A simple method for preparation of DNA-carbon nanotubes hybrid hydrogel based on a two-step procedure including: (i) solubilization of multi-walled carbon nanotubes (MWCNT) in aqueous solution of DNA, and (ii) chemical cross-linking between solubilized MWCNT via adsorbed DNA and free DNA by ethylene glycol diglycidyl ether is reported. We show that there exists a critical concentration of MWCNT below which a homogeneous dispersion of MWCNT in hybrid hydrogel can be achieved, while at higher concentrations of MWCNT the aggregation of MWCNT inside hydrogel occurs. The strengthening effect of carbon nanotube in the process of hydrogel shrinking in solutions with high salt concentration was demonstrated and significant passivation of MWCNT adsorption properties towards low-molecular-weight aromatic binders due to DNA adsorption on MWCNT surface was revealed.