B. B. Meshkov

Complexation of a gaseous spin probe with cyclodextrins bound to the silica microspheres: Molecular dynamics of the complexed probes and the effect of aromatic hydrocarbon vapors on it

The formation of guest-host complexes of a gaseous radical TEMPO with cyclodextrins bound to the silica microspheres (MSs) either covalently with a chemical linker or by physical adsorption has been investigated. For the former complexes, two conformational states with strongly different rotational mobilities have been found similar to those described in [9] for aqueous MS suspensions. These states are assigned to the adsorbed (s) and desorbed (w) complexes, the fraction of s-states in the gaseous phase being noticeably higher than that in the suspensions. The binding of TEMPO to the copolymers of carboxymethyl β-CD (PCCD) or sulfated β-CD (PSCD) with epichlorhydrin adsorbed on MS results in dipole-dipole interactions (DDI) between the radicals. The local concentrations of the radicals have been estimated from the spectral parameters of DDI. The concentrations considerably exceed the average ones, indicating the preferred radical location in mesopores. The TEMPO rotational mobility in complexes with PCMCD is satisfactorily described by the model of anisotropic molecular rotation in the liquid-crystal-like orienting potential in the temperature interval of 206–273 K but deviates from this model at T > 290 K. The rotational diffusion coefficients are in the range of 106.8–107.6 with an activation energy of 2.5 ± 0.15 kcal/mole and order parameter D20 in the interval of 0.85-0.55. The dependence of the portion of the s-states and rotational mobility of the w-states on the vapors of the aromatic hydrocarbons and on the CD structure was discovered for complexes with the covalently bound CD. The changes in molecular dynamics in the presence of benzene, toluene, and naphthalene were displayed for both types of sensor layers: covalently or noncovalently bound to MS. The character of these changes is specific for these hydrocarbons, so it may be used for their identification.

The interaction of double-charged metal ions with monolayers and bilayers of phospholipids

Electrophoretic mobilities of hexadecane/water emulsions containing dimyristoyl-phosphatidylcholine (DMPC) or egg yolk lecithin (EYL) monolayers at the interface and those of liposomes prepared from the same lipids were measured as functions of the concentrations of Ca2+, Mn2+, Cu2+, and Ni2+ cations in the aqueous phase. The surface potentials, surface charge densities (σ), and the Langmuir adsorption isotherms for various distances from the charged surface to the slip plane (d) were calculated on the basis of the Gouy-Chapman theory for 1∶2 electrolytes and the values of ζ-potentials. The binding constants (K) and parametersd were determined under the assumption that the maximum σ values correspond to one ion per phospholipid molecule at the interface. In the case of DMPC, the ion binding constants (L mol−1) at 25°C are 230 and 87 for Ca2+, 31.5 and 21 for Mn2+, 11 and 6 for Cu2+, and 7.5 and 5.3 for Ni2+ in liposomes and emulsions, respectively. The affinities of Cu2+ and Ni2+ ions for EYL monolayers and bilayers are lower than those for DMPC mono- and bilayers. Thed parameters for all ions are smaller than the radii of the hydrated ions. In the case of Ca2+, Cu2+, and Ni2+, thed values for mono- and bilayers are different. The differences in K values between monolayers and bilayers as well as those between DMPC and EYL mono- and bilayers can be attributed to the differences in the local environment and orientation of the interfacial phosphate groups in these systems.

A study of adsorption and molecular dynamics of spin-labeled molecules on the surface of silica nanoparticles

Stable nitroxyl radicals of different structures, hydrophobicities, and electric charges were used as spin probes for studying the adsorption and molecular dynamics of the adsorbed molecules on the surface of LEVASIL silica nanoparticles. Neutral hydrophobic probes, namely, spin labeled derivatives of indole, are not adsorbed on the nanoparticles; however, the microviscosity and hydrophobicity of their environment differ from those in aqueous solutions. pH in the LEVASIL suspension is measured using a probe with an amino group. A study of the adsorption of a series of positively charged spin probes with hydrocarbon substituents of different lengths showed that the hydrophobic interactions do not contribute to their binding to the nanoparticle surface. The binding constants, the average number of negatively charged adsorption centers per nanoparticle, and the surface potential were determined from the adsorption isotherms of the radical cations. The rotational mobility parameters of the adsorbed radicals were estimated after analysis of the EPR line shape. A dependence of these parameters on the total spin probe concentration is observed. This is explained by the rapid dynamic equilibrium (chemical exchange) between the adsorbed and unbound spin probes. The chemical exchange rates are estimated. A slow increase in adsorption in the equilibrium nanoparticle suspension (with a characteristic time of ∼104 min) and a change in adsorption isotherms (accompanied by a change in the dynamical parameters of the adsorbed particles) are detected. Slow changes in these parameters are attributed to the existence of sufficiently deep energy traps (pores) where the adsorbed radicals slowly diffuse. The rotational mobility of the radicals in these pores is less than in the surface adsorption centers.

Structure and molecular dynamics of host–guest complexes of TEMPO with cucurbituryl: An ESR and DFT study

Host–guest complexes are of interest as promising nanodevices for molecular recognition and chemosensors. In this work, the structure and molecular dynamics of complexes of the nitroxyl radical TEMPO (I), as models of indicator and analyte, with cucurbituril CB[7] in solution and in the solid phase have been studied by ESR and DFT methods. The kinetic accessibility of the NO group of I for water-soluble reagents has been determined. By simulation of the ESR spectra of the complex, the rotational diffusion coefficients and the anisotropy of its rotation have been determined. To study the rotational mobility of the guest in the CB[7] cavity, solid solutions of I@CB[7] in the CB[7] matrix have been obtained. The ESR spectra indicate rapid jump-like rotation of I about an axis oriented along the normal to the CB[7] portals. The formation energy and the spatial structure of the complex have been calculated by the DFT method; a change in the spin density on the NO group with changing the orientation of I in the CB[7] cavity has been found.

Detection of low concentrations of volatile amines in aqueous solutions using pH-dependent fluorophores

The fluorescence of 2-haphthole (NL) and 1-naphthylamine (NA) has been studied depending on the concentration of amines: dimethylamine (DMA), trimethylamine (TMA), and pyridine (Pyr). The fluorescence spectrum of NL has two well-resolved bands with the maxima at 355 and 415 nm, corresponding to the protonated and deprotonated forms of 2-naphthole, the relative intensity of which depends on the concentration of aliphatic amines, but is essentially independent of the Pyr concentration. The parallel measurements of the pH of the medium showed that there is an unequivocal correlation between pH, ratio of intensities of two bands of the NL fluorescence, and concentration of aliphatic amines. It has been shown that NA has only one fluorescence band (447 nm); however, its intensity (quantum yield) is high and it is sensitive to the concentration of all three amines and pH. The pH-Metric titration of NA makes it possible to determine pK of protonation of this sensor and the pH value corresponding to the maximum slope of the titration curve, or to the maximum slope of the dependence of fluorescence intensity on the concentration of amine. This point (pH ≈ 4.2) is used as the initial one in the determination of low concentrations of amines. The effect of β-cyclodextrin (β-CD), crown ether 18-crown-6, and NA concentration on the NA sensitivity to the concentration of amines has been studied. The sensitivity of NA to Pyr is lower than to DMA, but higher than for TMA. In the presence of 2 mM of β-CD in the solution, the sensitivity of NA in relation to three amines is approximately similar.

Detection of volatile aromatic hydrocarbons from the gas phase by fluorescence and absorption spectra of guest-host complexes with cyclodextrins in gel matrices

An approach for determining volatile aromatic hydrocarbons based on the use of polyacrilamide gel layers containing cyclodextrin molecules is developed. Using the examples of naphthalene, toluene, and o-xylene, the principal possibility of detecting these hydrocarbons at the level of the maximum allowable concentration due to the formation of supramolecular complexes of inclusions into cyclodextrins is demonstrated. It was shown that the study of the kinetics of gaseous analyte incorporation in gels makes it possible to obtain information on their diffusion coefficient and analyze analyte mixtures.

Structure and Dynamics of Ternary Complexes of Cucurbit[8]uril with Spin-Labeled Indicators and Biologically Active Analytes

Ternary host–guest complexes have been first obtained from cucurbituril CB[8] as a host molecule and two guest molecules: nitroxyl probes of different structures and biologically important amino acids (AA) and aromatic compounds. To characterize the binding of the guests, parameters of the polarity of the environment and the rotational mobility of the spin probes have been used. These parameters have been shown to depend on the nature of the analytes. For the ternary complexes, in addition to the usual triplet ESR spectra from nitroxyl probes (S3), supramolecular ensembles consisting of three equivalent ternary complexes (“triads”) have been found, whose ESR spectra have a seven-component hyperfine structure (S7) due to delocalization of the unpaired electron over three nitrogen nuclei. The relative intensity of the S7 spectra increases with increasing NaCl concentration in the solution, and also depends significantly on the nature of the analyte and the spin probe. Quantum chemical calculations have shown that (1) to determine the stability of the complexes, it is necessary to allow for the van der Waals interaction, and (2) the complexes involving the zwitterionic form of AA are much more stable than those with the neutral form of AA.

EPR and fluorescence study of molecular dynamics of lipids in bilayers adsorbed on porous silica gel

Lipid bilayers immobilized on highly porous silica gel monoliths (SLBs) have been proposed as templates for chemosensors. Procedures for preparation and characterization of SLBs have been developed and applied. Rotational dynamics and orientational ordering at different depths in a SLB membrane and, for comparison, in bilayer liposomes have been studied for the first time using n-PC phospholipid spin labels.

Investigation of volatile aliphatic and aromatic amine detection using a fluorescent pH indicator ethyl eosin in polymer matrices

Changes in the fluorescence spectra (F) of ethyl eosin (EE) embedded into polymer matrices of different compositions, gas permeability, and polarity under the action of gaseous aliphatic and aromatic amines (dimethylamine (DMA), trimethylamine (TMA), and pyridine (Pyr)), depending on the pH and the structure of polymer matrix, were investigated. Organic methanesulfonic acid was used to adjust pH in a polymer matrix. Cellulose acetate-propionate (CAP), polyvinyl trimethylsilane (PVTMS), polyethylene oxide (PEO), and polyvinylpyrrolidone (PVP), as well as mesoporous silica gel microspheres, were used as polymer matrices. Absorption and fluorescence maxima of EE in solution and in most polymers are in the region of 505 nm and 550 nm, respectively. The maximum of EE fluorescence intensity increases in all polymer matrices except for PVP in the presence of amines, while the F decreases in the PVP matrix and silica gel microspheres. The fluorescence response to DMA is higher than to TMA and Pyr for all the polymers; the F response is two-phase in the case of Pyr: an increase in F followed by a decrease. Characteristic response times for different matrices change in a sequence: CAP < PVP < PEO < MS < PVTMS. These differences are probably due to the different contributions to the process of the analyte diffusion, its affinity to the polymer, and the internal polarity of the polymer matrix for different polymers.

Transfer of Electronic Excitation Energy from Naphthalene to Fluorophore Indicator on the Surface of Silica Microspheres with Covalently or Physically Bound Cyclodextrins

In order to use electronic excitation energy transfer (TE) for the detection of aromatic hydrocarbons (AHs) in a gas phase, TE from naphthalene (N) to a fluorophore indicator on the surface of silica microspheres (SMs) modified by covalently or physically adsorbed cyclodextrins (CDs) has been studied. It is found that dansylglycine (DG) is an appropriate acceptor of energy, because its absorption spectrum is shifted to the red region of the spectrum and overlaps well with the fluorescent spectra of AHs, while upon excitation at 290 nm its fluorescence is low. It has been demonstrated that, upon the introduction of SMs into the vapors of N and the excitation of N at 290 nm, the fluorescence of DG grows 500 nm more intensively than that of N. A method of quantitative variation of the N vapors pressure and, correspondingly, of the concentration of adsorbed N, based on the measurement of weight loss of the N solution in a nonvolatile solvent (DOS), has been developed. Using this method, the dependencies of intensity of fluorescence was determined for N and DG, as well as the efficiency of TE on the concentration of the adsorbed N. Under the assumption of the Forster mechanism of TE, the Forster radius (23.4 Å) and the average distances between N and DG were estimated. It has been demonstrated that, upon excitation at the absorption band of N, the intensity of DG fluorescence linearly increases with the DG concentration, which makes it possible to use TE for the detection of low concentrations of N. The kinetics of fluorescence at the rapid introduction of the N vapors was studied: first the sorption of N on the surface and fast growth of monomeric F occurs; further, in ∼200 s, excimer fluorescence and TE arise. For SMs with adsorbed polymeric CDs, the efficiency of TE from N to DG is several times lower than for SMs with covalently bound CDs.