V. A. Livshits

Studies of possibility of detecting aromatic hydrocarbons in gas phase on the basis of fluorescence of host-guest complexes with cyclodextrins immobilized on silica microspheres

Microsperes (MSs) of porous silica gel modified by aminopropyl groups (APs) on which negatively charged polymer cyclodextrins (PCDs) were sorbed (copolymers of carboxymethyl-β-CD (KMCD) or sulfo-β-CD (SCD) with epichlorohydrin) are used for the detection of vapors of benzene (B), toluene (T), and naphthalene (N). For comparison, MSs with β-CD covalently bound through an AP linker and CD-free AP-MS are used. The N-PCD complexes have binding constants Kb higher by more than an order of magnitude than those of the β-CD monomers and higher fluorescence quantum yields. A method for determining the concentrations of adsorbed PCD is developed. Adsorption isotherms for both PCDs are almost linear in the PCD concentration range of 0–30 mM. The amount of KMCD polymer corresponds to a monolayer surface coverage of MSs at the initial concentration of 30 mM as opposed to the much lower surface density of covalently bound β-CD (0.2/nm2). The fluorescence spectra of hydrocarbons sorbed on MSs from saturated vapors differ substantially from those in aqueous solutions representing broad bands shifted to high wavelengths λ due to the formation of excimers. Fractions of excimers and shapes of their spectra are determined by the subtraction of monomer spectra from the overall spectra of hydrocarbons on MSs. For N on MSs with polymer CD and MSs with covalently bound β-CD, excimers of different types are observed. The kinetics of spectral changes is studied for the inflow of N vapors in the range of 25–1000 s.

Adsorption and molecular dynamics of spin probes on hydrophobizated silica gel microparticles: EPR spin-label study

The molecular dynamics parameters and hydrophobicity of the surface of Silasorb silica gel particles covalently modified with alkyl CnH2n + 1 groups differing in length (n = 2, 8, 18) were studied by electron paramagnetic resonance using spin probes of different structures, sizes, and electrical charges. It has been shown that the hydrocarbon shells of the silica gel microparticles in liquid suspensions provide high rotational and conformational mobility and a very high hydrophobicity of the environment for the incorporated molecules. Some regularities in the dependences of the rotational mobility, orientation ordering, and environmental hydrophobicity have on the hydrocarbon shell thickness have been established for spin probes of different structures: a radical containing an aromatic residue which models a fluorescent sensor molecule, lipid spin probes sensitive to the microviscosity and orientation ordering, and positively charged probes sensitive to electrostatic interactions. It has been found that the rotational mobility of the lipid spin probes sharply decreases upon removing the solvent and recovers in the presence of water or water-ethanol vapors. The high values of rotational mobility and environmental hydrophobicity for the spin probes located in the hydrocarbon shell of the Silasorb particles, which are dispersed in suspensions or in the solvent vapors, allow one to conclude that these microparticles can be perspective matrices for molecular optical chemosensors for analytes in solutions or the gas phase.

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.

Self-organization and phase structure of ternary lipid membranes: A spin label EPR study

The phase structure of membranes formed by ternary lipid mixtures, palmitoyl sphingomyelin (PSM)/palmitoyl oleoyl phosphatidylcholine (POPC)/Cholesterol (Chol), the composition of which simulates that of cell membranes, was studied by EPR using spin-labeled phospholipids. It is shown that, in the two-component POPC/Chol membranes and ternary membranes with a low (less than 24%) fraction of PSM, no phase boundaries are observed and the transition from the liquid disordered (ld) phase to the liquid ordered (lo) phase occurs continuously. In contrast, two-component EPR spectra and tie lines are detected in the other two-component systems, PSM/POPC and PSM/Chol, as well as in the ternary systems with a low content of Chol or POPC, respectively. These features are indicative of the coexistence of the gel phase (Lβ) and two liquid phases, ld or lo, respectively. The tie lines are approximately parallel to the Gibbs triangle sides Chol = 0 and POPC = 0, respectively. In the central part of the Gibbs triangle, where a sufficiently fast rotational diffusion of the spin labels indicates the membrane liquid state, we also observed two-component EPR spectra with quite different magnetic anisotropy and tie lines, indicating the coexistence of ld and lo phases. The boundaries are found for all the two-phase coexistence regions. The localization of the three-phase (Lβ, ld, and lo) coexistence region between the two-phase regions (Lβ-ld, Lβ-lo, and ld-lo) was proposed on the basis of an EPR-line shape analysis.

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.

Electron paramagnetic resonance study of the effect that the covalent binding of cyclodextrin receptors to silica-gel microparticles has on the molecular dynamics and complexation of spin-labeled guests

The molecular dynamics of the spin probes of different structures in complexes with cyclodextrins (CDs) which were covalently attached to silica-gel microparticles has been studied by electron paramagnetic resonance (EPR). Two states of the bound spin probes have been found which differ appreciably in rotational mobility and environment polarity. Our results allow one to conclude that the strongly immobilized state is that of CD spin probe complexes which are adsorbed on the hydrophobic regions of the modified silica-gel surface, whereas the weakly immobilized state corresponds to the desorbed complex, the mobility of which is controlled by the conformational flexibility of the linker. The binding constants K were determined for a hydrophobic probe (spin-labeled indole) and two water-soluble spin probes with silica-gel microparticles containing the covalently bound β-CD and phenyl-βCD. The absolute value of the effect that the microparticle surface has on the K value is quite small, and its sign depends on the probe structure and the mode of its intercalation into the CD complex. The competitive binding of the spin-labeled molecules and analytes with CDs has been demonstrated, which can be utilized for the production of the paramagnetic sensors.

Supramolecular complexes of spin-labeled and luminescent molecules with cyclodextrins

Studies on the structure, molecular dynamics, and spectral-luminescent properties of the guesthost cyclodextrin (CD) complexes which were carried out in the Photochemistry Center and the Institute of Chemical Problems, Russian Academy of Sciences (RAS), in the period of 2000–2011 are reviewed. In a series of studies, the binary and ternary CD complexes in aqueous solutions, in the solid phase, and on silicawater and silica-gas interfaces were investigated. The spin-labeled analogs of the biologically active molecules (indoles, fatty acids, and cholesterol) were used as models of the functional guest molecules in these complexes. The use of various EPR approaches made it possible to determine the character of rotational motion and its quantitative parameters (rotational frequencies and angular amplitudes) for guest molecules in the CD cavities and to reveal the effects of the second and third guest molecules and of the covalent binding of CDs to silica microspheres on molecular dynamics, as well as the hydrophobicity of the environment for the spin-labeled molecules. The properties of the β-CD and γ-CD complexes of naphthalene and its derivatives were studied by fluorescence and phosphorescence methods; the effects of the cage compounds (adamantane, carborane, and cyclohexane (CH)), the low-molecular-mass compounds (acetone), and the adsorption on silica of the polymer CD on the appearance and the line shape of the naphthalene excimer fluorescence (EF) and the long-lived phosphorescence were analyzed.

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.