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Biomedical subjects

Igor Efimov

Publications and source records attributed to Igor Efimov.

7 recordsLinked to original sources
PubMed2006

Correlation of viscoelastic properties with solvation of regioregular poly(3-decylthiophene) films.

Viscoelastic properties of regioregular poly(3-decylthiophene) films cast on gold electrodes and exposed to acetonitrile/LiClO4 solution were studied using high-frequency acoustic impedance. Values of shear moduli, G = G' + jG'', were determined under conditions of potentiodynamic and potentiostatic electrochemical control, as functions of potential (0.0 < E/V < 0.8), temperature (5 < T/ degrees C < 70), and angular frequency (omega = 2pi f; 10 < f/MHz < 110). The effect of potential was small, of temperature was significant, and of frequency was dominant. The principle of time-temperature equivalence was used to construct master relaxation curves. Application of activation, Williams-Landel-Ferry, and Rouse-Zimm models shows the material to be quite different from other thiophene-based conducting polymers, namely, poly(3,4-ethylenedioxythiophene) and regioregular poly(3-hexylthiophene). Detailed exploration of the data reveals novel insights into the compositional origins--notably with regard to solvation--of the shear modulus behavior.

Journal Article↗
PubMed2005

Time-scale- and temperature-dependent mechanical properties of viscoelastic poly(3,4-ethylenedioxythiophene) films.

The viscoelastic properties of thin films of poly(3,4-ethylenedioxythiophene) (PEDOT) have been studied using the method of acoustic impedance. The films were deposited on the Au electrodes of 10 MHz AT-cut quartz thickness shear mode resonators and exposed to acetonitrile solutions of 0.1 M TEABF4 and LiClO4. For p-doped films, admittance spectra as a function of potential (E), temperature (T), and time scale (frequency, via harmonics, in the range 10-110 MHz) were acquired. Shear modulus components extracted from these responses surprisingly showed virtually no variation with E (and thus film solvation) or with T, but the variation with frequency was dramatic. This qualitative behavior and the numerical values of the shear moduli contrast strongly with recently reported data for the related poly(3-hexylthiophene) system, which shares the same conducting spine but differs substantially in the substitution pattern. Accordingly, the models and interpretation for PEDOT are quite different: film dynamics are determined by free-volume effects, and side-chain motion is not a significant factor. Qualitatively similar potential and time-scale effects were seen for n-doped PEDOT, but the scope of the measurements was limited by film stability.

Journal Article↗
PubMed2005

Time-temperature superposition for viscoelastic properties of regioregular poly(3-hexylthiophene) films.

Shear moduli were determined for chemically polymerized and solvent cast regioregular poly(3-hexylthiophene) films, using thickness shear mode acoustic wave resonators. The results are strikingly different to those for electropolymerized regiorandom poly(3-hexylthiophene) films. The time scale of the measurement was varied directly by use of higher harmonics of the acoustic wave resonator and indirectly via temperature. The significant variations in shear modulus with effective time scale can be "normalized"onto a stress master relaxation curve by using the concept of time-temperature superposition; this is the first time this has been demonstrated for electroactive films. The shift factors required to effect this normalization do not follow the classical Williams-Landel-Ferry (WLF) equation developed for long-range backbone motions of bulk polymers. Instead, they follow an Arrhenius-like behavior, commonly used to describe secondary motions of polymer side-chains. The activation enthalpy associated with this is independent of applied potential, is the same as for as cast (undoped) films, and is similar to that for rotation about a carbon-carbon single bond. These all point to the hexyl side-chains as the origins of the observed phenomena, consistent with the "melting point" separating two temperature-dependent phases and with the different molecular packing arrangements that would necessarily apply to regioregular and regiorandom materials.

Journal Article↗
PubMed2005

Relationship between charge-transfer interactions, redox potentials, and catalysis for different forms of the flavoprotein component of p-cresol methylhydroxylase.

Thirty-three variants of the flavoprotein component of p-cresol methylhydroxylase that contain noncovalently or covalently bound flavin adenine dinucleotide (FAD) analogues were studied. A very good correlation was found between the efficiency of p-cresol oxidation by these proteins and E(CT), the energy for the maximum wavelength for the charge-transfer band of the complex between the bound flavin and 4-bromophenol, a substrate mimic. The correlation covers a range of k(cat) values that spans over 5 orders of magnitude and values of E(CT) that span 900 mV, and the analysis of the data provided a value of the transfer coefficient, alpha, of 0.31. This study demonstrates clearly that the redox properties of both the bound substrate and the flavin cofactor must be taken into account to explain the relative catalytic efficiencies of the variant flavoproteins.

Catalysis↗
PubMed2004

A study of the spectral and redox properties and covalent flavinylation of the flavoprotein component of p-cresol methylhydroxylase reconstituted with FAD analogues.

The spectral and redox properties are described for the wild-type and Y384F mutant forms of the flavoprotein component (PchF) of flavocytochrome, p-cresol methylhydroxylase (PCMH), and cytochrome-free PchF that harbor FAD analogues. The analogues are iso-FAD (8-demethyl-6-methyl-FAD), 6-amino-FAD (6-NH(2)-FAD), 6-bromo-FAD (6-Br-FAD), 8-nor-8-chloro-FAD (8-Cl-FAD), and 5-deaza-5-carba-FAD (5-deaza-FAD). All of the analogues bound noncovalently and stoichiometrically to cytochrome-free apo-PchF, and the resulting holoproteins had high affinity for the cytochrome subunit, PchC. Noncovalently bound FAD, 6-Br-FAD, or 6-NH(2)-FAD can be induced to bind covalently by exposing holo-PchF to PchC. The rate of this process and the redox potential of the noncovalently bound flavin may be correlated. In addition, the redox potential of each FAD analogue was higher when it was covalently bound than when noncovalently bound to PchF. Furthermore, the potential of a covalently bound or noncovalently bound FAD analogue increased on association of the corresponding holo-PchF with PchC, and the activity increased as the flavin's redox potential increased. It was discovered also that 4-hydroxybenzaldehyde, the final p-cresol oxidation product, is an efficient competitive inhibitor for substrate oxidation by PchF since it binds tightly to this protein when the flavin is oxidized, although it binds more loosely to the enzyme with reduced flavin. Finally, the energies of the charge-transfer bands for the interaction of bound flavin analogues with 4-Br-phenol (a substrate mimic) increased as the potential decreases, although a simple global correlation was not seen. This is the case because the energy is also a function of the redox properties of the bound mimic. The implications of these findings to covalent flavinylation and catalysis are discussed.

Binding Sites↗
PubMed2004

Insight into covalent flavinylation and catalysis from redox, spectral, and kinetic analyses of the R474K mutant of the flavoprotein subunit of p-cresol methylhydroxylase.

Each flavoprotein subunit (PchF) of p-cresol methylhydroxylase (PCMH) has flavin adenine dinucleotide (FAD) covalently tethered to Tyr384. The PCMH structure suggests that Arg474 in PchF is required for self-catalytic covalent flavinylation and for substrate oxidation. The replacement of Arg474 with Lys was carried out to probe the subtleties of the role of Arg474 in these processes. In nearly all of the aspects examined, the mutant protein showed compromised properties relative to the wild-type protein, including the tenacity of noncovalent FAD binding to the apo-protein, the rate of covalent flavinylation, the affinity of the covalent flavoprotein for PchC (the cytochrome subunit), the k(cat) for substrate oxidation, and the affinity for substrate analogues in the formation of FAD-charge-transfer complexes (CT complexes). Nevertheless, because the mutant retains these attributes, the comparison allows for an examination of the role of this residue in the various properties of the enzyme. A correlation is proposed to exist between nu(m), the frequency for the absorbance maximum of the CT complex with a substrate analogue, and k(cat), the steady-state rate constant for oxidation of p-cresol by various forms of PCMH and PchF; both nu(m) and k(cat) can be expressed as functions of the ionization potential of the donor (I(D)) and the electron affinity of the acceptor (E(A)). This correlation is a better predictor of the rate constant for substrate oxidation than is the magnitude of the redox potential, E(m,7), of the bound FAD, which was determined for the various mutant enzyme species and compared with those of the wild type.

Animals↗