Optical transmittance of carbon suspensions in polymer matrixes under powerful pulsed laser irradiation

The effect of optical limiting is investigated in the suspensions of carbon microparticles in aqueous gelatin gel and epoxy resin. Both transient and permanent changes of optical transmittance are observed after the irradiation by a Q-switched YAG:Nd³⁺ laser pulses. The experimental results are explained with taking into account the formation of micro-bubbles filled with water steam and with gaseous products of decomposition of the matrix. In the epoxy resin suspensions, the laser-induced permanent changes of transmittance are caused by pyrolysis of epoxy oligomers in the vicinity of laser-heated carbon particles

A Map of Dielectric Heterogeneity in a Membrane Protein: the Hetero-Oligomeric Cytochrome b 6 f Complex

The cytochrome b6f complex, a member of the cytochrome bc family that mediates energy transduction in photosynthetic and respiratory membranes, is a hetero-oligomeric complex that utilizes two pairs of b-hemes in a symmetric dimer to accomplish trans-membrane electron transfer, quinone oxidation–reduction, and generation of a proton electrochemical potential. Analysis of electron storage in this pathway, utilizing simultaneous measurement of heme reduction, and of circular dichroism (CD) spectra, to assay heme–heme interactions, implies a heterogeneous distribution of the dielectric constants that mediate electrostatic interactions between the four hemes in the complex. Crystallographic information was used to determine the identity of the interacting hemes. The Soret band CD signal is dominated by excitonic interaction between the intramonomer b-hemes, bn and bp, on the electrochemically negative and positive sides of the complex. Kinetic data imply that the most probable pathway for transfer of the two electrons needed for quinone oxidation–reduction utilizes this intramonomer heme pair, contradicting the expectation based on heme redox potentials and thermodynamics, that the two higher potential hemes bn on different monomers would be preferentially reduced. Energetically preferred intramonomer electron storage of electrons on the intramonomer b-hemes is found to require heterogeneity of interheme dielectric constants. Relative to the medium separating the two higher potential hemes bn, a relatively large dielectric constant must exist between the intramonomer b-hemes, allowing a smaller electrostatic repulsion between the reduced hemes. Heterogeneity of dielectric constants is an additional structure–function parameter of membrane protein complexes

Pathways of transmembrane electron transfer in cytochrome bc complexes : dielectric heterogeneity and interheme Coulombic interactions

The intramembrane cytochrome <i>bc</i>₁ complex of the photosynthetic bacterium Rhodobacter capsulatus and the cytochrome <i>b</i>₆<i>f</i> complex, which functions in oxygenic photosynthesis, utilize two pairs of <i>b</i>-hemes in a symmetric dimer to accomplish proton-coupled electron transfer. The transmembrane electron transfer pathway in each complex was identified through the novel use of heme Soret band excitonic circular dichroism (CD) spectra, for which the responsible heme–heme interactions were determined from crystal structures. Kinetics of heme reduction and CD amplitude change were measured simultaneously. For <i>bc</i>₁, in which the redox potentials of the transmembrane heme pair are separated by 160 mV, heme reduction occurs preferentially to the higher-potential intermonomer heme pair on the electronegative (n) side of the complex. This contrasts with the <i>b</i>₆<i>f</i> complex, where the redox potential difference between transmembrane intramonomer p- and n-side hemes is substantially smaller and the n–p pair is preferentially reduced. Limits on the dielectric constant between intramonomer hemes were calculated from the interheme distance and the redox potential difference, Δ<i>E</i>_m. The difference in preferred reduction pathway is a consequence of the larger Δ<i>E</i>_m between n- and p-side hemes in <i>bc</i>₁, which favors the reduction of n-side hemes and cannot be offset by decreased repulsive Coulombic interactions between intramonomer hemes