Electronic conformational coupling

For most of the unbranched alkyl substituted polymers, in solvents of high refractive index, the smooth transition described by the above model changes to an abrupt, first order like transition (8). This has been described in terms of electronic-conformational coupling ( n ). The thermochromism for the n-propyl methyl polymer is summarized in Figure for two solvents. 

The spatial separation between the components of the electron transport chain and the site of ATP synthesis was incompatible with simple interpretations of the chemical coupling hypothesis. In 1964, Paul Boyer suggested that conformational changes in components in the electron transport system consequent to electron transfer might be coupled to ATP formation, the conformational coupling hypothesis. No evidence for direct association has been forthcoming but conformational changes in the subunits of the FI particle are now included in the current mechanism for oxidative phosphorylation. 

If the molecular tumbling rate is slow enough that larger electron-electron dipolar couplings are not motionally averaged, Fourier deconvolution can be used to analyze dipolar interactions in fluid solution.18 Distances in doubly spin-labelled rhodopsin were measured by Fourier deconvolution of CW line-shape changes in room temperature solution.78 The broadening function was modelled as the sum of Pake patterns from a distribution of distances. As a reference point for the distance measurements one label was attached at the cytoplasmic termination of transmembrane helix 1. The second label was attached near the cytoplasmic termination of transmembrane helix 7 or in the short helix 8. The distances and conformational flexibility in the dark state are... 

Radical anions derived from 2,5-diformylthieno[3,2-6]thiophene (39) as well as (40) were studied in connection with the conformational analysis of heteroaromatic carbonyl compounds. Different ESR signals were given by distinct rotational isomers. Information on interconversion of the rotamers could not be obtained since the radicals were unstable at the temperatures necessary for interconversion. On the other hand, ketyl radicals derived from ketones (41) and (42) are relatively more stable at the temperatures needed for the study of the conformational mobility in these systems. The ESR spectra of the bis-thienothienyl ketyls from (41) and (42) at room temperature show that the unpaired electron is coupled to three pairs of equivalent protons. Both spectra exhibited a certain amount of asymmetry, which was enhanced by lowering the temperature. At -10°C the highfield part of the spectra split into new lines arising from two species which have similar hyperfine splitting, but different g factors. These have been identified as the rotational isomers of the radicals. The two preferred conformations are cis-trans and trans-trans. An examination... 

The idea of conformational coupling of ATP synthesis and electron transport is especially attractive when we recall that ATP is used in muscle to carry out mechanical work. Here we have the hydrolysis of ATP coupled to motion in the protein components of the muscle. It seems reasonable that ATP should be formed as a result of motion induced in the protein components of the ATPase. Support for this analogy has come from close structural similarities of the F, ATPase P subunits and of the active site of ATP cleavage in the muscle protein myosin (Chapter 19). 

No generally applicable Q s can be given for methylene protons, for example of the propionates (Fig. 2), since the electron-proton coupling depends strongly on the sterical conformation of the molecule (16). 

Yet, the information about the electron-vibration coupling is known because the adiabatic approximation has parametrized the electronic structure for each nuclear conformation. 

Soliton — Solitons (solitary waves) are neutral or charged quasiparticles which were introduced in solid state physics in order to describe the electron-phonon coupling. In one-dimensional chainlike structures there is a strong coupling of the electronic states to conformational excitations (solitons), therefore, the concept of soliton (-> polaron, - bipolaron) became an essential tool to explain the behavior of - conducting polymers. While in traditional three-dimensional -> semiconductors due to their rigid structure the conventional concept of - electrons and -> holes as dominant excitations is considered, in the case of polymers the dominant electronic excitations are inherently coupled to chain distortions [i]. 

Harris, T. K., Davidson, V. L., Chen, L. Y., Mathews, F. S., and Xia, Z. X., 1994, Ionic strength dependence of the reaction between methanol dehydrogenase and cytochrome 05511 evidence of conformationally coupled electron transfer. Biochemistry 33 12600912608. 

The continuum electrostatic approach has been considered also appropriate for studying biomolecular systems when electronic polarisation effects, typically neglected by the additive pair-wise potentials commonly use in MD and MC, are dominant in relation to the conformational flexibility, and when changes in protonation states of tritable sites [193-198] or electron-proton coupling phenomena [199-204] occur. 

As already explained in Chapter 3.17 Section 3.2.3, C NMR data provides information on the electronic environment of C atoms in the ring framework in particular, but also of course on exocyclic substituents. Detailed statements can often be made about ring conformation, couplings, and deshielding processes, for example. In noncyclic BMe compounds a more or less linear relationship between B and C NMR shifts is observed <75CB383l >. However, in OEt- and SMe-substituted... 

Several mechanisms have been proposed to account for the coupling of electron transport and ATP production. The mechanism that served as the point of departure in all discussions is chemiosmotic coupling, which was later modified to include a consideration of conformational coupling. 

Earlier theoretical work on the excited-state dynamics of pyrazine focused on the vibronically induced anharmonic couplings in the Si state, on the interactions between gerade and ungerade rm states or on selected PE functions and electron-vibrational coupling parameters. In a series of papers the Munich group has characterized the S2 and 51-state PE surfaces in the conformational subspace relevant for the short-time photophysics following the S2 — So and 5i — So transitions.In the course of this work, the treatment has steadily improved, as regards the accuracy of the electronic-structure calculation as well as the number and description of the vibrational modes considered. The most accurate calculations are based on the CASSCE/MRCI method with a basis set of DZP quality. 

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