Relative orientation, of donor and

Perhaps the most Important effect of conformational variations In electron transfer reactions would be to alter the distances and the relative orientations of donors and acceptors. In photosynthetic RC s, where the primary donors and acceptors lie within 4-5A of each other ( ), small structural displacements (, 5A) may significantly affect rates of back reactions. If they occur rapidly (24), (Conformational movements on a picosecond time scale are not Inconsistent with resonance Raman data on photo-dlssoclated heme-CO complexes (25)), On a longer time scale, protein rearrangements triggered by and propagating from the chromophores may also help subsequent reactions such as the transport of protons that Is Initiated by the primary photochemical event In the R,C, (26),... 

The tris-bipyridine complexes on the other hand are encapsulated by the oxalate network. Thus in the co-doped systems a [Cr(bpy)3]3+ complex happening to sit in the first acceptor shell of a given donor is much closer to this donor than a [Cr(bpy)3]3+ complex sitting in the second shell, n-n overlap between ligand orbitals of the donor and an acceptor in the first shell ensure efficient energy transfer on the sub-microsecond timescale mediated by exchange interaction. Additionally, the relative orientation of donor and acceptor plays an important role for the n-n overlap. For acceptors further away, for which there is no exchange pathway, dipole-dipole interaction takes over. With a critical radius of the order of 11 A, this is much less efficient and the overall quantum efficiency is thus less than unity. 

Here, (fret is a collection of various constants, 0D is the donor emission quantum yield 7ad is the overlap integral between the emission spectrum of the donor and the absorption spectrum of the acceptor is the dipole orientation factor, which depends upon the relative orientations of donor and acceptor n is the refractive index of the medium td the observed donor lifetime (note that (/id/td is the radiative rate constant of the donor) and 7 da is the donor acceptor separation. 

For FRET, the donor emission and acceptor absorption spectra must have sufficient spectral overlap. As the donor excitation energy is transferred to the acceptor via an induced dipole-dipole interaction, efficiency depends on the distance between and orientation of donor and acceptor fluorophores (33). Ideal dyes are photostable, have little intensity fluctuation, and are relatively small in size, to minimize perturbation of the chemokine receptor. 

The reaction center protein (RC) stores the energy of a photon as reduced quinone and oxidized cytochrome . The protein is designed so that ov r 98% of the photons absorbed yieid product, but these products store less than 500 meV of the 1,400 meV initially absorbed. As electron transfer rates are controlled by both the distance between donors and acceptors and the free energy difference between the reactants and products, the protein must control both factors to achieve the near unity quantum yield (1-5). The positions of the cofactors relative to each other are now well known thanks to the solution of the x-ray crystal structure of RCs from Rp. viridis (6, 7) and from Rb. sphaeroides (8, 9). This information has allowed consideration of the effects of the distance between and orientation of donor and acceptor on the electron transfer rates (10, 11). Knowledge of the structure also allows exploration of the interaction of cofactors with the protein that determine the reaction free energy (12). 

The conductive LB films of a CT complex which has alkyl chains in both donor and acceptor molecules was first obtained using C17DMTTF and Cj TCNQ [171]. Relatively homogeneous film was obtained with a conductivity of 0.5 S/cm. The orientation of donor and... 

One thus sees that in addition to thermodynamic factors the physical properties of the donor and acceptor molecule and purely geometrical problems of the relative orientation of the molecules, are of importance. For this reason a comparative discussion of the differences observed is very difficult (cf. Briegleb, 1961). 

The relative orientations of the donor and acceptor stacks in (TTF)(TCNQ) give no reason to assume the existence of low energy orbital overlap between the stacks, in agreement with the observed difference of two orders of magnitude between the conductivity along... 

This structure differs from those of a-(BEDT-TTF)2MHg(SCN)4 salts with respect to both the donor and acceptor sublattices [7]. The donor layers in the present salt have a P -type arrangement (Fig. 1) and are built from three different BEDO-TTF donors (A, B and C). There are three different types of intermolecular interactions with the slab of organic molecules, the relative orientation of which allows us to describe this layer as being composed of a series of parallel stacks of slipped donors along the (2a-b)-direction, as a series of step-chains along the a+2b) -direction, or as a series of parallel... 

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