Distance transfer

Figure C3.2.5. Strongest tunnelling patliways between surface histidines and tire iron atom in cytochrome c. Steps in patliways are denoted by solid lines (covalent bonds), dashed lines (hydrogen bonds), and tlirough-space contacts (dotted lines). Electron transfer distance to His 72 is 5 A shorter tlian in His 66, yet tire two rates are approximately...

Fig. 41. Empirical correlation between 0-0 distance, barrier height and hydrogen-atom transfer distance in OH-O fragment.

Polymerisation casting involves mixing monomer or low molecular weight polymer with a polymerisation initiator, pouring the mix into the mould and allowing polymerisation to occur in situ. A variation is to impregnate fibres with initiated monomer or other low molecular weight material and polymerise to produce composite structures. The main problem is due to the heat of polymerisation. Unless heat transfer distances are kept short or unless the reaction is carried out very slowly it can easily get out of hand. 

That such a process is today commercially important is a measure of the success of chemical engineers in overcoming heat transfer problems involved with masses incapable of being stirred. An idea of the extent of the problem can be gauged from the fact that it takes six hours to cool a sample of polystyrene from 160°C using a cooling medium at 15°c when the heat transfer distance is two inches. 

Suspension polymerisation of styrene is widely practised commercially. In this process the monomer is suspended in droplets 5 -Min. in diameter in a fluid, usually water. The heat transfer distances for the dissipation of the exotherm are thus reduced to values in the range s-fisin. Removal of heat from the low-viscosity fluid medium presents little problem. The reaction is initiated by monomer-soluble initiators such as benzoyl peroxide. 

Munkonge et al. [411] obtained distances of 20A for the quenching of the fluorescence of NCD-Ca " -ATPase either by doxylstearate (5-NS), or by FITC-labeled phosphatidylethanolamine (FITC-PE) incorporated into the bilayer although the authors concluded that the NCD label was located 20A above the surface of the bilayer, the data actually support the conclusion of Scott [132] placing the NCD = 20A below the surface of the bilayer. The energy transfer distance of... 

The intramolecular distances measured at room temperature with the AEDANS FITC pair were similar in the Ca2Ei and E2V states [297]. Ca and lanthanides are expected to stabilize the Ej conformation of the Ca -ATPase, since they induce a similar crystal form of Ca -ATPase [119,157] and have similar effects on the tryptophan fluorescence [151] and on the trypsin sensitivity of Ca -ATPase [119,120]. It is also likely that the vanadate-stabilized E2V state is similar to the p2 P state stabilized by Pi [418]. Therefore the absence of significant difference in the resonance energy transfer distances between the two states implies that the structural differences between the two conformations at sites recorded by currently available probes, fall within the considerable error of resonance energy transfer measurements. Even if these distances would vary by as much as 5 A the difference between the two conformations could not be established reliably. 

Reduction in transfer distances between raw material storage and process and between individual unit operations... 

Incorporation of an artificial flavin nucleobase and of a cyclobutane pyrimidine dimer building block into DNA DNA double strands, DNArPNA hybrid duplexes, and DNA-hairpins, provided compelling evidence that an excess electron can hop through DNA to initiate dimer repair even at a remote site. The maximum excess electron transfer distance realised so far in these defined Donor-DNA-Acceptor systems is 24 A. New experiments are now in progress to clarify whether even larger transfer distances can be achieved. 

The interactions V P and involved in Eqs. (13) determining the zeroth-order electron states in the transitional configuration represent the interactions of the electron with the polarizations P and PB, respectively. At long transfer distances the perturbation leading to the electron transfer has the form... 

The results for the symmetric system are given in Table 2. A comparison of Tables 1 and 2 shows that the dependence of nAnB on and rj influences the position of the transitional configuration and this effect increases with increase in the transfer distance. The physical reason for the change of the path of the transition in this case is that the system prefers to shift from the saddle point to the... 

The height of the potential barrier decreases with the decrease of the transfer distance. Therefore, the contribution of the transitions between excited vibrational states increases and so does the transition probability. However, short-range repulsion between the reactants increases with a decrease of R, and the reaction occurs at an optimum distance R which is determined by the competition of these two factors. In principle, we may imagine the situation when the optimum distance R corresponds to the absence of a potential barrier for the proton. However, we should keep in mind that the transitions between certain excited states may become entirely adiabatic at short distances.40,41 In this case, the further increase of the transition probability with the decrease of R becomes quite weak, and it cannot compensate for the increased repulsion between the reactants, so that even for the adiabatic transition, the optimum distance R may correspond to sub-barrier proton transfer. 

First calculations of the optimum distance between the reactants, R, taking into account the dependence of the probability of proton transfer between the unexcited vibrational energy levels on the transfer distance have been performed in Ref. 42 assuming classical character of the reactant motion. Effects of this type were considered also in Ref. 43 in another model. It was shown that R depends on the temperature and this dependence leads to a distortion of the Arrhenius temperature dependence of the transition probability. 

We next focus on the use of fixed-site cofactors and coenzymes. We note that much of this coenzyme chemistry is now linked to very local two-electron chemistry (H, CH3", CH3CO-, -NH2,0 transfer) in enzymes. Additionally, one-electron changes of coenzymes, quinones, flavins and metal ions especially in membranes are used very much in very fast intermediates of twice the one-electron switches over considerable electron transfer distances. At certain points, the chains of catalysis revert to a two-electron reaction (see Figure 5.2), and the whole complex linkage of diffusion and carriers is part of energy transduction (see also proton transfer and Williams in Further Reading). There is a variety of additional coenzymes which are fixed and which we believe came later in evolution, and there are the very important metal ion cofactors which are separately considered below. 

An interesting approach to measuring rates of electron transfer reactions at electrodes is through the study of surface bound molecules (43-451. Molecules can be attached to electrode surfaces by irreversible adsorption or the formation of chemical bonds (461. Electron transfer kinetics to and from surface bound species is simplified because there is no mass transport and because the electron transfer distance is controlled to some degree. 

Figure 13.16 (a) Polypeptide fold and (b) electron transfer distances in E. coli quinol-fumarate reductase, (c) intercofactor distances in the Wolinella succinogenes enzyme. (From Iverson et al., 2002. Reproduced by permission of the Journal of Biological Chemistry.)... 

This general approach has, however, serious limitations. The position of the site for attack (and therefore the electron transfer distance involved) is very conjectural. In addition, the vexing possibility, which we have encountered several times, of a dead-end mechanism (Sec. 1.6.4) is always present. One way to circumvent this difficulty, is to bind a metal complex to the protein at a specific site, with a known (usually crystallographic) relationship to the metal site. The strategy then is to create a metastable state, which can only be alleviated by a discernable electron transfer between the labelled and natural site. It is important to establish that the modification does not radically alter the structure of the protein. A favorite technique is to attach (NH3)5Ru to a histidine imidazole near the surface of a protein. Exposure of this modified protein to a deficiency of a powerful reducing agent, will give a eon-current (partial) reduction of the ruthenium(III) and the site metal ion e.g. iron(III) heme in cytochrome c... 

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