Matrix through-space

The observations clearly indicated that bromine groups shield the carbene center better than methyl groups, as expected. One may wonder whether through-space interaction between the triplet carbene center and the o-bromine groups may play a role in stabilizing the triplet. The EPR data clearly indicate, however, that there is no such interaction at least in a matrix at low temperature. 

Figure 18-5 A current concept of the electron transport chain of mitochondria. Complexes I, III, and IV pass electrons from NADH or NADPH to 02, one NADH or two electrons reducing one O to HzO. This electron transport is coupled to the transfer of about 12 H+ from the mitochondrial matrix to the intermembrane space. These protons flow back into the matrix through ATP synthase (V), four H+ driving the synthesis of one ATP. Succinate, fatty acyl-CoA molecules, and other substrates are oxidized via complex II and similar complexes that reduce ubiquinone Q, the reduced form QH2 carrying electrons to complex III. In some tissues of some organisms, glycerol phosphate is dehydrogenated by a complex that is accessible from the intermembrane space.

According to the chemiosmotic theory, flow of electrons through the electron-transport complexes pumps protons across the inner membrane from the matrix to the intermembrane space. This raises the pH in the matrix and leaves the matrix negatively charged with respect to the intermembrane space and the cytosol. Protons flow passively back into the matrix through a channel in the ATP-synthase, and this flow drives the formation of ATP. 

Rates of non-adiabatic intramolecular electron transfer were calculated in Ref. [331] using a self-consistent perturbation method for the calculation of electron-transfer matrix elements based on Lippman-Schwinger equation for the effective scattering matrix. Iteration of this perturbation equation provides the data that show the competition between the through-bond and through-space coupling in bridge structures. 

The pre-exponential factor A in Eq. 1 is a weak function of the temperature and the reorganization energy, and strongly dependent upon the electronic coupling matrix element V. In the simplest case, V may be assumed to be exponentially dependent upon the through-space donor-acceptor separation r. This yields a distance dependence for electron transfer of ... 

To form the relatively undissociated water, 2 protons per electron pair, transported through complex IV, are removed from the mitochondrial matrix. An additional 2 protons per electron pair transported are extruded from the matrix by complex IV. The total number of protons lost by the mitochondrial matrix through the action of complexes I, III, and IV is thus 8-10 per electron pair, depending on the authority cited. The reason protons are extruded across the inner mitochondrial membrane is 2-fold complex IV apparently acts as a true proton pump with specific protein(s) of that complex acting as the transport particle(s). Complexes I and III, on the other hand, are associated with the so-called vectoral proton translocation process those enzymatic reactions that release protons (e.g., reoxidation of UQH2) take place at or near the intermembrane space surface on the inner mitochondrial membrane. This allows protons to be discharged into the intermembrane space rather than into the mitochondrial matrix. Overall, the pH differential between the cytosol and the mitochondrial matrix is about 1, or a 10-fold difference in [H+] (alkaline inside). 

Fig. 21 Some through-bond coupling pathways in 15(4). The t3 matrix element is responsible for the through-space interactions (represented by a wavy line). The McConnell splitting energy contribution from each pathway is given, as are the signs of the interactions. Note that tt is negative for all values of i.

Figure 19.25. Comparison of Photosynthesis and Oxidative Phosphorylation. The light-induced electron transfer in photosynthesis drives protons into the thylakoid lumen. The excess protons flow out of the lumen through ATP synthase to generate ATP in the stroma. In oxidative phosphorylation, electron flow down the electron-transport chain pumps protons out of the mitochondrial matrix. Excess protons from the intermembrane space flow into the matrix through ATP synthase to generate ATP in the matrix.

---