Tunneling charge transfer bands of donor-acceptor pairs attached to proteins

6 TUNNELING CHARGE TRANSFER BANDS OF DONOR-ACCEPTOR PAIRS ATTACHED TO PROTEINS 

It has been noted by Potasek [105] that electron tunneling in the donor-acceptor pair D-A may lead to the appearance of a charge transfer band in the absorption spectrum of this pair. The author obtained the following formula describing the dependence of the extinction coefficient, , of this band on the energy, E, of the absorbed light quantum 

In this formula, V is the electron matrix element for electron tunneling transition, l is the distance between the centres of the D and A particles, a is the width of the charge transfer band, and EmSLX is the position of the maximum of this band. Emax = Eu — EA + A, where (ED — EA) is the difference of the redox potentials of the donor and the acceptor and A is the energy spent on the excitation of the vibrational degrees of freedom. 

The appearance of similar absorption bands has also been observed upon the formation of a complex between the reduced form of cytochrome c and the simple inorganic acceptor Fe(III)(CN)6[106]. The tunneling distance evaluated from the intensity of this band amounts to 7—10 A. However, more recent experiments have failed to detect such a band [107]. The situation is more favourable in the system [cytochrome c/P870] of the Chromatium reaction centre, where the intensity of the charge transfer band centred at 200 nm could be correlated with the data obtained in kinetic experiments [108]. 

The results reported in refs. 105 and 108 show that, in principle, it is possible to use the data on the absorption spectra of a donor-acceptor pair for estimating the distances of electron tunneling stimulated by light. It should be emphasized that, in this case, the illumination is performed in the band of the tunneling charge transfer from the donor to the acceptor without exciting the electron transitions within the donor and the acceptor molecules themselves. 

6 Tunneling Charge Transfer Bands of Donor-Acceptor Pairs Attached to Proteins 

A band of this type has been observed for an enzyme-substrate complex ES where the enzyme was represented by the oxidized form of peroxidase cytochrome c, cyt(Fe(III)) and the substrate was the reduced form of cytochrome c, cytj (Fe(II)) [298]. Indeed, on mixing the solution of cyt(Fe(I I)) and cytj (Fe(II)) there appeared a new absorption band with the absorption maximum at Emax = 1.4 eV, the extinction coefficient e = 0.35 M-1 cm-1, and the width a = 0.2 eV. This band was referred [298] to charge transfer via electron tunneling, [cyt(Fe(III))/ cyt, (Fe(II))] - [cyt(Fe(II))/cytl(Fe(III))]. From a comparison of the data on the intensity of this band with the results of fluorescence measurements, the distance between the iron atoms Fe(III) and Fe (II) in the [cyt(Fe(III))/cyt1(Fe(II))] complex has been estimated to be R 15-20 A and the edge-to-edge tunneling distance Rt = 7 A. 

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