A detailed investigation of the applicability of the vibronically coupled electron tunneling theory in biomolecules can be mate by a quantitative study of a weak charge-transfer optical absorption band that has been predicted by this theory. The measurement of the position, width, and molar extinction coefficient of this band is examined in the bound model system cytochrome c-Fe(CN)6 at room temperature and demonstrates that the theory is quantitatively applicable in this system. The size of the parameters measured is typical of those relevant for biological electron transfers. The comparisons lend credibility to the generality of vibronically coupled electron theory in biomolecules and its short transfer distances.

The nature of electron transfer between the bound complex cytochrome c and cytochrome c peroxidase has been investigated. Experimental verification of the predicted charge-transfer band provides evidence of electron tunneling as the mechanism of transfer between these molecules in solution at room temperature. The measured transfer distance is ≃ 7 angstroms between heme edges, which results in a distance of ≃ 15 to 20 angstroms between iron atoms.

Evidence for the constructs central to vibronically coupled electron transfer has been obtained. Our experiments show the existence of a weak (f t 10-v) charge-transfer absorption band in the near infrared for the bound donor-acceptor complex, cytochrome c-Fe(CN 6. Such a charge-transfer band had been predicted from the theory of such transfers. The experimental method, using a form of excitation modulation spectroscopy, measures only the optical absorption that induces charge transfer between the donor and the acceptor (and does etect other absorptions) and allows the study of chargetransfer bands whose absorbances are small compared to the sample absorbance. The energy position and oscillator strength of the band agree with the general predictions of this vibronically coupled tunneling theory. We suggest that, in this system at room temperature, the electron transfer can be described by this tunneling theory. This model system result gives credence to the short electron transfer distances the theory has predicted for biological electron transfers. Electron transfer is an essential process in photosynthetic and aerobic life systems. The molecular mechanism by which solar energy is captured and converted into chemical energy necessitates electron transfer. Photoexcitation of chlorophyll results in a series of electron transfers in which an electron is moved from a high-potential cytochrome to a low-potential molecule on the opposite side of the membrane. Absorption of light results in the creation of an electron-hole pair necessary for useful chemical work. Selectivity must prevent those spontaneous "downhill" transfers that result in electron flow from the low-potential molecule back to the high. Enzyme composition and conformational mobility must be optimized to achieve efficient electron transfers. The molecular mechanism of electron transfer in photosynthesis and in respiration are identical in principle. An understanding of the mechanism of electron transfer in biomolecules is of fundamental importance to an explanation of the functions and meanings of the complex intermolecular relationships of components in an electron transfer chain.There is direct experimental support for the idea that, at low temperatures, tunneling is involved in electron transfer. However, attempts to describe biomolecular electron transfer as an electron tunneling from a bound state of energy on one molecule to an electronic continuum of energies (1, 2) miss the essence of the tunneling problem between two local states, such as two biomolecules. By representing the second site as a metal instead of as a single energy level, one makes a conceptual error which results in an answer permitting huge tunneling distances-30-80 A. To correct this error, a theory based on electron tunneling between two states with the energy conserved by vibronic coupling in the individual molecules (3-5) has been introduced. In this theory, the sites are correctly represented as having a single electronic level. The presence of the Abbreviations: C...

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An optical modulation apparatus is described for the measurements of very weak (ε<1 M−1 cm−1) charge transfer optical absorption bands between electron donor–acceptor complexes. Such small bands, which are of interest in understanding the mechanism of electron transfer in physical, chemical, and biological systems, are undetectable by more traditional methods of optical spectroscopy due to their small size and their superposition on much larger background sample absorptions. An additional feature of the method is that the kinetics of the electron returning to its equilibrium state following photo-induced charge transfer can be measured.