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The temperature dependence of the mean square displacement of the iron atom in reduced and oxidized cytochrome c has been studied by Mössbauer spectroscopy. The flexibility of the protein, labeled by the modes coupling to the iron, is diminished upon reduction. The differences in flexibility are sufficient to explain the differences in physicochemical properties between the oxidized and the reduced forms.

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New experimental data [Berg, A. I., Noks, P. P., Kononenko, A. A., Frolov, E. N., Khrymova, I. N., Rubin, A. B., Likhtenstein, G. I., Goldanskii, V. I., Parak, F., Bukl, M. & Mossbauer, R. (1979) Mol. Biol. (USSR) 13, 81-89; Berg, A. I., Noks, P. P., Kononenko, A. A., Frolov, E. N., Uspenskaya, N. Y., Khrymova, I. N., Rubin, A. B., Likhtenstein, G. I. & Hideg, K. (1979) MoL Biol( USSR) 13, [469][470][471][472][473][474][475][476][477] provide evidence that the electron tunneling process is connected to a special type of conformational transition (segmental transition) of protein macromolecules in photosynthetic membranes. This problem is investigated with a simple mechanical model. It is shown that the segmental degree of freedom can play the role of the strongly interacting accepting mode for the e ectron tunneling process. The temperature dependences of the electron tunneling rate and the recoilless -/-ray absorption of membrane-bound 5 Fe, as an indicator of the intramolecular mobility, are calculated. The problem of energy storage in proteins is also discussed. Electron tunneling in biology was first observed in 1966 (1), and it has since been discussed in many papers (2-11). Attention has been paid to the temperature dependence of the electron tunneling rate and to the connection between electron transfer and oscillations of the accepting mode. We have investigated the kinetics of electron tunelling between the quinone acceptors of photosynthetic reaction centers in chromatophore membranes from Rhodospirillum rubrum (12) and subchromatophore preparations from Rhodopseudomonas sphaeroides (13). The characteristics of the observed kinetic curves were compared with those of intramolecular mobility measured by electron spin resonance spin label and Mossbauer spectroscopy methods under various conditions of temperature and relative humidity. It was shown that both the efficiency of the electron transfer process and the parameters of intramolecular mobility change sharply within the same temperature range-between about 190 The observed sharp decrease of the probability of the Mossbauer effect without any noticeable changes in the line shape at T > 220 K indicates that a soft mode (10-100 cm-1) with large amplitude (A) and (or) small frequency (w) of the vibrations of the 57Fe atom appears. The amplitude A can be estimated from A2 (x2) , 2 -(0.14)2 A2 [1] in which X = 27rX is the wavelength of the resonance ,y-quantum. The frequency cw can be estimated from hw < Cr = e2/2 MFeC2 -2 X 10-3 eV, in which Er is the recoil energy of the 57Fe atom and ee = 14.4 keV, the energy of the resonance y-quantum. Thus, w < E$r/h = 3 X 1012 sec-1.[2] Estimates 1 and 2 are interrelated. Frequencies w and amplitudes A of such orders of magnitude have been discussed recently in connection with the "enzyme machine" concept (15)(16)(17)(18)(19) and the ideas of Frohlich (20, 21). According to these hypotheses, an enzyme protein is a structure with one particular macroscopic degree of freedom-i.e., it is a machine. The exis...