Electron transfer rate calculation


This applet requires Java V. 5.0; if the calculater fails to load, try upgrading.

This Java Applet calculates a value for the rate constant of an electron transfer reaction occurring in a protein environment between two redox centers separated by an electron transfer Distance, with driving force deltaG, and reorganisation energy Lambda. Values for these parameters can be varied by either changing the value in the appropriate text box, or using the slider.

The applet offers a choice between two methods:

  1. Moser-Dutton: The calculation is performed using the equation recommneded by Moser, Dutton and colleagues (1, 2) for exergonic processes:

    with &beta = 1.4, and &gamma = 3.1 to reflect quantum mechanical corrections as recommended.
  2. Marcus: The calculation is performed using the classical Marcus value (3, 4) for &gamma of F/(4 x 2.303RT), which has a value of 4.22 at 298 oK.

Distance is the distance between nearest atoms in the conjugated ring system for structures such as hemes and chlorophylls.

Values for &beta, and for temperature (in oK), can be changed through text boxes that come up when you click in Parameter Values.
The parameter &beta is the slope of the curve of logk v. distance.
The effect of changing temperature on Marcus calculations is explicit. In the Moser-Dutton calculation, the value of 3.1 is scaled by 298/K, rather than through use of the Hopfield approximation (see 3).

Treatment of endergonic reaction

Endergonic reactions are treated just like exergonic reactions. No attempt has been made to implement the treatment recommended by Page et al. (2), since this is unnecessary (5-7), and can give erroneous results, as can be demonstrated by derivation of the Marcus relationship from parabolas for either an exergonic or an endergonic process.

References

  1. Moser, C.C., Keske, J.M., Warncke, K., Farid, R.S., and Dutton, P.L. (1992) Nature 355, 796-802
  2. Page, C.C., Moser, C.C., Chen, X. and Dutton, P.L. (1999) Nature 402, 47-52
  3. Marcus, R. (1965) J. Chem. Phys. 43, 679-701
  4. DeVault, D. (1980) Q. Rev. Biophys. 13, 387-564
  5. Crofts, A.R. (2004) Biochim. Biophys. Acta 1655, 77-92
  6. Crofts, A.R. (2005) "The bc1 complex: what is there left to argue about?" in Biophysical and Structural Aspects of Bioenergetics, ed. by Wikstrom, M. In press. Royal Society of Chemistry, Cambridge, U.K.
  7. Crofts, A.R. and Rose, S. (2007) Marcus treatment of endergonic reactions: A commentary. Biochim. Biophys. Acta 1767, 1228-1232. Reprint

Generated by Stuart Rose and Antony Crofts