Photoinduced Electron Tunneling in Protein Molecules

The temperature dependence of the rate constant of electron transfer over large distance from the first triplet state of Zn porphyrin to Rum(NH3)5 covalently attached to histidine-33 in Zn-substituted cytc was studied in Ref. [318]. A temperature independent triplet quenching process with the rate constant 3.6 s-1, was observed at 10-100 K and tentatively attributed to electron transfer facilitated by nuclear tunneling. 

Temperature independent electron tunneling was observed also in Ref. [319], where the rate of electron transfer over large distance in mixed-metal hemoglobin hybrids [MP, FeIU(CN )P], where M = Zn or Mg, was measured in the temperature range from ambient to 100 K. The electron transfer from the triplet state of MP to Fera was not effected by the freezing of the cryosolvent, which may indicate that coupling of electron transfer to low-frequency solvent modes may be minimal. For both M, but especially for M = Mg, the rate constant of the back reaction is nearly temperature independent. 

The temperature dependence of the intramolecular electron transfer from Ru11 to Cu11 in Ru-modified stellacyanin from Rhus vernisifera was studied in Ref. [320] in the temperature interval 298-112.7 K. The activation enthalpy AH was found to be 19.1 3.1 kJ/mol. The entropy of activation found for the intramolecular electron transfer process AS = —201 + 40 J/mol K was in a good agreement with the calculated value AS = —193 J/mol K for intramolecular electron transfer in Ru-stellacyanin-Cu over the distance R = 16 A estimated from the tentative three-dimensional model of stellacyanin molecule [320]. 

---