ELECTRIC FIELD EFFECTS ON PHOTOSYSTEM II ELECTRON TRANSFER

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We have developed ways of measuring electron transfer dynamics in specially designed capacitors made from membrane bound reaction center protein. This has demanded the design of critical coupling protocols between femtosecond pulses and the creation of fields across the structure. From the spectroscopic standpoint the great challenge is to improve the signal-to noise in fs experiments where the sample cannot be moved or flowed. Furthermore, the coupling of such experiments to pulsed electric fields and concerning demonstrations of the presence of transient fields is nontrivial. The free energy of the initial charge separation in bacterial photosynthetic reaction centers has been modified by placing oriented Langmuir-Blodgett films of the purified protein between external electrodes in a planar capacitor and applying a field of nearly 106 V/cm. The near-infrared transient absorption changes associated with the decay of the excited state of the bacteriochlorophyll d imer and t he initial charge separation was measured with 300 fs time resolution with and without applied field. The surprisingly small field induced rate changes of the oriented systems compared to unoriented systems suggest that modulation of the energy gap between excited bacteriochlorophyll dimer and the charge separated state with bacteriochlorophyll monomer reduced is the principal influence of electric field on rate. The field induced quantum yield failure observed at longer timescales appears to be associated with modulation of the bacteriopheophytin to bacteriochlorophyll dimer charge recombination.