INITIAL CHARGE SEPARATION KINETICS OF BACTERIAL PHOTOSYNTHETIC REACTION CENTERS IN ORIENTED LANGMUIR-BLODGETT-FILMS IN AN APPLIED ELECTRIC-FIELD

The free energy of the initial charge separation in bacterial photosyn thetic 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 10(6) V/cm. The nea r-infrared transient absorption changes associated with the decay of t he excited state of the bacteriochlorophyll dimer and the initial char ge separation was measured with 300 fs time resolution with and withou t applied field. The surprisingly small field induced rate changes of the oriented systems compared to unoriented systems suggest that modul ation 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 asso ciated with modulation of the bacteriopheophytin to bacteriochlorophyl l dimer charge recombination.