DELAYED FLUORESCENCE STUDY ON P(ASTERISK)Q(A)-]P(-) CHARGE SEPARATIONENERGETICS LINKED TO PROTONS AND SALT IN REACTION CENTERS FROM RHODOBACTER-SPHAEROIDES()Q(A)()

The energetics of the first stable charge separated state, P(+)Q(A)(-) relative to that of PQ(A) was examined in isolated RC from Rhodobact er sphaeroides by delayed fluorescence. The temperature dependence of the delayed fluorescence indicates that the charge separation is a hig hly enthalpy-driven process (Delta H = - 818 +/- 20 meV at pH 8) and t he free energy gap between PQA and P(+)Q(A)(-) drops with increasing pH (40 +/- 4 meV between pH 6 and IO). The FH-dependence of the free e nergy change of the P(+)Q(A)(-) State runs parallel to the (integrated ) net proton uptake due to the PQ(A)/P(+)Q(A)(-) redox change in a wid e pH range and under different ionic conditions. Elevation of the ioni c strength increases the delayed fluorescence intensity and decreases the (dark and light) pK(a) values as well as the light-induced Delta p K(a) changes of the protonatable groups of the protein. The observed d ependence of the energetics of P(+)Q(A)(-) an the concentration and co mposition of mobile ions is discussed in terms of binding and screenin g of protonatable groups and surface charges as dominant modes of elec trostatic interaction between RC and salt.