Temperature and -.DELTA.G.degree. dependence of the electron transfer from BPh.cntdot.- to QA in reaction center protein from Rhodobacter sphaeroides with different quinones as QA

Abstract

The rate of electron transfer from BPh{sup {sm bullet}{minus}} to Q{sub A} (k{sub 1}) was determined at 14, 35, 113, and 298 K in reaction center protein from Rhodobacter sphaeroides R-26 in which the native Q{sub A}, UQ{sub 10}, was removed and activity reconstituted with 22 other quinones. The majority of these had in situ midpoints lower than that of UQ{sub 10}, diminishing the {minus}{Delta}G{degree} for Q{sub A} reduction. The electron-transfer rate was determined from measurement of the quantum yield of (BChl){sub 2}{sup {sm bullet}+}Q{sub A}{sup {sm bullet}{minus}} ({Phi}{sub Q}). At 295 K, {Phi}{sub Q} was obtained from excitation-flash saturation measurements, monitoring (BChl){sub 2}{sup {sm bullet}+}Q{sub A}{sup {sm bullet}{minus}} formation optically. Between 14 and 113 K, {Phi}{sub Q} was determined from the amount of (BChl){sub 2}{sup {sm bullet}+}Q{sub A}{sup {sm bullet}{minus}}, measured by EPR, produced by excitation with a subsaturating flash. When the {minus}{Delta}G{degree} for the reduction of Q{sub A} by BPh{sup {sm bullet}{minus}} was diminished by as much as 150 meV, relative to that found in the native protein, {Phi}{sub Q} remained {>=} 0.9. However, as the exothermicity was decreased further, {Phi}{sub Q} diminished steeply. Thus, much of the 650 meV reaction {minus} {Delta}G{degree} found in the native RCmore » is required to maintain a near-unity quantum yield. Calculation of the rate of electron transfer from BPh{sup {sm bullet}{minus}} to Q{sub A} shows the reaction slows with decreasing exothermicity. However, over the range of {minus}{Delta}G{degree} studied, the rate changed little as the system was cooled from 295 to 14 K. These results were analyzed with the model of electron transfer as a nonadiabatic, multiphoton, nonradiative decay process.« less