Measurement of electrostatic potentials above oriented single photosynthetic reaction centers

Photosystem I (PS I) reaction centers are nanometer-size robust supramolecu lar structures that can be isolated and purified from green plants. Using t he technique of Kelvin force probe microscopy, we report here the first mea surement of exogenous photovoltages generated from single PS I reaction cen ters in a heterostructure composed of PS I, organosulfur molecules, and ato mically flat gold. Illumination of the reaction centers was achieved with a diode laser at lambda = 670 nm. Data sets consisting of 22 individual PS I s measured entirely under laser illumination, 12 PS Is measured entirely in darkness, and four PS Is in which the light-dark transition occurred in mi dscan of a single PS I were obtained. The average values of the light minus dark voltages relative to the substrate for the four PS Is were -1.13 +/- 0.14 and -1.20 +/- 0.19 V at diametrical peripheries' and -0.97 +/- 0.04 V at the center. Under illumination, the potentials of the central region of the PS Is were typically more positive than the periphery by 6-9 kT, where kT is the Boltzmann energy at room temperature. These energies suggest a po ssible mechanism whereby negatively charged ferredoxin, the soluble electro n carrier from PS I to the Calvin-Benson cycle, is anchored and positioned at the reducing end of PS I for electron transfer. The results are placed i n context with the prior experimental literature on the structure of the re ducing end of PS I.