Thermodynamics of the primary and secondary photochemical reactions in Chromatium

Abstract

Thermodynamic properties of the primary and secondary photosynthetic electron transfer reactions in Chromatium vinosum have been studied with the aid of short actinic light flashes. Midpoint redox potentials measured for the primary electron acceptor (X), P870, and cytochromes C555 and C552 agreed with those which previous workers have obtained with other techniques. The secondary electron acceptor (Y) has a midpoint redox potential at pH 7.7 ( E m7.7) of approx. −90 mV, and is present in a pool of approx. two molecules per molecule of X. The reaction between X − ( E m7.7 approx. −130 mV) and Y is a single-electron process, which occurs at the same rate whether the Y pool is completely oxidized or partially reduced. The possibility was considered that the E m values might be sensitive to electrostatic interactions between electron carriers within a photosynthetic unit. However, the redox titrations of cytochrome C555 appear to be unaffected by the oxidation state of X. Similarly, oxidation of one of the two cytochrome C555 hemes appears not to perturb the titration of the other heme. As a result, values of ΔG°, ΔH°, and ΔS° derived from redox titrations of the electron carriers in separate photosynthetic units probably represent the true standard state values for the photochemical reactions. Measurements of the E m values as functions of temperature allowed a resolution of the free energy changes ( ΔG°) into the entropy and enthalpy changes ( ΔS° and ΔH°), for each of the reactions. Surprisingly, an entropy decrease accounts for all of the free energy which is stored in the primary photochemical reaction. Overall values of ΔG°, ΔH°, and ΔS° for the transfer of an electron from cytochrome C555 to Y are +9.9 kcal/mole, −0.7 kcal/mole, and −35.9 cal×°K −1×mole −1, respectively. The E m values of the cytochromes are independent of pH. Those of P870, X, and Y all depend on pH, indicating the uptake of approx. 1 2 H + in each of the half-cell reductions. N-Methylphenazonium methosulfate (PMS), at a concentration of 100 μM, does not alter the redox titrations of P870, X, or either cytochrome C555 or cytochrome C552. However, it does interfere with titrations of Y, raising the apparent midpoint potential and increasing the pH dependence of the titration, in a manner which does not directly correlate with the chemical reduction of PMS itself.