Fast changes of enthalpy and volume on flash excitation of Chromatium chromatophores.

We have used a capacitor microphone transducer to measure volume changes in suspensions of Chromatium chromatophores 100 μs to 20 ms after flash excitation. Volume changes in photochemical reactions can arise both from volume differences between reactants and products, and from enthalpy changes which heat or cool the solution. By measuring the volume changes at two temperatures, one can resolve the total changes into their two components. In Chromatium, light-driven electron transfer from cytochrome C555 to the primary or secondary electron acceptors causes a contraction of approx. 33 A3 per electron transferred. The relaxation of the initial volume change is altered by the presence of gramicidin, valinomycin, o-phenanthroline, or phosphate. From the effects of these agents, we conclude that the volume change is localized at the photochemical reaction center, and that part of the relaxation involves transfer of a proton from the solvent to the chromatophores. Electron transfer from cytochrome C555 to the primary or secondary electron acceptors does not cause a significant enthalpy change. We conclude that the free energy increase that accompanies this reaction is due to a negative entropy change.