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The molecular machines of living cells work at physiological temperatures and thus experience thermal motion, yet their function, which requires correct alignment of parts and steering of reactions, is executed precisely. Proteins do not lose control, even when electrons are the carriers of function, and quantum dynamics, with its sensitivity to interference effects and minute spatial changes, rules. Swanlund Professor of Physics Klaus Schulten believes that proteins exploit such thermal motion to create "molecular machines" (K. Schulten, "Electron Transfer: Exploiting Thermal Motion, Science 290, 61–62 [2000]).

The figure shown above represents the arrangement of prosthetic groups involved in electron transfer reactions in the photosynthetic reaction center (RC) of the purple bacterium Rhodopseudomonas (Rps.) viridis. The protein matrix is shown in translucent blue with a thin, black, tubular backbone. Excitation energy is transferred from the light-harvesting proteins to a pair of bacteriochlorophylls (P) (purple); the energy drives the transfer of electrons through BPh to quinone Q_A and quinone Q_B. The thermal motion of the protein matrix is strongly coupled to the electron transfer through Coulomb interaction and through alterations in the tunneling pathways of the electron.

The simulation of this process was produced with VMD, a molecular visualization program for displaying, animating, and analyzing large biomolecular systems using 3-D graphics and built-in scripting, which was created by the Theoretical Biophysics Group at Illinois. For further information about this this exciting work or VMD, contact Professor Klaus Schulten (Department of Physics and the Beckman Institute).