  The enzyme FoF1-ATP synthase provides the 'chemical energy currency'
adenosine triphosphate (ATP) for living cells. Catalysis is driven by
mechanochemical coupling of subunit rotation within the enzyme with
conformational changes in the three ATP binding sites. Proton translocation
through the membrane-bound Fo part of ATP synthase powers a 10-step rotary
motion of the ring of c subunits. This rotation is transmitted to the gamma and
epsilon subunits of the F1 part. Because gamma and epsilon subunits rotate in
120 deg steps, we aim to unravel this symmetry mismatch by real time monitoring
subunit rotation using single-molecule Forster resonance energy transfer
(FRET). One fluorophore is attached specifically to the F1 motor, another one
to the Fo motor of the liposome-reconstituted enzyme. Photophysical artifacts
due to spectral fluctuations of the single fluorophores are minimized by a
previously developed duty cycle-optimized alternating laser excitation scheme
(DCO-ALEX). We report the detection of reversible elastic deformations between
the rotor parts of Fo and F1 and estimate the maximum angular displacement
during the load-free rotation using Monte Carlo simulations
