FoF1-ATP synthase catalyzes the synthesis of adenosine triphosphate (ATP).The F1 portion can be stripped from the membrane-embedded Fo portion of theenzyme. F1 acts as an ATP hydrolyzing enzyme, and ATP hydrolysis is associatedwith stepwise rotation of the gamma and epsilon subunits of F1. This rotarymotion was studied in great detail for the last 15 years using single F1 partsattached to surfaces. Subunit rotation of gamma was monitored byvideomicroscopy of bound fluorescent actin filaments, nanobeads or nanorods, orsingle fluorophores. Alternatively, we applied single-molecule F\"orsterresonance energy transfer (FRET) to monitor subunit rotation in the holoenzymeFoF1-ATP synthase which was reconstituted in liposomes. Now we aim to extendthe observation times of single FRET-labeled F1 in solution using a modifiedversion of the anti-Brownian electrokinetic trap (ABELtrap) invented by A. E.Cohen and W. E. Moerner. We used Monte Carlo simulations to reveal thatstepwise FRET efficiency changes can be analyzed by Hidden Markov Models evenat the limit of a low signal-to-background ratio that was expected due to highbackground count rates caused by the microfluidics of the ABELtrap.
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