The actin protein was purchased from Cytoskeleton Inc. (rabbit skeletal muscle, >95% pure, product number AKL99). All protein stocks were centrifuged at 100,000 g for five minutes upon reconstitution in deuterated water (D 2 O). These protein stocks were stored between 0 • C and 4 • C and used within seven days. The G-actin concentration was determined by measuring the solution absorbance at 290 nm against a background solution of 50 mM KCl, 20 mM imidazole (pH 7.4) with a Nanodrop One (ThermoScientific, Wilmington, DE, USA) using extinction coefficients of 26,600 M -1 cm -1 . Note that the actin was reconstituted in deuterated water (D 2 O) instead of water (H 2 O) to eliminate the strong vibrational water band (OH) in the amide I stretch region.