Micro-mechanical response and power-law exponents from the longitudinal fluctuations of F-actin solutions

Soft Matter, 2023,19, 3652-3660 We investigate the local fluctuations of filamentous actin (F-actin), with focus on the skeletal thin filament, using single-particle optical trapping interferometry. This experimental technique allows us to detect the Brownian motion of a tracer bead immersed in a complex fluid with nanometric resolution at the microsecond time-scale. The mean square displacement, loss modulus, and velocity autocorrelation function (VAF) of the trapped microprobes in the fluid follow power-law behaviors, whose exponents can be determined in the short-time/high-frequency regime along several decades. We obtain 7/8 subdiffusive power-law exponents for polystyrene depleted microtracers at low optical trapping forces. Microrheologically, the elastic modulus of these suspensions is observed to be constant up to the limit of high frequencies, confirming the origin of this subdiffusive exponent on the local longitudinal fluctuations of the polymers. Deviations from this value are measured and discussed in relation to the characteristic lengths scales of these F-actin networks and probes' properties, and also in connection with the different power-law exponents detected in the VAFs. Finally, we observe that the thin filament, composed by tropomyosin (Tm) and troponin (Tn) coupled to F-actin in the presence of Ca$^{2+}$, returns exponent values less dispersed than F-actin alone, which we interpret as a micro-measurement of the filament stabilization.