Measurement of viscoelastic properties of the cellular cytoplasm using optically trapped Brownian probes

Rahul Vaippully, Vaibavi Ramanujan, Saumendra Bajpai and Basudev Roy

Calibration of optically trapped particles in-vivo has been complicated given the frequency depen-dence and spatial inhomogeneity of the cytoplasmic viscosity, and the requirement of accurateknowledge of the medium refractive index. Further, it has been demonstrated that the mediumviscosity is dependent upon the measurement probe leading to reliability issues for measurementswith even micrometer sized particles. Here, we employ a recent extension of Jeffery's model ofviscoelasticity in the microscopic domain to fit the passive motional power spectra of micrometer-sized optically trapped particles embedded in a viscoelastic medium. We find excellent agreementbetween the 0 Hz viscosity in MCF7 cells and the typical values of viscosity in literature, between2 to 16 mPa sec expected for the typical concentration of proteins inside the cytoplasmic solvent.This bypasses the dependence on probe size by relying upon small thermal displacements. Ourmeasurements of the relaxation time also match values reported with magnetic tweezers, at about0.1 sec. Finally, we calibrate the optical tweezers and demonstrate the efficacy of the techniqueto the study of in-vivo translational motion.

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