No abstract

A laser tweezer (LT) along with advanced imaging techniques has been widely applied to manipulate and study living as well as nonliving microscopic objects. In this study we present yet another novel application of LTs for a precise measurement of the viscosities of fluids in a micro-volume flow. We have demonstrated this novel application by measuring the viscosity of a fetal bovine serum (FBS) using a LT constructed from a single intensity gradient laser trap. By calibrating the LT using dielectric silica micro-beads in a fluid with a known viscosity, specifically water, and by suspending same size of silica beads in the FBS and trapping with the same trap, we have determined the viscosity of the FBS at different temperatures. We have used the relationship between the trapping and Stoke's drag force for a constant drag speed to determine the viscosity. We have also analyzed the viscosities determined in comparison with corresponding viscosities measured using an Ostwald viscometer.

Hydrogen bonds play an important role in RNA structure and dynamics. Fluctuations in base pair openings could be the starting point of unfolding processes or could indicate potential docking sites for ligands or proteins. The effects of hydrogen bond formation and breaking kinetics in RNA base pairs on the linear and coherent third order infrared spectra of small UUCG tetraloops in solution can be described by Markovian, not necessarily Gaussian, fluctuations. We have simulated these spectra using the stochastic Liouville equations. Slow fluctuations are described phenomenologically. Fast fluctuations are characterized by an N-state jump model for hydrogen bond configurations, where N depends on the specific tetraloop. Bases in the RNA strands that exhibit high levels of fluctuation are isotope labeled and the chemical exchange 2DIR spectra are calculated. The existence and evolution of the resultant cross peaks at different waiting times provides information on the coupling interactions between base pairs in the loops, which will be used to help characterize unfolding mechanisms of the RNA strands.