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Polymers and Self Assembly: From Biology to Nanomaterials Poster Session II
27-POS
Board 27
Viscoelastic Properties of DNA in Vivo Measured by Fluorescence Correlation
Spectroscopy
Rudra Kafle, Liebeskind Molly,
Jens-christian Meiners
.
University of Michigan, Ann Arbor, USA.
Mechanical quantities like the elasticity of cells are conventionally measured by directly probing
them mechanically. This, however, is often impractical, and even impossible when subcellular
structures inside living cells are concerned. We use a purely optical method instead; an
adaptation of fluorescence correlation spectroscopy (FCS) to measure such mechanical quantities
in chromosomal DNA in live E. Coli cells. FCS is a fluorescence technique conventionally used
to study the kinetics of fluorescent molecules in a dilute solution. Being a non-invasive
technique, it is now drawing increasing interest for the study of more complex systems like the
dynamics of DNA or proteins in living cells and cell membranes. Unlike an ordinary dye
solution, the dynamics of macromolecules like proteins or entangled DNA in crowded
environments is often slow and subdiffusive in nature. This in turn leads to longer residence
times of the attached fluorophores in the excitation volume of the microscope and artifacts from
photobleaching abound that can easily obscure the signature of the molecular dynamics of
interest. We present a method of calculating the intensity autocorrelation function from the
arrival times of the photons on the detector that maximizes the information content while
correcting for the effect of photobleaching to yield an autocorrelation function that reflects only
the underlying dynamics of the sample. This autocorrelation function in turn is used to calculate
the mean square displacement of the fluorophores attached to DNA. By using a suitable integral
transform of the mean square displacement, we determine the viscoelastic moduli for lambda-
DNA solution and the DNA in E. coli. We note that these mechanical quantities evaluated by an
optical technique are in good agreement with similar quantities measured by completely different
techniques.