Prof. Friedrich Kremer
Institute for Experimental Physics, Uni-Leipzig
Date: Friday, April 18
Time: 15:00
Room: 225
Title & Abstract:
"Experiments with single polymers and with single colloids"
Optical tweezers are experimental tools with extraordinary resolution in
positioning (± 1 nm) a micron-sized colloid and in the measurement of
forces (±50 fN) acting on it. This enables one to carry out experiments
on single polymer (DNA) -- chains and with single colloids: (i) The
force-extension dependence of DNA chains of different length (2000 base
pairs (bp), to 6000 bp) in media of varying ionic concentration is
measured and compared with the worm-like-chain model. (ii) The action of
the DNA-binding protein TmHU (from Thermotoga maritima) on a single
chain of DNA is studied. It is shown that the condensation of DNA takes
place in discrete steps of 4 -- 5 nm length having a net binding
enthalpy of about 16 k_B T (k_B : Boltzmann constant). This is in
reasonable agreement with a microscopic model that estimates the
extension of the binding sites of the protein and evaluates the
energetics mainly for bending the DNA in course of interaction. (iii)
Experiments are reported in which the forces of interaction between two
identical DNA-grafted colloids are measured on approach between each
other. Parameters to be varied are the grafting density of the
DNA-brushes (0,09 - 0,032 chains/nm^2 ), the lengths of the
double-stranded DNA-chains (1000 bp, 4000 bp and 6000 bp), the
concentration and the valence of the ions in the surrounding medium.
Further experiments are carried out to measure under similar conditions
the interaction between one DNA-grafted and one blank colloid. The
results are discussed with respect to the different brush regimes. (iv)
Measurement of the flow resistance of a single colloid grafted with DNA
of lengths between 1000 bp to 6000 bp. The experiments can be described
by a modified Stokes law with an effective hydrodynamic radius being
dependent on the flow velocity, the length of the chains, and the
grafting-density.
References:
Salomo, M., K. Kegler, M. Struhalla, J. Reinmuth, W. Skokow, U. Hahn and
F. Kremer;"The elastic properties of single double-stranded DNA chains
of different length",J. of Colloid & Polymer Science Vol. 284, No.11,
pp. 1325-1331 (2006)
Gutsche, C., M. Salomo, Y.W. Kim, R.R. Netz and F. Kremer;"The flow
resistance of single DNA-grafted colloids as measured by Optical
Tweezers",J. Microfluidics and Nanofluidics, ISSN:1613-4982 (Paper)
1613-4990 (Online),(2006)
Salomo, M., C. Gutsche, K. Kegler, M. Struhalla, K. Kroy, J. Reinmuth,
W. Skokow, C. Immisch and F .Kremer; "The binding of TmHU to single
ds-DNA as observed by Oprtical Tweezers", J. Mol. Biol. 359, 769-776 (2006)
K.Kegler, M.Salomo and F.Kremer; "Forces of interaction between
DNA-grafted colloids: An Optical Tweezer Measurement" , Phys.Rev.Lett.
98,058304 (2007)
Salomo M., U. Keyser, K. Kegler, M. Struhalla, C. Immich, U. Hahn and F.
Kremer; "Kinetics of TmHU binding to DNA as observed by optical
tweezers", Microscopy Research & Technique 70, No. 11, 938-943 (2007)
C.Gutsche, U.F.Keyser, K.Kegler, F.Kremer and P.Linse; "Forces between
single pairs of charged colloids in aqueous salt solutions", Phys. Rev.
E 76, 031403 (2007)
Kegler, K.; M. Konieczny, G. Dominguez-Espinosa, C. Gutsche, M. Salomo,
F. Kremer and C. N. Likos
"Polyelectrolyte-Compression Forces between Spherical DNA Brushes" Phys.
Rev. Lett. 100, 118302 (2008)**