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)**