Soft Matter Seminar: Understanding the Forces Driving DNA Condensation: From Simple Ions To Complex Proteins
Friday, February 19, 2010 - 3:00pm - 4:15pm
Understanding the strength and specificity of interactions among biologically important macromolecules that control cellular functions requires a quantitative knowledge of intermolecular forces. As most in vivo DNA exists in a highly condensed state, controlled DNA condensation and assembly are particularly critical for biology with separate repulsive and attractive forces determining the extent of DNA compaction. How these forces depend on the charge and the nature of the condensing ion is unknown, but is fundamental for understanding the physics of DNA-DNA interactions. We have directly measured these thermodynamic forces using osmotic stress coupled with x-ray scattering. In this talk, I will discuss the dependence of the repulsive and attractive forces on cation length, or equivalently charge, for three separate homologous cationic species. Force-distance curves for all condensing agents are well described by a two exponential fit with 2.4 and 4.8 Å decay lengths demonstrating the universality of these forces for DNA assembly. The repulsive short range 2.4 Å decay length force amplitudes are found to be species dependent but nearly independent of charge within each species. Strikingly, the attractive force amplitudes for all samples, including the heterogeneous arginine-rich protein protamine, collapse to a single curve varying linearly with 1/N, where N is the cation charge. The second half of my talk will focus on model hexaarginine peptides incorporating uncharged amino acids to investigate the role of amino acid chemistry, position and length to the forces in the heterogeneous protein protamine that replaces histones in late-stage spermatogenesis. Better understanding the cation mediated attractions and repulsions helps us to elucidate the interplay between ion entropy and the correlations that are common to nearly all theories for counter-ion induced attractions. Recent attempts to extend our work from in vitro to in vivo systems such as spermatozoa nuclei will be briefly discussed.
Host: Daniel Blair