|M.Sc Student||Konorty Marina|
|Subject||Microstructural Aspects of Complexetion between DNA and|
Positively Charged Colloids
|Department||Department of Chemical Engineering||Supervisor||Professor Emeritus Yeshayahu Talmon|
The electrostatic interaction between DNA and cations, or positively charged particles, has been studied in relation to DNA condensation, DNA packaging, and during recent years, in relation to molecular electronics. Despite the extensive effort put into this subject, there is still no good microscopic understanding of DNA-colloid complexation.
In this work we have used direct-imaging cryo-transmission electron microscopy (cryo-TEM), coupled with dynamic light scattering and optical density measurements to study the complexes formed between stained or unstained DNA and positively charged nanoparticles. We showed that unstained DNA in its uncondensed form, can be clearly imaged by cryo-TEM. We also investigated the influence of uranyl acetate, tetrachloro platinum complex (K2PtCl4), and the protein RecA, on the observed DNA structure in solution.
We studied complexes formed between lambda DNA and two kinds of positively charged gold nanoparticles: 16 nm diameter nanoparticles (G16) with an estimated charge of a few thousands, and 1.4 nm diameter nanoparticles with an average charge of +4 (G1.4). Our results show both DNA-G16 and, DNA-G1.4 systems to exhibit a similar phase diagram.
Cryo TEM of the G1.4-DNA system, at relatively high nanoparticles/DNA concentration (“phase 1”) reveled extended DNA fibers with homogeneous coverage of G1.4 nanoparticles that occasionally coiled up to form toroid resemble complexes. We found our experimental phase diagram of the G1.4-DNA system, to coincide well with Manning’s theoretical phase diagram.
Comparison of the result for the DNA-G16 and, DNA-G1.4 systems to DNA-small counterion systems shows similar behavior in the sense of stability, and in the nature of phase 1, forming well-defined complexes. Thus the surprising extent of similarity between the two investigated DNA-collodial systems and DNA-small counterion systems may suggest the generality of the DNA-counerparticle system.