![]() ![]() Cations added to the solution form a "cloud" of positive charges around the DNA. Each strand of DNA has an enormous charge density (charge per unit volume), so the two strands tend to push each other apart. Q: Why does NaCl increase the stability of DNA duplexes, although you might expect salts to interfere with hydrogen bonds, rather than strengthen them?Ī: The Na+ neutralizes the charge. As e goes down, F goes up and *BANG*, anion and cation form an ion pair and promptly swoon out of solution. Adding organic solvent *decreases* the dielectric constant of the solution. ![]() Where Q is the charge on each ion and r is the distance between them. ![]() Water has a high dielectric constant (e), which from Coulomb's Law tells us that the electrostatic force (F) between two ions of opposite charge is very low in water: In dilute aqueous solution, DNA and counterions like Na+ and Mg++ are more or less in the free ion form rather than the ion pair form (that is, they are surrounded by one or more layers of water molecules). For a precipitation, you're interested in forming ion pairs between the polyanion (DNA) and the cation (Na+, Mg++, spermidine, protamine, etc). Q: What is the role of ethanol in precipitating DNA from aqueous solutions?Ī: The purpose of adding salts is to neutralize the charge on the sugar-phosphate backbone of the DNA, but ethanol's task is a little more complex than "removing" the water. A thorough discussion was presented by Scott Keeney, of the Biochemistry and Molecular Biology Department, University of California Berkeley I have shortened the opening questions and extracted his answer from the newsgroup and present them below for your convenience. The anions and cations form ion pairs and this results in precipitation of DNA.Ī discussion on this topic was posted to the "-reagnts" bulletin board in July 1992. Adding ethanol decreases the dielectric constant of the solution. The DNA and counterions are more or less in the free ion form rather than the ion pair form in dilute aqueous solution, and are surrounded by one or more layers of water molecules. ![]() For a precipitation, you need to form ion pairs between the polyanion (DNA) and the cation (usually sodium or ammonium). Where it facilitates interactions between antithrombin (an inhibitor of bloodĬoagulation) and thrombin (a clot-forming protein).The purpose of adding salts is to neutralize the charge on the sugar-phosphate backbone of the DNA. Polysaccharide that is heteropolysaccharide with anti-clotting properties. This is because the glucose residue at the center of the glycogen 'spiral' is covalently linked to a protein called glycogenin (see adjacent figure). Amylopectin contains one free glucose at the reducing end of the 'tree branch' whereas glycogen lacks a free reducing end. Another difference is between the macromolecular structures of both. The more branch points, the more ends are available for glucose retrieval and storage. The glucose units can be added or removed only from the non-reducing ends of amylopectin and glycogen. branch point is after every 6-10 residues. In glycogen, the branching occurs more frequently i.e. There are certain differences between starch (amylopectin, more specifically) and glycogen. ![]()
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