Physicochemical Properties of N4 Virus Solutions

The effect of formaldehyde and inorganic salts such as NaCl, NaBr, NaClO4, LiBr, LiClO4, and MgCl2 on the stability of N4 virus and of the DNA-free protein shell has been investigated by viscosity, sedimentation, infectivity, optical density, and electron microscopy measurements. Calorimetric data for the process of ejection, from the viral particle, of native DNA in solution are also presented. Inorganic salts generally increase the temperature for DNA ejection while they may decrease (NaClO4, LiBr, LiClO4) or increase (NaCl) the temperature of degradation of the protein shell. These effects, when interpreted in terms of polyelectrolyte-salt and isoelectric protein-salt interactions, suggest mechanisms responsible for virus stabilization and DNA ejection. The transformation: folded DNA inside the virus → double helical DNA in solution is an exothermic one and the heat effect is 25 Cal per mole of bases of DNA. In the presence of formaldehyde, the processes of DNA ejection and protein degradation can no longer be separated. Formaldehyde produces irreversible inhibition of DNA ejection and stabilizes the protein shell structure, apparently by a cross-linking mechanism.