Structure of viral φ29 DNA condensed by simple triamines: A light‐scattering and electron‐microscopy study

Abstract

Abstract The structures of viral φ29 DNA condensed by triamines, principally spermidine, in 10 −3 M NaCl were investigated by static and dynamic light scattering and electron microscopy. All of the results for DNA condensed in 30 μ M spermidine at neutral pH are quantitatively consistent with a toroidal structure with a mean outer diameter of 1850 Å. At pH 10.2, however, condensed structures of a completely different size and shape are observed for the first time. These structures are also more irregular in shape and more polydisperse than those at neutral pH. This conformational change is believed to result from a change in the mode of spermidine binding that is coupled to, or associated with, the (premature) titration of protons on the base‐ring nitrogens of guanine and thymine. Besides spermidine, certain homologs of spermidine, in which the butyl moiety of spermidine was replaced by longer pentyl through octyl moieties, were also studied. Though all of the triamines condensed the DNA at 30 μ M , aggregation became a more prevalent occurrence as the length of the end chain increased. This suggests that crosslinking may play an important role in the condensation process. Finally, these aggregates are dissociated to a considerable extent at pH 10.2, and the resulting compact structures appear to be quite similar, independent of the triamine used to condense the DNA. The observed partial breakdown of aggregates is also consistent with the hypothesis of a change in mode of triamine binding at pH 10.2.