DNA collapse

Most trivalent metal cations would be anticipated to cause DNA collapse (aggregation) into condensed toroidal structures that are considered to be a model for DNA packaging in viruses and chromosomes. [Co(NH3)6] + induces this effect at five times lower concentration than spermidine, [+H3N-(CH2)4-NH3+-(CH2)3-NH3+], whcrcas divalent metal ions and putrescine, [+H3N-(CH2)4-NH3+], are incapable of causing DNA condensation in aqueous solution. 7302 polyvalent metal compounds, including Ap+ ions, also can relax supercoiled DNA, probably through single-strand DNA cleavage (see Section 7). 

In the case of DNA collapse in a solution of neutral polymer incompatible with DNA molecules, two main effects have to be taken into account. The first effect is the osmotic pressure of the counterions on the DNA coil. The second effect is connected with the difference in polymer concentration inside and outside the DNA coil. It was found that the concentration of neutral polymer within the effective volume of DNA is always lower than that in external solution. This difference could be negligible in the case of good compatibility observed at low polymer concentration. In this case the polymer molecules practically freely penetrate inside the DNA coil, so that the water/polymer composition inside the DNA coil is the same as the composition in the external solution. The second regime is the regime of practically perfect segregation between the DNA chain and the polymer. The polymer segregates from the DNA coil and imposes additional osmotic pressure,... 

Fluorescence microscopy allows us also to get the data for the dependence of the width of the transition region (i.e., the region of coexistence of the coil and globule states) on the collapsed agent. For the case of multivalent ions, our calculations predict that the width of the transition region is the narrowest one for the multivalent cations of highest valence. The experimental results obtained for the case of DNA collapse in the solutions of polyamine of different valences are in good correspondence with theoretical results. 

Two natural collapse processes are the folding of proteins into their compact native states in water, and the compaction of DNA molecules for insertion into virus heads and cell nuclei. While this homopolymer collapse model illustrates the principle of coil-to-globule transitions, neither protein folding nor DNA collapse follow it exactly, because both polymers also have electrostatic interactions and specific monomer sequences. 

Stoichiometric [40], whereas for single-stranded DNA it is not [41]. Studies on spermine and lipospermine binding to DNA were conducted and reported [42] to assess the importance of contribution of hydrophobic effects on DNA collapse. While ITC measurements showed a larger DNA binding affinity for the hydro-phobic lipospermine than spermine, electron micrographs showed that lipospermine was incapable of condensing DNA into toroidal structures. 

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