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X-ray diffraction studies on cation-collapsed DNA.

J A Schellman, N Parthasarathy

Journal of Molecular Biology

|May 25, 1984

Summary

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Polyamines like spermidine collapse DNA into a compact toroidal structure. The specific chemical structure of the collapsing agent, not ionic strength, dictates DNA arrangement in this ordered state.

Area of Science:

Biophysics
Structural Biology
Molecular Biology

Background:

Polyamines (spermidine, spermine, putrescine) induce tertiary DNA collapse into toroidal structures.
The precise structural details and stabilizing forces within collapsed DNA particles remain unclear.

Purpose of the Study:

To investigate how the chemical structure of collapsing agents influences DNA's tertiary conformation.
To elucidate the forces stabilizing the collapsed DNA state.

Main Methods:

Collapsing DNA using aliphatic triamines and diamines with varying methylene bridge lengths.
Analyzing collapsed DNA structures using Bragg spacing measurements.
Modeling DNA packing using a hexagonal lattice.

Main Results:

Related Experiment Videos

Bragg spacing and interhelical spacing systematically varied with methylene bridge length.
Ionic strength did not affect Bragg spacing, indicating polycation structure dictates arrangement.
A hexagonal array model for DNA helices with counterions in interstices was supported.

Conclusions:

The chemical structure of polyamines, not electrostatic forces, determines the ordered hexagonal arrangement of collapsed DNA.
A stable, highly ordered three-dimensional DNA phase is formed via polycation-induced hexagonal packing.