| Polymers, which are chained molecules with carbons as the backbone,
are the materials for engineering products and the building blocks of life.
We briefly describe our molecular dynamic simulation studies of randomly
charged polymers (polyampholyte), focusing on the structure formation by
the electrostatic forces, which model the folding of proteins and the DNA.
Charged polymers, sometimes called electrolyte polymers, are roughly
divided into two categories, polyelectrolyte, with monomers of the same
charge sign, and polyampholyte with randomly arranged monomers of both
signs. This simple category works well and represents the typical
characteristics of charged polymers. For example, the DNA is strongly and
negatively charged polyelectrolyte, with an electronic charge (-e) residing
in every nucleic acid of either A, G, C or T.
A Coulomb crystal (top) and amorphous
(bottom), showing inside polymer chains,
the positive and negative monomers in
the bird's-eye view and the crosscut, from
the left to right panels, respectively.
In water at room temperature, the electrostatic energy exceeds by
several times thermal energy. Polyelectrolyte takes a stretched state
due to repulsion among the monomers of like-sign charges, against
bending forces between monomers. On the other hand, polyampholyte
takes a loose globule because monomers of randomly sequenced
charges not only cancel average forces but positive and negative
monomers correlate to form pairs. Thus, we can control the softness
of charged polymers by changing temperature, pH and salt density.
The biochemical processes might be making good use of these properties.
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