Some Electrostatic Problems From Biological Soft Matter

Biology is based on large molecules in a water environment. A result of this is that most biological molecules carry electric charges in the presence of water. For example, DNA has one of the strongest anionic linear charge densities of any polymers (linear macromolecules). This is related to the fact that it carries two ionizable phosphate groups per base pair (for double stranded DNA). In spite of this strong negative charge, trivalent and tetravalent cations can induce attractive forces between DNA molecules in solution. In this presentation, I will review our current understanding of this phenomenon in the context of strong electrostatic correlation effects. The fact that charges are quantized (charges occur in multiples of the electronic charge) is an important factor in mediating these interactions. The fact that salt is only soluble in water below some critical concentration (salting-out instability) provides a hint of the underlying physics. Hydrophobic objects are those which prefer not to be in contact with water. An application of the ideas that we have discussed for charged systems will allow us to understand the forces between hydrophobic surfaces.