17 Thermodynamic Considerations for Evolution by RNA

Abstract

The only definitive statement that thermodynamics makes about evolution is that ultimately it will result largely in CO₂, H₂O, and N₂. Fortunately, the sun allows us to live in a world that is kinetically controlled due to an input of energy. Nevertheless, since many equilibria are reached rapidly compared with the age of the earth, thermodynamic principles can be used to predict what reactions are possible in this transitory world. In this chapter, we argue that known thermodynamic properties of RNA suggest it is well adapted to play a central role in the evolution of information transfer and catalysis. This is because mononucleotides contain many functional groups and are therefore capable of strong, specific binding interactions. The evidence for the strength of the binding interactions comes largely from optical melting studies of oligonucleotides (Turner et al. 1988). Since evolution presumably started with rather short molecules, oligonucleotides are reasonable model systems for studying the interactions important in early times. In the following, we discuss the interactions thought to be important for organizing RNA, describe some implications for catalysis and information transfer, and speculate on implications for evolution.

DISCUSSION
Fundamental Interactions Determining RNA Structure
Molecular association and organization are essential for information transfer and catalysis. The thermodynamic measure of the strength of an association or the extent of organization is the change in free energy, ΔG°, since the equilibrium constant for either association or organization into a specific conformation relative to the random coil is given by K = e^−ΔG°/RT Here R...