Models of RNA folding in planetary environments

Multiple lines of evidence suggest that RNA performed all of the biological functions in the first life forms on earth. These functions included cleavage, ligation, polymerization, recognition, binding, and replication. In order to perform these functions, populations of RNA molecules with unevolved sequences must have been able to fold into compact three dimensional shapes, in unregulated environments, and without the help of proteins. Folding into compact tertiary structures is difficult because of the high charge density of RNA. Consequently, the ranges of temperature, salinity, pH, and pressure that allow RNA to fold into functional shapes is very restricted. We use thermodynamic arguments and Brownian dynamics simulations to compute the range of these environmental parameters that will allow RNA to fold. This is a non-trivial calculation due to the formation of an ion atmosphere around RNA that reduces its electric field. The results can be used to clarify the environments in which the transition to life is possible. Our preliminary calculations suggest that environments with low temperatures ($0-50\circ C$) and high salt concentrations (greater than 100mM) are the most favorable for unassisted RNA folding and thus the transition to RNA-based life. Applications of our results include determining the environments on early earth where life formed, assesing the habitability of Europa, Titan, and (using modeled parameters) extrasolar planets.