12 The Role of Metal Ions in RNA Biochemistry

Approximately two-thirds of the elements in the periodic table can be cate-gorized as metals. Besides luster, malleability, and conductivity, one of the fundamental characteristics of metals is their low ionization potential. As a result, the ionic forms of these elements predominate in the bio-sphere. Considering the diverse properties of these ions, it is not surpris-ing that through the process of evolution, metal ions have been co-opted into numerous roles in biology. Metal ions are required for so many bio-chemical reactions that it is likely that they also had an important role in the RNA world. To understand both modern and prebiotic RNA bio-chemistry, it is therefore essential to have a basic understanding of these inorganic elements. Metal ions were abundant in the primordial soup. It is believed that 3.8 × 109 years ago, the ocean was between 80˚C and 100˚C with a pH possibly as low as 6 (Bengston 1994). Table 1 shows the concentrations of the most common metal ions in today’s seas and in blood plasma. Although the concentrations of most of these ions in the prebiotic ocean are not known, the higher temperature and lower pH relative to the cur-rent ocean would have solvated a variety of ions and leached metal ions from the mineral-rich ocean beds. Therefore, the concentrations would have been significantly higher than the current values. One important additional difference is the extremely low concentration of easily oxi-dized metal ions such as Fe(II). Ferrous ion has been predicted to have been very abundant in the primordial ocean, as high as 0.1 mM (Bengston 1994). Interestingly, this change is probably due to the fact that early life forms produced molecular oxygen from photosynthetic reactions that in turn caused a cascade of oxidation reactions to occur. The conversion of Fe(II) to the more acidic Fe(III) resulted in the formation of oxyhydrox-ide polymers that precipitated out of the ancient oceans, depleting the ion