Before revision:  For ribozymes, divalent cations are known to be more efficient than monovalent ions in driving them to a compact state although often Mg^{2+} ions are needed for catalytic activity .
After revision:  For ribozymes, divalent cations are known to be more efficient than monovalent ions in driving them to a compact state although often Mg^{2+} ions are needed for catalytic activities .