Stopped-flow fluorescence traces of the A1095AP U-turn hairpin construct. Black trace is control addition of buffer only. Adding MgCl2 to 3, 5, 8, 20, 80,100 mM results in an immediate increase in fluorescence intensity but no subsequent transitions. Experiments with other hairpins showed similar rapid intensity changes. [RNA] = 100 nM in 100 mM KCl, 10 mM sodium cacodylate pH 6.5. 20° C.