We determined the concentration of folded hairpins by fluorescence spectroscopy. To do so, we compared the fluorescence intensity of our sample with fuel at 30 °C to two reference points, i.e., the fluorescence intensity of a sample where all the hairpins were folded and fluorescence intensity at 30 °C before the addition of fuel. We prepared the sample where all the hairpins were folded by the addition of 20 mM MgCl2 while the corresponding α at 30 °C is known from the melting curve of each H (Supporting Table ). From these two reference points, the α was calculated using the equation described in Supporting Notes 2.