Samples were loaded into borosilicate glass capillaries with internal section of 4 × 0.2 mm 2 (CM Scientific). Capillaries were cleaned by first sonicating in 2% Hellmanex III water solution (HellmaAnalytics) at 90 • C for 15 minutes. Surfactant was removed by two rounds of thorough rinsing and sonication in ultra-pure water (Milli-Q). Prior to filling capillaries, oligonucleotide mixtures prepared in Eppendorf tubes were heated to 95 • C using a heating block to ensure complete denaturation of any aggregates formed upon initial mixing. After loading into capillaries, a small amount of sample was removed and either side was capped off with mineral oil before sealing permanently onto a microscope slide with epoxy glue (Araldite). Sealed capillaries were then loaded into a fully programmable, Peltier controlled, water bath and heated to 95 • C for 30 min, before cooling to 20 • C at -0.01 • C min -1 .