I am trying to do some experiments, where my hairpin stem is 18 bp in length, but my loop itself is 125 nt long (something like the attached figure). I was wondering if it's possible at all for a DNA to self anneal and form a hairpin if the loop length is much larger than the stem lengh, like in this case. I am unable to see any hairpin formation - I am checking for hairpin formation by measuring the temperature-dependent UV absorbance during melting of a pre-formed hairpin.