  A theory is presented of DNA hairpins enclosed in a nanochannel. A hairpin
becomes constrained as it approaches the wall of a channel which leads to an
entropic force causing the hairpin to tighten. The free energy of the hairpin
computed in the classical limit is significantly larger than what one would
expect. As a result, the distance between hairpins or the global persistence
length is often tens of micrometers long and may even reach mm sizes for 10 nm
thin channels. The hairpin shape and size, and the DNA elongation are computed
for nanoslits, and circular and square nanoschannels. A comparison with
experiment is given.
