  A model of self-avoiding walk with suitable constraints on self-attraction is
developed to describe the conformational behavior of a short RNA or a single
stranded DNA molecule that forms hairpin structure and calculate the properties
associated with coil-hairpin transition by enumerating all possible
conformations of a chain of N monomers in two and three dimensions. The first
and last five monomers of the chain have been allowed to pair and form the stem
of the hairpin structure while the remaining monomers can form a loop. The
coil-hairpin transition is found to be first order with large entropy change.
While the rate of unzipping of the hairpin stem is found to be independent of
the length of the loop and the dimensionality of the space, the rate of closing
varies greatly with loop length and dimensionality of the space.
