  We derive and solve a differential equation satisfied by the probability
distribution of the work done on a single biomolecule in a mechanical unzipping
experiment. The unzipping is described as a thermally activated escape process
in an energy landscape. The Jarzynski equality is recovered as an identity,
independent of the pulling protocol. This approach allows one to evaluate
easily, by numerical integration, the work distribution, once a few parameters
of the energy landscape are known.
