  We establish a framework for assessing whether the transition state location
of a biopolymer, which can be inferred from single molecule pulling
experiments, corresponds to the ensemble of structures that have equal
probability of reaching either the folded or unfolded states (Pfold = 0.5).
Using results for the forced-unfolding of a RNA hairpin, an exactly soluble
model and an analytic theory, we show that Pfold is solely determined by s, an
experimentally measurable molecular tensegrity parameter, which is a ratio of
the tensile force and a compaction force that stabilizes the folded state.
Applications to folding landscapes of DNA hairpins and leucine zipper with two
barriers provide a structural interpretation of single molecule experimental
data. Our theory can be used to assess whether molecular extension is a good
reaction coordinate using measured free energy profiles.
