  The WW domain of the human Pin1 protein for its simple topology and the large
amount of experimental data is an ideal candidate to assess theoretical
approaches to protein folding. The purpose of the present work is to compare
the reliability of the chemically-based Sorenson/Head-Gordon (SHG) model and a
standard native centric model in reproducing through molecular dynamics
simulations some of the well known features of the folding transition of this
small domain. Our results show that the G\={o} model correctly reproduces the
cooperative, two-state, folding mechanism of the WW-domain, while the SHG model
predicts a transition occurring in two stages: a collapse followed by a
structural rearrangement. The lack of a cooperative folding in the SHG
simulations appears to be related to the non-funnel shape of the energy
landscape featuring a partitioning of the native valley in sub-basins
corresponding to different chain chiralities. However the SHG approach remains
more reliable in estimating the $\Phi$-values with respect to G\={o}-like
description. This may suggest that the WW-domain folding process is stirred by
energetic and topological factors as well, and it highlights the better
suitability of chemically-based models in simulating mutations.
