  We discuss the use of a structure based C$\alpha$-Go model and Langevin
dynamics to study in detail the mechanical properties and unfolding pathway of
the titin I27 domain. We show that a simple Go-model does detect correctly the
origin of the mechanical stability of this domain. The unfolding free energy
landscape parameters $x_u$ and $\Delta G^{\ddagger}$, extracted from
dependencies of unfolding forces on pulling speeds, are found to agree
reasonably well with experiments. We predict that above $v=10^4$ nm/s the
additional force-induced intermediate state is populated at an end-to-end
extension of about $75 \mathring{A}$. The force-induced switch in the unfolding
pathway occurs at the critical pulling speed $v_{crit} \approx 10^6-10^7$ nm/s.
We argue that this critical pulling speed is an upper limit of the interval
where Bell's theory works. However, our results suggest that the Go-model fails
to reproduce the experimentally observed mechanical unfolding pathway properly,
yielding an incomplete picture of the free energy landscape. Surprisingly, the
experimentally observed intermediate state with the A strand detached is not
populated in Go-model simulations over a wide range of pulling speeds. The
discrepancy between simulation and experiment is clearly seen from the early
stage of the unfolding process which shows the limitation of the Go model in
reproducing unfolding pathways and deciphering the complete picture of the free
energy landscape.
