  This paper compares simulation results and experimental published data
concerning the interaction between $\alpha_2\beta_1$ integrin and collagen.This
phenomenon is characterized by a competition among steric, reciprocal
orientation constraints of the two structures and long range electrostatic
force. The published experimental results shown that the
$\textrm{k}_{\textrm{on}}$ rate, for the 42-mer collagen fragment and the
$\alpha_2\textrm{I}$ integrin domain complex, ranges in order of magnitude from
$10^4-10^5$ M$^{-1}$s$^{-1}$. This is the lower bound of the interval defined
by the diffusion limited regime and the orientational constraint. The
electrostatic affinity between 21-mer collagen fragment and the
$\alpha_2\textrm{I}$ integrin domain due to the divalent cations
($\textrm{Mg}^{2+}$, $\textrm{Co}^{2+}$, $\textrm{Mn}^{2+}$ and
$\textrm{Ca}^{2+}$) has been expressed in terms of the Pearson correlation
coefficient between the electrostatic desolvation potential of the ligand and
the electrostatic interaction potential of the receptor. The obtained values of
the Pearson coefficient clearly reveals the complementarity between the
$\alpha_2\textrm{I}$ integrin domain and 21-mer collagen in the metal ion
dependent site. Atomistic Brownian dynamics simulations applied in the
diffusion limited regime, correctly reproduce the binding kinetics, of the
$\alpha_2\textrm{I}$ integrin domain and the 21-mer collagen in presence of
$\textrm{Mg}^{2+}$, $\textrm{Co}^{2+}$ and $\textrm{Mn}^{2+}$, according to
published experimental data.
