\[ \begin{align} p&=\frac{W_{bound}}{W_{unbound}+W_{bound}} &=\frac{ \frac{P}{N_{NS}} e^{\frac{\epsilon_S - \epsilon_{NS}}{\kappa_BT} } }{1+\frac{P}{N_{NS}} e^{\frac{\epsilon_S - \epsilon_{NS}}{\kappa_BT} } suppose that \frac{P}{N_{NS}} e^{\frac{\epsilon_S - \epsilon_{NS}}{\kappa_BT} } \ll 1 &\approx \frac{P}{N_{NS}} e^{\frac{\epsilon_S - \epsilon_{NS}}{\kappa_BT} } &\propto e^{\frac{\epsilon_S - \epsilon_{NS}}{\kappa_BT} } \end{align} \] \(x=x+1\)