At the distance of closest approach ($r_0$), the initial kinetic energy ($K$) of the projectile is completely converted into electrostatic potential energy ($U$). The formula for potential energy between two charges $q_1$ ($Z_1e$) and $q_2$ ($Z_2e$) separated by a distance $r_0$ is: $U = \frac{1}{4\pi\epsilon_0} \frac{q_1 q_2}{r_0} = \frac{1}{4\pi\epsilon_0} \frac{(Z_1e)(Z_2e)}{r_0}$. Since $K = U$, the energy of the projectile is proportional to the product of the charges ($Z_1 Z_2$). Therefore, $Energy \propto Z_1 Z_2$.