Electrostatic potential maps of gp120 and the gp120-CD4 complex. Electrostatic potentials are shown for the solvent accessible surfaces. (A) The electrostatic potential map on gp120. Blue indicates electropositive areas whereas red represents electronegative areas. (B) After CD4 binds to gp120, the electropositive surface area (blue) increases markedly while the most negatively charged (red) area on gp120 (arrow) becomes blocked by sCD4.