Structural determinants for receptor subtype specificity. Top: Two non-conserved residues provide structural diversity to the ligand binding site (left: alpha4-beta2, center: alpha4-beta4, right: alpha7-alpha7). The shape difference of the Van der Waals boundary is shown at residue 183 of subunit 1 (gold), and 135 of subunit 2 (cyan), next to docked nicotine (magenta). Bottom: Electrostatic potential coloring shows a rather limited variation in charge distribution between the alpha4-beta2 dimer (left) and the alpha7-alpha7 dimer (right). Charge color coding is red: negative, gray: hydrophobic, blue: positive.