FIGURE 2.

Quantum chemistry of iodo-aromatic system. A, Tyr side chain with iodine in its 3- or 5-ring position (green and yellow, in and out, respectively) with carbon-atom positions labeled. Rotation angles for rigid-body modeling involve rotations around the C_α–C_β bond (χ₁) and C_β–C_γ bond (χ₂). The insulin dimer with B26 side chains (red) and side chains of Tyr^B16, Phe^B24, Phe^B25, and their dimer-related mates (blue). The right-hand side provides a cross-eyed stereo view of the modified dimer with the B chain helix removed for clarity. B, electrostatic potential (ESP) surface maps of phenol, iodophenol, and iodophenyl at the 0.001 e bohr⁻³ isodensity. The color scale of the surface potential ranges from −2.12 e⁻² (red) through 0 (green) to 2.12 e⁻² (blue). In the upper row the iodine (facing the viewer) exhibits the effect of the electron-donating –OH on the σ-hole. The lower row shows effects of iodine on the π-system of the phenol ring. In the 1st row the surface is opaque, and in the 2nd row the surface is transparent. Angle β represents the σ-hole size as delimited by black dashed lines. δ⁺ and δ⁻ represent respective regions of positive and negative charge around the iodine. C, ESP contours of iodophenyl (left) and 2-iodophenol (right), at different isovalues, calculated in the plane of the aromatic ring. The halogen boundary represents a region of an electron isodensity of 10⁻³ e bohr⁻³ (111). Isocontours in the left and right panels are at the same heights but in uneven separations. The σ-hole size, defined by an angle β (B), was calculated from the angular profile of the ESP on the intersection line of the 2D grid and halogen boundary where the ESP changes its sign (55); positive ESP and negative ESP regions are shown in blue and red, respectively. The black dashed arrow indicates directionality of the C–I bond.