Fig. 5.

Molecular models of the lumi (A) and meta I (B) intermediates of mouse UV based on the assumption that a counterion switch occurs during the lumi (E108 counterion) to meta I (E176 counterion) transformation. The chromophore, binding site residues, and water were allowed to seek minimum energy, but the protein backbone and outer residues were fixed. The electronic spectra of the two intermediates are simulated by using MNDO-PSDCI molecular orbital theory in the C and D, and the calculated absorption band maxima are indicated in nanometers. The dipole moments of the binding site residues, μ_bs, are given in the upper right of A and B and the dipole moment vector is shown by using red-to-blue arrows. The MNDO-PSDCI calculations included all 36 single and 666 double excitations within the π electron manifold of the chromophore. The heights of the vertical bars in C and D are proportional to the oscillator strengths of the π* ← π transitions, where solid blue bars indicate B_u⁺-like states, dashed blue bars indicate B_u^–-like states, solid red bars indicate A_g⁺-like states, and dashed red bars indicate A_g^–-like states. The simulated chromophore spectra were generated assuming Gaussian profiles with full-widths at half-maxima of 4,000 cm^–1.