Fig. 1.

(Upper) Reaction free-energy surface of L29W-S108L mutant MbCO at low temperatures with bound state A and photoproduct states B and C. (Inset) Double-well potential governing the CO dynamics in state C. Solid lines, sine-squared potential; dashed lines, harmonic oscillator potential with quantized energy levels. (Lower Left) Structure of L29W mutant MbCO (10) showing the heme group and side chains lining the Xe 4 cavity as well as the CO ligand in states A (bound to the heme), B (on top of the heme), and C (in the Xe 4 cavity). (Lower Right) Depiction of CO in the Xe 4 cavity with its IR transition dipole fluctuating in a cone around the direction of the electric field.