Molecular tunneling in heme proteins

At low temperatures the rebinding of carbon monoxide to heme proteins, after photodissociation by a laser flash, occurs via quantum‐mechanical tunneling. Quantum‐mechanical tunneling and the classical over‐the‐barrier process, which dominates at higher temperatures, are both described by an energy spectrum. The barrier widths can be determined from the energy dependence of the tunneling rates. Rebinding of carbon monoxide to the beta chain of hemoglobin is treated in detail, and we find that in this case the barrier width d(E) depends on the barrier height and is given by d(E) = 0.05 nm ×(E/Epeak)1.5, with Epeak = 4.0 kj/mole, Epeak being the peak of the energy spectrum. Data on rebinding of carbon monoxide to the octapeptide formed by proteolytic digestion of cytochrome c are presented. In this system an additional fast component to the rebinding process is observed. The absorption spectrum and the approximately temperature independent behavior of this fast component suggest quantum‐mechanical tunneling from an excited state.