Isotope Effect in Molecular Tunneling

The intramolecular binding of carbon monoxide ($^{13}\mathrm{C}$$^{16}\mathrm{O}$/$^{12}\mathrm{C}$$^{16}\mathrm{O}$ and $^{12}\mathrm{C}$$^{18}\mathrm{O}$/$^{12}\mathrm{C}$$^{16}\mathrm{O}$ mixtures) to myoglobin at 20 and 60 K is observed with time-resolved Fourier-transform infrared spectroscopy. The binding rates (at \ensuremath{\approx} 1 ks) for the molecules $^{12}\mathrm{C}$$^{16}\mathrm{O}$, $^{12}\mathrm{C}$$^{18}\mathrm{O}$, and $^{13}\mathrm{C}$$^{16}\mathrm{O}$ are in the ratio 1:0.84:0.65 at 20 K, 1:0.87:0.84 at 60 K. The large isotope effect confirms earlier evidence for molecular tunneling. The fact that $^{12}\mathrm{C}$$^{18}\mathrm{O}$ binds faster than $^{13}\mathrm{C}$$^{16}\mathrm{O}$ implies that structure effects are significant in molecular tunneling.