Direct observation of ligand transfer and bond formation in cytochrome c oxidase by using mid-infrared chirped-pulse upconversion

We have implemented the recently demonstrated technique of chirped-pulse upconversion of midinfrared femtosecond pulses into the visible in a visible pump-midinfrared probe experiment for high-resolution, high-sensitivity measurements over a broad spectral range. We have succeeded in time-resolving the CO ligand transfer process from the heme Fe to the neighboring Cu _B atom in the bimetallic active site of mammalian cytochrome c oxidase, which was known to proceed in <1 ps, using the full CO vibrational signature of Fe-CO bond breaking and Cu_B-CO bond formation. Our differential transmission results show a delayed onset of the appearance of the Cu_B-bound species (200 fs), followed by a 450-fs exponential rise. Trajectories calculated by using molecular-dynamics simulations with a Morse potential for the Cu_B-C interaction display a similar behavior. Both experimental and calculated data strongly suggest a ballistic contribution to the transfer process.