Proton coupled electron transfer in myoglobin compound II auto-reduction revealed by temperature dependent rate behavior.

Myoglobin Compound II auto-reduction has been hypothesized to be sequential proton transfer followed by an electron transfer rate-limited via the protonation of the ferryl oxo with a pKa ≤ 2.7 via various spectroscopic techniques or 4.7 via UV-Visible spectroscopy and kinetic studies. However, by exploring the subtle pH and temperature dependence of kobs, we have found the Compound II auto-reduction to be best modeled with a pre-equilibrium proton transfer that includes both the sequential proton electron transfer and concerted proton-coupled electron transfer, gated by the distal Histidine64 as a proton donor. Parameters from the temperature dependence studies allow an extension of the proposed proton-coupled electron transfer model to fit the kobs to over full temperature range of 20⎼50 °C.