Hemeproteins Bathed in Ionic Liquids;Examining the Role of Water and Protons in Redox Behavior and Catalytic Function

Abstract

We investigate the changing behaviors of myoglobin and nitric oxide synthase (NOS) in the near-absence of bulk water and/or protons by using ionic liquid butyl methyl imidazolium tetrafluoroborate as a non-aqueous milieu. Through direct charge transfer and metalloprotein-mediated catalytic reduction of oxygen and nitric oxide, we shed light on diverging aspects on how the two hemeproteins face the scarcity of water and/or protons in bulk. Isotopic effect investigations using D2O further elucidates kinetic aspects of proton transfer. Finally, in the case of NOS oxygenase, pterin cofactor binding and NOS-mediated catalytic oxidation of L-arginine in ionic liquids interrogates proton and water availability as a modulating factor affecting electrochemically-driven production of nitric oxide. Overall, our results indicate that the catalytic and redox properties of NOS and other heme-proteins change in a unique way as a function of available water. The redox and catalytic behavior of each metalloprotein is rationalized in terms of its inherent structural aspect