Direct electrochemistry and environmental sensing of rice hemoglobin immobilized at graphite electrodes

Non-symbiotic hemoglobin from rice (Oryza sativa L.), OsHb-1, with hexacoordinated rather than pentacoordinated heme and high affinity for oxygen, may have a particular role in O2 and environmental sensing. Here, a 21 kDa monomer OsHb-1 was electrochemically studied at graphite electrodes and further probed in analysis of environmental species such as hydrogen peroxide, cyanide, and superoxide. Redox potential of the OsHb-1 heme iron was found to be −136 mV vs. SCE, at pH 6.5, while the rate constant ks for the heterogeneous electron transfer (ET) between graphite and OsHb-1 immobilized in the Nafion membrane at the carbon nanotubes-modified electrodes was below 0.2 s−1. Despite sluggish ET, OsHb-1 efficiently, with current densities exceeding 2 mA cm−2 at −0.3 V, electrocatalyzed reduction of O2 starting from the potentials of OsHb-1 heme. The bioelectrocatalytic reduction of O2 was partially inhibited by CN− thus enabling its sensing down to a 100 pM level. Peroxidase-like activity of OsHb-1 and the reaction of the superoxide anion with the heme iron of OsHb-1, in de-oxygenated solutions, were studied and analysed in terms of OsHb-1 reactivity. The results obtained indicate OsHb-1 is a sensitive tool for environmental biosensing and toxicity screening.