Direct Electrochemistry and Environmental Sensing of Rice Hemoglobin Immobilized at Graphite Electrodes

Abstract

Non-symbiotic hemoglobin from rice (Oryza sativa L.), OsHb-1, with its rather hexacoordinated than pentacoordinated heme and high affinity for oxygen, may have a particular role in O2 and environmental sensing. Here, the 21 kDa monomeric 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 immobilised 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¿s heme. The bioelectrocatalytic reduction of O2 was inhibited by CN- thus enabling its sensitive, 100 pM analysis. Peroxidase-like activity of OsHb-1 and the reaction of 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