Odor binding protein as probe for a refractive index-based biosensor: new perspectives in biohazard assessment

The odorant-binding proteins (OBPs) are abundant low-molecular weight soluble proteins, which are secreted by the olfactory epithelium in the nasal mucus of vertebrates. These protein reversibly bind odorants with dissociation constants in the micromolar range. For this reason, they are good candidates as biological elements in the development of biosensors. Vertebrate OBPs belong to the lipocalin superfamily. Even if the members of this superfamily display low sequence similarity, all of them show a conserved folding pattern, that is an 8-stranded β-barrel flanked by an α-helix at the C-terminal end of the protein chain. The β-barrel defines a central apolar cavity, called calyx, whose role is to bind and transport hydrophobic odorant molecules. The detection of hazard exposure is becoming a priority in the third millennium, and OBPs are good candidates for detecting traces of explosive molecules in different environments such as luggage's storage rooms and public places. In this context, the measurement of refractive index of odor-binding protein in absence and in presence of odorant molecules have been performed in order to assess its usefulness as a probe for detection of hazardous agents. The work is instrumental to explore the possibility to realize a biosensor where the concentration of searched for substances is analyzed as a variation of the protein refractive index by means of suitable optoelectronic devices.