Engineered (Lys)(6)-Tagged Recombinant Sulfide-Reactive Hemoglobin I for Covalent Immobilization at Multiwalled Carbon Nanotubes

[Image: see text] The recombinant HbI was fused with a poly-Lys tag ((Lys)(6)-tagged rHbI) for specific-site covalent immobilization on two carbon nanotube transducer surfaces, i.e., powder and vertically aligned carbon nanotubes. The immobilization was achieved by following two steps: (1) generation of amine-reactive ester from the carboxylic acid groups of the surfaces and (2) coupling these groups with the amine groups of the Lys-tag. We analyzed the immobilization process using different conditions and techniques to differentiate protein covalent attachment from physical adsorption. Fourier transform infrared microspectroscopy data showed a 14 cm(–1) displacement of the protein’s amide I and amide II peaks to lower the frequency after immobilization. This result indicates a covalent attachment of the protein to the surface. Differences in the morphology of the carbon substrate with and without (Lys)(6)-tagged rHbI confirmed protein immobilization, as observed by transmission electron microscopy. The electrochemical studies, which were performed to evaluate the redox center of the immobilized protein, show a confinement suitable for an efficient electron transfer system. More importantly, the electrochemical studies allowed determination of a redox potential for the new (Lys)(6)-tagged rHbI. The data show that the protein is electrochemically active and retains its biological activity toward H(2)S.