Direct electron transfer and conformational change of glucose oxidase on carbon nanotube-based electrodes

To gain insights into the direct electron transfer (DET) mechanism of multi-walled carbon nanotubes (MWCNTs), we investigated the conformational changes that occur in proteins when they interact with MWCNTs. We used glucose oxidase (GOD) as an example. Using cyclic voltammetry measurements, the GOD that was immobilized on the MWCNT-modified carbon paper electrode exhibited apparent direct electrochemistry compared to that on the bare electrode without MWCNTs. The structural transformation of GOD upon adsorption on the MWCNTs was characterized spectrally. GOD was not denatured, and only small shifts of the wavenumber of the β-sheet structure were observed. There was a consistent tendency for the amount of α-helix to decrease and the β-sheet to increase. The α-helix content dropped from 21.2% to 19.6% as measured using Fourier transform infrared spectroscopy and from 27.1% to 25.9% as measured using circular dichroism. The reduction in the amount of α-helix led to a less shielded GOD active site and weakened the resistance of the electron transfer. These MWCNT-induced conformational changes could account for the DET between GOD and the MWCNT-modified electrode surface.