Influence of the Surface Functional Group Density on the Carbon-Nanotube-Induced α-Chymotrypsin Structure and Activity Alterations

Because of the special properties of carbon nanotubes (CNTs), their applications have been introduced to many fields. The biosafety of these emerging materials is of high concern concomitantly. Because CNTs may initially bind with proteins in biofluids before they exert biological effects, it is of great importance to understand how the target proteins interact with these exogenous nanomaterials. Here we investigated the interaction between alpha-chymotrypsin (alpha-ChT) and carboxylized multiwalled CNTs in a simulated biophysical environment utilizing the techniques of fluorescence, UV-vis, circular dichroism spectroscopy, zeta potential, atomic force microscopy, and bicinchoninic acid analysis. It was demonstrated that CNTs interacted with alpha-ChT through electrostatic forces, causing a decrement in the alpha-helix and an increment in the beta-sheet content of the protein. The protein fluorescence was quenched in a static mode. The increase in the surface modification density of CNTs enhanced the protein absorption and decreased the enzymatic activity correspondingly. alpha-ChT activity inhibition induced by CNTs with low surface modification density exhibited noncompetitive characteristics; however, a competitive feature was observed when CNTs with high surface modification density interacted with the protein. An increase of the ionic strength in the reaction buffer may help to reduce the interaction between CNTs and alpha-ChT because the high ionic strength may favor the release of the protein from binding on a CNT surface modified with functional groups. Accordingly, the functionalization density on the CNT surface plays an important role in the regulation of their biological effects and is worthy of concern when new modified CNTs are developed.