Superior activity of structurally deprived enzyme−carbon nanotube hybrids in cationic reverse micelles

Abstract

In the present work, we report the superior activity of hydrophobically adsorbed enzymes onto single-walled carbon nanotubes (SWNTs) in the reverse micelles of cationic surfactants. Horseradish peroxidase and soybean peroxidase adsorbed onto SWNTs endure a notable loss in secondary structure and catalytic activity. This structurally and functionally deformed enzyme−SWNT when confined in CTAB reverse micelles showed ∼7−9-fold enhancement in activity compared to that was in water and also importantly ∼1500−3500 times higher activity than that of the enzymes in aqueous−organic biphasic mixtures. The activation observed for this nanobiocomposite is due to the (i) possible localization of enzyme−SWNT hybrid at the micellar interface; (ii) facile transport of substrates across the microscopic interface of reverse micelles; and (iii) greater local concentration of substrates at the augmented interfacial space in the presence of SWNT. This interfacial localization of the SWNT−protein hybrid was tested using FITC-tagged protein (BSA) by fluorescence spectroscopy. FTIR and CD spectroscopy established that the enzyme notably loses its native structure as it gets adsorbed onto the CNTs. However, this loss in the secondary structure is neither aggravated nor recovered when the enzyme-SWNT resides at the reverse micellar interface. So, localization of the surface-active peroxidase−CNT hybrids at the interface is the main reason for significant enzyme activation. The generality of the activation of the enzyme−CNT hybrid by reverse micelles was tested using amphiphiles with varying headgroup sizes, where an overall enhancement in activity was observed with an increase in headgroup size. Activation of this nanobiocomposite would find utmost importance in material science as the activity of structurally deprived enzyme in reverse micelles surpassed (∼1.7-fold) even the activity of the native enzyme in water.