Fluorescence study reveals differential interaction of four unrelated proteins with graphene oxide

Molecular interactions of four completely unrelated proteins were studied with graphene oxide (GO) in aqueous buffer, using fluorescence spectroscopy. The proteins chosen are small monomeric lysozyme, a large, multidomain, membrane skeletal protein spectrin, and two tetrameric proteins, an enzyme alcohol dehydrogenase (ADH) and the mannose binding Concanavalin A, differing in nature and source of abundance. GO induced Stern–Volmer fluorescence quenching and steady-state anisotropy measurements revealed the binding to be protein specific. The biphasic pattern of the Stern–Volmer quenching indicate a combination of both static and dynamic quenching involved in the process. Acrylamide quenching experiments show significant differences between the quenching constants (K_SV) and the fraction of accessible tryptophan (α) residues in presence of GO. Increase in fluorescence anisotropy indicated binding of proteins, both in their native and denatured forms. The bindings are predominantly enthalpy driven. The apparent binding constants of the GO–Protein complexes are of the order of 10⁻⁴ M showing moderate, endothermic binding to GO. ADH showed the strongest binding with an increase in enthalpy of ΔH + 32.4 kJ.mol⁻¹.