Bioluminescence of the Ca2+-binding photoprotein aequorin (AEQ) is largely enhanced by the co-presence of titanate nanosheets (TNS). A stable and transparent colloidal solution of TNS with a small hydrodynamic diameter of less than 10 nm is synthesized through a simple hydrolysis reaction of titanium tetraisopropoxide followed by a dialysis against pure water. Huge agglomerates of AEQ formed in an aqueous solution are peptized in the presence of TNS. The deflocculation of AEQ is considered to be because of a weak electrostatic interaction with the TNS, which is indirectly demonstrated by the enhanced thermal stability of AEQ. As a result, the TNS addition causes an excellent affinity of Ca2+ towards the binding sites (so called EF hands) in AEQ and subsequently efficient bioluminescence. A similar phenomenon is also confirmed for the chemiluminescence of luminol catalyzed by an oxidoreductase. These findings support the fact that the TNS addition is useful to simultaneously solve the faint emissions and instability of photoproteins.
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