Surface Properties of “Jellyfish”: Langmuir Monolayer and Langmuir-Blodgett Film Studies of Recombinant Aequorin

In this paper, we studied the surface properties of recombinant aequorin at the air−water interface. Using the Langmuir monolayer technique, the surface properties of aequorin were studied, including the surface pressure and surface potential−area isotherms, compression−decompression cycles, and stability on Trizma Base (Tris/HCl) buffer at pH 7.6. The results showed that aequorin formed a stable Langmuir monolayer and the surface pressure−area isotherms were dependent on both pH and ionic strength. At a pH higher or lower than 7.6, the limiting molecular area decreased. The circular dichroism (CD) spectra of aequorin in aqueous solutions explained this result: when the pH was higher than 7.6, the α-helix conformation changed to unordered structures, whereas at a pH lower than 7.6, the α-helix conformation changed to β-sheet. The addition of calcium chloride to the Tris/HCl buffer subphase (pH 7.6) caused an increase of the limiting molecular area of the aequorin Langmuir monolayer. The fluorescence spectra of a Langmuir−Blodgett (LB) film of aequorin in the presence of calcium chloride indicated that the aequorin transformed to the apoaequorin.