Infrared reflection-absorption spectroscopy and polarization-modulated infrared reflection-absorption spectroscopy studies of the aequorin langmuir monolayer

The Langmuir monolayer of aequorin and apoaequorin was studied by infrared reflection-absorption spectroscopy (IRRAS) and polarization-modulated IRRAS techniques. The α-helices in the aequorin Langmuir monolayer were parallel to the air - water interface at zero surface pressure. When the surface pressure increased to 15 mN·m^_1, the α-helices became tilted and the turns became parallel to the air-water interface. As for apoaequorin, the α-helices were also parallel to the air-water interface at 0 mN·m^-1. However, the α-helix became tilted and the turns became parallel to the air-water interface quickly at 5 mN·m ^-1. With further compression of the apoaequorin Langmuir monolayer, the orientation remained the same. The different behaviors of aequorin and apoaequorin at the air-water interface were explained by the fact that aequorin formed dimers at the air - water interface but apoaequorin was a monomer. It is more difficult for a dimer to be tilted by the compression of the Langmuir monolayer.