Surface chemistry and spectroscopic studies of the native phenylalanine dehydrogenase Langmuir monolayer at the air/aqueous NaCl interface.  This study investigates the main aspects of the surface behavior of the native phenylalanine dehydrogenase (PheDH) enzyme at the air/aqueous interface employing a saline subphase to induce the enzyme surface activity. Surface chemistry experiments were performed in order to determine the surface packing and stability of the formed layer, while spectroscopic experiments provided information regarding its secondary structure conformation. It was found that the PheDH enzyme forms a fluid film, which is quite homogeneous throughout its entire compression, being stable for long periods of time with no significant evidence of aggregates or irreversible domains during interfacial compression/decompression processes. The main secondary structures of the interfacial PheDH film were accessed via in situ reflectance-absorbance infrared spectroscopy, indicating a majority presence of α-helices, which were maintained after the film transfer to solid muscovite supports. The immobilized films presented a homogeneous and regular deposition, with controlled roughness and a mean thickness in the range of 8-10 nm.