Secondary Structure of Organophosphorus Hydrolase in Solution and in Langmuir−Blodgett Film Studied by Circular Dichroism Spectroscopy

The secondary structure of organophosphorus hydrolase (OPH) has been studied with circular dichroism (CD) spectroscopy in the far-UV region. The effect of pH on the secondary structure of OPH solution was examined over the pH range from 3.56 to 9.60. As shown on the CD spectra, the secondary structure of OPH is well defined when the pH value is near the isoelectric point (7.6); however, it is destroyed when the pH values are increased or decreased further. This is explained by the loss of helical structure. The pH effect on CD spectra contributes to clarify the optimum pH needed to obtain a stable OPH Langmuir film at the air−water interface and its correlation to the secondary structure of the enzyme. Comparative study of the thermal treatment on the secondary structure of OPH in solution, Langmuir−Blodgett film, and dry film shows that the molecular arrangement plays a dominant role in the thermal stability of OPH. With use of the CDPro software package a quantitative estimation of the secondary structure from the CD spectra of OPH solution was obtained. Results show that there is a decrease in the percentage of the α-helical and an increase of β-strands with the change of pH or temperature.