Infrared Reflection−Absorption Spectroscopy and Polarization-Modulated Infrared Reflection−Absorption Spectroscopy Studies of the Organophosphorus Acid Anhydrolase Langmuir Monolayer

Abstract

The secondary structure of the organophosphorus acid anhydrolase (OPAA) Langmuir monolayer in the absence and presence of diisopropylfluorophosphate (DFP) in the subphase was studied by infrared reflection-absorption spectroscopy (IRRAS) and polarization-modulated IRRAS (PM-IRRAS). The results of both the IRRAS and the PM-IRRAS indicated that the alpha-helix and the beta-sheet conformations in OPAA were parallel to the air-water interface at a surface pressure of 0 mN.m-1 in the absence of DFP in the subphase. When the surface pressure increased, the alpha-helix and the beta-sheet conformations became tilted. When DFP was added to the subphase at a concentration of 1.1 x 10(-5) M, the alpha-helix conformation of OPAA was still parallel to the air-water interface, whereas the beta-sheet conformation was perpendicular at 0 mN.m-1. The orientations of both the alpha-helix and the beta-sheet conformations did not change with the increase of surface pressure. The shape of OPAA molecules is supposed to be elliptic, and the long axis of OPAA was parallel to the air-water interface in the absence of DFP in the subphase, whereas the long axis became perpendicular in the presence of DFP. This result explains the decrease of the limiting molecular area of the OPAA Langmuir monolayer when DFP was dissolved in the subphase.

Surface FTIR Techniques to Analyze the Conformation of Proteins/ Peptides in H2O Environment

“…The s-and p-polarized IRRAS results of the Langmuir monolayer of oganophosphorus acid anhydrolase (OPAA) are shown in Figure 3a and 3b, respectively [35]. OPAA contains most of the typical conformations (i.e., unstructured conformation, α-helix, β-sheet, turns and so on) as discussed above.…”

“…Interestingly, the amide I band of unstructured conformation at 1640 cm -1 was not detected in p-polarized IRRAS. This result is reasonable because the carbonyls in unstructured conformation should be randomly distributed and this orientation is not preferred by p-polarized IRRAS, as discussed in the above section about the selection rules of IRRAS [35]. Similarly, some other proteins with high fraction of α-helix have been analyzed by IRRAS and important information about the change of both the conformation and orientation of the proteins has been addressed [35].…”

“…The technique of IRRAS for the Langmuir monolayer was developed in 1985 [33]. Subsequently, some proteins were shown to be also able to form a Lanmguir monolayer and IRRAS has been consequently used to analyze protein samples in the Langmuir monolayer [14,[34][35][36][37][38], which covers the surface of H 2 O. IRRAS can not only detect the FTIR signal of proteins, but also valuate the orientation of the proteins sample in the Langmuir monolayer due to the selection rules as discussed below [14,[34][35][36]38].…”

“…For example, the PM-IRRAS of OPAA is shown in Figure 8. The positive amide I band in Figure 8 indicates that the carbonyls in various conformations are parallel to the air-water interface [35].…”

α-Synuclein in α-helical conformation at air–water interface: implication of conformation and orientation changes during its accumulation/aggregation

“…, stability or aggregation) of amphiphile biomacromolecules at the air–water interface. 9-11 We recently observed that Aβ molecules accumulated at the air–water interface were predominantly present in the α-helical conformation. 9 A kinetic model was proposed to explain the Aβ adsorption behavior.…”