Anisotropic Optical Constants of Bacteriorhodopsin in the Mid-Infrared: Consequence on the Determination of α-Helix Orientation

In this paper, we present the first determination of the anisotropic optical constants of a protein (bacteriorhodopsin or bR) monolayer in the mid-infrared range. This procedure has allowed us to calculate a mean tilt angle of the α-helices' axis with respect to the normal of the surface. Using these optical constants, we have been able to reproduce the polarized attenuated total reflection (ATR) spectra of a bR monolayer deposited on a germanium crystal. On the other hand, the optical constants have to be modified to reproduce the polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) spectrum of a bR monolayer spread at the air/water interface. This modification of the optical constants indicates that the α helices are more tilted when the bR monolayer is spread on the water surface than when it is deposited onto a dry solid substrate.