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 alpha-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 alpha helices are more tilted when the bR monolayer is spread on the water surface than when it is deposited onto a dry solid substrate.

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