Distortion of the amide-I and -II bands of an α-helical membrane protein, pharaonis halorhodopsin, depends on thickness of gold films utilized for surface-enhanced infrared absorption spectroscopy

► SEIRA spectroscopy has been applied on pharaonis halorhodopsin. ► We observed normal, bipolar, and inverted shapes of the amide-I and -II bands. ► The distortion of the bands differs in cases of p- and s-polarization. ► The physical origin of this phenomenon has been revealed.