Orientation of intrinsic proteins in photosynthetic membranes. Polarized infrared spectroscopy of chloroplasts and chromatophores.

In order to estimate the degree of orientation of the alpha-helices of intrinsic proteins in photosynthetic membranes, polarized infrared spectroscopy has been used to measure the dichroism of the amide I and amide II absorption bands of air-dried oriented samples of purple membranes, chloroplasts and chromatophores from Rhodopseudomonas sphaeroides. Using purple membrane, in which the orientation of the alpha-helices is precisely known (Henderson, R. (1977) Annu. Rev. Biophys. Bioeng. 6, 87-109), as a standard to calibrate our measurements and estimating the mosaic spread (extent of orientation) of the membranes from linear dichroism measurements performed in the visible spectral range, it is concluded that in photosynthetic membranes, the alpha-helices of intrinsic proteins are tilted at less than 40 degrees with respect to the normal to the plane of the membrane.