Far Ultraviolet Circular Dichroism and Infrared Absorption of Thylakoids

Abstract

In suspensions of stroma freed chloroplasts, the far ultraviolet circular dichroism of the thylakoid proteins is masked by linear optical anisotropy, due to the regular stacking of thylakoids. The circular dichroism spectra of small fragments of thylakoid membranes have the shape, characteristic of a protein with consiberable α-helix content. The same is true for the vesicle like thylakoids of the purple bacterium Rhodopseudomonas spheroides, suspended in water. The turbid thylakoid suspension of the blue green alga Oscillatoria chalybea equally yields a protein spectrum, which, however, is somewhat distorted by light scattering. Corresponding results are obtained by means of infrared absorption spectroscopy. The location and shape of the amide I and amide II bands of suspensions of stroma freed chloroplasts in D2O and of solid films of chloroplasts are compatible with α-helix containing protein. It was found, that a considerable amount of the amide groups is protected against deuteration. No considerable difference is observed concerning the position and shape of amide I and amide II bands from solid films, regardless, whether the lipid absorption was eliminated either by extraction with dry acetone, or by difference spetroscopy. Disorganisation of the thylakoids by dodecyl sulfate causes only minor changes in the circular dichroism. Extraction of lipids from solid films with water containing acetone or heat denaturation results in the appearence of β-structure. Freeze dried chloroplasts are soluble in hexafluoroisopropanol. This solvent causes an increase of α-helix. Consequences of these experiments for the understanding of the molecular structure of the thylakoid membrane are discussed.