PROTEIN AND CHLOROPHYLL IN PHOTOSYSTEM-II PROBED BY INFRARED-SPECTROSCOPY

The infrared spectra of photosystem II (PS II) enriched submembrane fractions isolated from spinach are obtained in water and in heavy water suspensions. Other spectra are obtained after a photooxidation reaction was performed on PS II to bleach the pigments. The water bands are removed by computer subtraction and the amide bands (A, B, I, II, and III) of the protein are identified. Computer enhancement techniques are used to narrow the bandwidth of the bands so that the weak chlorophyll bands, buried in the much stronger protein bands, can be observed. Comparing the spectra of native and photooxidized PS II preparations in water and in heavy water, we determine that three polypeptide domains are present in the native material. The first domain, which contains 22% of the polypeptides, is situated in the peripheral region of the PS II system. The polypeptides in this region are unfolded and devoid of chlorophyll. The second domain contains 41% of the polypeptides, is more organized, and contains the chlorophylls. The third domain has an alpha-helix configuration, does not contain chlorophyll, and is not affected by the photooxidation reaction or by the proton/deuteron exchange. Three different types of chlorophyll organisation are identified: two have their ketone carbonyl groups non-bonded, differing from one another only in their hydrophobic milieux; the third is weakly bonded to another unidentified group. Other forms of chlorophyll organisation are present but could not be observed because their absorption is buried in the protein amide I band.