Low temperature spectral properties of subchloroplast fractions purified from spinach

Spinach (Spinacia oleracea L.) chloroplasts solubilized by digitonin were separated into five fractions by sucrose density gradient centrifugation. Three of the fractions, F(I), F(II), and F(III), corresponding to photosystem I, photosystem II, and the chlorophyll a/b complex, were purified further by two steps of diethylaminoethyl-cellulose chromatography followed by electrofocusing on an Ampholine column. The polypeptide patterns of the fractions were examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and the spectral properties of the fractions at -196 C determined by absorption spectra, fourth derivative curves of the absorption spectra, fluorescence emission spectra, and fluorescence excitation spectra. The activity of purified F(II) (photosystem II) was also assayed by the photoreduction of dichlorophenol-indophenol at room temperature using 1,5-diphenylcarbohydrazine as the electron donor and by the photoreduction of C-550 at -196 C. The different fractions showed unique polypeptide patterns and unique sets of low temperature-absorbing forms of chlorophyll. The fluorescence emission spectra of F(I), F(II), and F(III) at -196 C were also unique with maxima at 734, 685 and 681 nm, respectively. F(I) showed negligible emission at wavelengths shorter than 700 nm and the long wavelength tails of F(II) and F(III) in the 730 nm region were relatively small (approximately 10% of emission of their wavelength maxima). Addition of 0.1% Triton to F(I) and F(II) caused the longer wavelength absorbing forms of chlorophyll to shift to 670 nm and the fluorescence emission maxima (of both fractions) to shift to 679 nm at -196 C with an increase in the yield of fluorescence especially in the case of F(I).