Light-induced changes in circular dichroism (CD) were studied in thylakoids isolated from spinach. The followingfeatures of CD responses occurring in the time range of 10 s to 1–3 min were noted: (i) The kinetics and relativeamplitudes of the responses are similar over broad spectral ranges surrounding the major CD bands, i.e., between 670 and760 nm and between 480 and 550 nm. This applies not only to randomly oriented samples but also to magnetically aligned membranes having markedly different CD spectra in the dark, (ii) Photosystem I is much more effective than photosystem II andcan drive a 40-80% decrease in CD signal relative to the dark control level, (iii) Photosystem I driven changes are fully inhibited by nigericin or NH4Cl but are largely insensitive to gramicidin. CD changes driven by photosystem II, on the other hand, are sensitive to all of these reagents. (iv) The CD responses can be shown to originate in circular differential scattering rather than in circular differential absorbance. They can also be distinguished from light-induced, nonpolarized scattering changes. The data are qualitatively evaluated with respect to the theory of circular differential scattering of large helically organized macroaggregates, the size of which is commensurate with the wavelength of the measuring beam [Bustamante, C., Maestre, M. F., & Keller, D. (1985) Biopolymers 24, 1595–1612]. The observed decrease of the large CD signal is ascribed to a partial loss of macrohelicity in the light-harvesting chlorophyll a/b protein complex, in response to a proton gradient and/or surface electrical field generated most effectively by photosystem I.
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