Chloroplasts isolated from pea leaves display an intense circular dichroism in the range 600 to 720nm. Circularly polarized light is also differentially scattered by chloroplasts, and this effect can be confused with circular dichroism. By using an instrumental modification it was possible to distinguish, and record separately, the ellipticities of the transmitted light (circular dichroism) and of the scattered light in the same c.d. instrument. By means of a light-scattering apparatus, the intensity of unpolarized light scattered by chloroplasts was measured as a function of wavelength and of angle. This measurement allowed the aforementioned ellipticities to be corrected for mutual interference. At a concentration of 4μg of chlorophyll/ml (the optimum practical concentration of chloroplasts at which there was no significant interaction of scattering and absorption effects) spectra of true circular dichroism (circular differential absorption) and circular differential scattering were obtained. The former showed maxima, positive at 688nm and negative at 676nm, with an intensity Δθ′8.3m°·litre·(mg of chlorophyll)-1·cm-1. The latter had a maximum at 683nm with an intensity of +47m° with respect to the solvent baseline; this value is independent of the concentration of chloroplasts in dilute suspensions. It is suggested that the intense circular dichroism of chloroplasts reflects specific chlorophyll–chlorophyll interactions in the light-harvesting pigment. The advantages of this method for determining the c.d. of scattering suspensions over those of other investigators are discussed.

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