Abstract
Abstract—The electron paramagnetic resonance Signal II of spinach chloroplasts was recorded under high‐resolution conditions. The signal is asymmetric, has five lines of unequal intensity and splitting, a derivative linewidth of 20.0 G, and a g‐value of 2.0051. The latter value varies between 2.0045 and 2.0052 since other signals contribute to the center portion of the spectrum. Signal II is present in dark‐adapted leaves and chloroplasts at a concentration of 50 per cent of the maximum light signal. It is not exhibited by stroma lamellae but is seen in grana thylakoid fractions. The maximum concentration of the material providing Signal II is found in a fraction (40 K) which has a very low Photosystem II activity. The unknown substance giving rise to Signal II may not be required for either Photosystem I or Photosystem II activity. Evidence presented here indicates that Signal II probably is not due to a free, mobile plastosemiquinone or a free, mobile plastochromanoxyl radical.
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