COMPARATIVE DECAY CHARACTERISTICS OF LIGHT GENERATED FREE RADICAL IN CHROMATOPHORES AND CHLOROPLASTS

Abstract— Chromatophores of Rhodospirillum rubrum when illuminated exhibit a free radical which ordinarily shows a biphasic decay. When chromatophores are dehydrated at room temperature the time course of the appearance of the light‐induced free radical is unaffected, but the decay pattern has been altered. Only the fast component remains, the slow component is no longer evident. Scanning the magnetic field reveals the presence of a dark signal which is stable as long as the preparation remains dehydrated. This signal has the same peak‐to‐peak line width of ˜ 10 G and the same g factor as the signal evident in the light. The amplitude of this signal is equal to the amplitude of the slow decay component seen in aqueous chromatophore suspensions. Chromatophores frozen in an aqueous medium at —150°C exhibit a behavior identical with dehydrated preparations. The effects produced by lyophilization or by freezing at low temperatures are entirely reversible. When a lyophilized preparation is re‐hydrated, the stabilized portion of the signal now decays in the dark; the same is observed when preparations frozen at —150°C are thawed. When such thawed or re‐hydrated preparations are illuminated again, they exhibit the usual light‐induced ESR signal showing a biphasic dark decay. A comparison was made between the behavior of the light‐induced ESR signal of chromatophores and that of system I of chloroplasts. This comparison revealed that there is a greater similarity in some of the decay characteristics of these signals than had been recognized previously. In chloroplasts, temperature insensitive, non‐enzymatic back‐reactions of the light‐induced free radical appear to be nil, and in chromatophores a distinct portion of the light‐induced free radicals exhibit the same characteristics. Another portion of the chromatophore free radicals must be able to back‐react by electron tunneling, a mechanism which appears to be absent in the chloroplast system. Copyright © 1969, Wiley Blackwell. All rights reserved