Thermooptic Effect in Chloroplast Thylakoid Membranes. Thermal and Light Stability of Pigment Arrays with Different Levels of Structural Complexity

Abstract

In chloroplast thylakoid membranes, chiral macrodomains, i.e., large arrays of pigment molecules with long-range chiral order, have earlier been shown to undergo light-induced reversible and irreversible structural changes; such reorganizations did not affect the short-range, excitonic pigment−pigment interactions. These structural changes and similar changes in lamellar aggregates of the main chlorophyll a/b light-harvesting complexes exhibited a linear dependence on the intensity of light that was not utilized in photosynthesis. It has been hypothesized that the light-induced rearrangements are driven by a thermooptic effect, i.e., thermal fluctuations due to the dissipation of excess excitation energies [Barzda, V., et al. (1996) Biochemistry 35, 8981−8985]. To test this hypothesis, we have utilized circular dichroism (CD) spectroscopy to investigate the structural stability of the chiral macrodomains and the constituent bulk pigment−protein complexes of granal thylakoid membranes against heat and prolonged, intense illumination. (i) In intact thylakoid membranes, the chiral macrodomains displayed high stability below 40 °C, but they were gradually disassembled between 50 and 60 °C; the thermal stability of the chiral macrodomains could be decreased substantially by suspending the membranes in reaction media that were hypotonic or had low ionic strength. (ii) The chiral macrodomains were also susceptible to high light: prolonged illumination with intense white light (25 min, 2500 μE m^-2 s^-1, 25 °C) induced similar, irreversible disassembly to that observed at high temperatures; in different preparations, lower thermal stability was coupled to lower light stability. (iii) The light stability depended significantly on the temperature: between about 5 and 15 °C, the macrodomains in the intact thylakoids were virtually not susceptible to high light; in contrast, the same preillumination at 35−40 °C almost completely destroyed the chiral macrodomains. (iv) As testified by the excitonic CD bands, the molecular organization of the pigment−protein complexes in all samples exhibited very high thermal stability between about 15 and 65 °C, and virtually total immunity against intense illumination. These data are fully consistent with the hypothesis of a thermooptic effect, and are interpreted within the frame of a simple model.