Thermooptic effect in chloroplast thylakoid membranes. Thermal and light stability of pigment arrays with different levels of structural complexity

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; s uch reorganizations did not affect the short-range, excitonic pigment-pigme nt interactions. These structural changes and similar changes in lamellar a ggregates of the main chlorophyll alb light-harvesting complexes exhibited a linear dependence on the intensity of light that was not utilized in phot osynthesis. It has been hypothesized that the light-induced rearrangements are driven by a thermooptic effect, i.e., thermal fluctuations due to the d issipation of excess excitation energies [Barzda, V., et al. (1996) Biochem istry 35, 8981-8985]. To test this hypothesis, we have utilized circular di chroism (CD) spectroscopy to investigate the structural stability of the ch iral macrodomains and the constituent bulk pigment-protein complexes of gra nal thylakoid membranes against heat and prolonged, intense illumination. ( i) In intact thylakoid membranes, the chiral macrodomains displayed high st ability below 40 degreesC, but they were gradually disassembled between 50 and 60 degreesC; 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 wer e also susceptible to high light: prolonged illumination with intense white light (25 min, 2500 muE m(-2) s(-1), 25 degreesC) induced similar, irrever sible disassembly to that observed at high temperatures; in different prepa rations, lower thermal stability was coupled to lower light stability. (iii ) The light stability depended significantly on the temperature: between ab out 5 and 15 degreesC, the macrodomains in the intact thylakoids were virtu ally not susceptible to high light; in contrast, the same preillumination a t 35-40 degreesC almost completely destroyed the chiral macrodomains. (iv) As testified by the excitonic CD bands, the molecular organization of the p igment-protein complexes in all samples exhibited very high thermal stabili ty between about 15 and 65 degreesC, and virtually total immunity against i ntense illumination. These data are fully consistent with the hypothesis of a thermooptic effect, and are interpreted within the frame of a simple mod el.