Changes in endothermic transitions associated with light-induced chlorophyllide formation, as investigated by differential scanning calorimetry

The changes in thermal transitions associated with protochlorophyllide (Pchlide) into chlorophyllide (Chlide) phototransformation were investigated using sensitive differential scanning calorimetry (DSC), Two groups of endothermic transitions, each composed of several components, were observed between 25 and 90 degrees C for prolamellar body (PLB) membranes isolated from etiolated wheat (Triticum aestivum) leaves. The first group, located in the 20-45 degrees C region, was strongly affected by a short light pulse given prior to measurement, A decrease of this group of transitions was observed shortly after illumination and it was hardly detected in PLBs measured after the Chlide Shibata shift. Calorimetric results were supported by 77-K fluorescence emission spectra measured for continuously heated samples at different temperatures. We interpreted the DSC band observed between 20 and 45 degrees C as due to the disaggregation of Pchlide- (or Chlide-)-reductase-NADPH complexes, More detailed analysis using Gaussian deconvolution showed that this band was composed of three transitions at 32, 39 and 41 degrees C. The second group of transitions was detected in the 45-80 degrees C region, the same as for mature thylakoids. Its main component at 60 degrees C was not affected significantly by a short light pulse. By comparison of thermograms obtained for PLBs with those for mature thylakoid membranes, this transition was identified as the ATPase denaturation band.