The vast majority of the photosynthetic pigments in plants do not take part in the actual charge separation of the photosynthetic light reaction. Instead they function as antenna molecules that absorb photons and transfer, via other pigment molecules in the antenna, the absorbed energy to the RCs.

Differential scanning calorimetry was employed to investigate the structure of spinach (Spinacia oleracea) chloroplast membranes. In a low ionic strength Hepes-buffered medium, major calorimetric transitions were resolved at 42.50C. (A), 60.60C (B), 64.90C (C1), 69.60C (C2), 75.80C (D), 84.30C (E), and 88.90C (F). A lipid melting transition was also commonly seen at 17°C in scans starting at lower temperatures. The D transition was demonstrated by four independent methods to derive from denaturation of the light harvesting complex associated with photosystem 11 (LHC-11). Evidence for this conclusion was as follows: (a) the endotherm of the isolated LHC-11 (74.0°C) was very similar to that of D (75.8°C); (b) the denaturation temperature of the 27 kilodalton LHC-11 polypeptide determined in intact chloroplast membranes by thermal gel analysis was identical to the temperature of the D transition at pH 7.6 and after destabilization by shifting the pH to 6.6 or by addition of Mg2+; (c) analysis of the stability of the LHC-11 complex by electrophoresis in native gels demonstrated that the complex dissociates during the D transition, both at pH 7.6 and 6.6; and (d) the 77 Kelvin fluorescence maximum of LHC-11 in chloroplasts was seen to shift to lower wavelengths (indicating gross denaturation of LHC-11), at the temperature of the D transition when examined at either of the above pHs. With this identification, five of the eight major endotherms of the chloroplast membrane have now been assigned.Differential scanning calorimetry has been employed by several laboratories recently to obtain information on the structure of the chloroplast membrane (4-6, 12, 16). Calorimetric scans of spinach thylakoids reveal eight endothermic transitions between 15 and 90°C which correspond to lipid melting or denaturation of major proteins or protein complexes within the thylakoid membrane.