Thermoluminescence emission at liquid helium temperatures from photosynthetic apparatus and purified pigments

Thermoluminescence (TL) emission below 77 K from photosynthetic pigment protein complexes and purified pigments was examined using liquid He. At least four TL components emitting at around 20, 50, 70 and 90 K were resolved on the glow curve from thylakoids. The 20, 50 and 70 K bands are newly observed TL components and designated as Z_α, Z_β and Z_γ bands, respectively. The 90 K band was found to be a different expression of the well-know Z band which was reported as the 110 K band in literatures. These TL bands were evidenced not to be related with charge separation and subsequent electron transfer in reaction centers but originate from light-harvesting chlorophyll (Chl) a and b by the following observations: (1) red light, which causes charge separation in reaction centers, was ineffective in inducing these TL components; (2) isolated LHCI and LHCII showed higher TL intensities than isolated PS I core and PS II core complexes; and (3) purified Chl a and Chl b in solid state exhibited essentially the same TL bands. Chl-Chl and Chl-ligand interactions in proteins have been discussed as possible chemical identities of the energy trap responsible for the TL bands.