Effects of acid and alkali on the light absorption, energy transfer and protein secondary structures of core antenna subunits CP43 and CP47 of photosystem II

The effects of acid and alkali treatment on the light absorption, energy transfer and protein secondary structure of the photosystem 11 core antenna CP43 and CP47 of spinach were investigated by the absorption spectra, fluorescence emission spectra and circular dichroism spectra. It has been found that acid treatment caused the appearance of absorption characteristic of pheophytin a (Pheo a), whereas alkali treatment induced a new absorption peak at 642 nm. The energy transfer between beta-carotene and chlorophyll a (Chl a) in CP43 was easily disturbed by alkali, whereas in CP47 was readily affected by acid. As to the effects on the secondary structure of proteins in CP43 and CP47, effects of acid were far less than those of alkali. Both acid and alkali disturbed the microenvironment of Chl a and interfered exciton interaction between Chl a molecules. It was suggested that acid and alkali affect the light absorption, energy transfer and protein secondary structure of CP43 and CP47 in a different way. H+ can permeate into the internal space of alpha-helix, change Chl a into Pheo a and disturb the microenvironment of pigments without damaging the secondary structure of protein, whereas OH- can induce the protein unfolding at first, then saponify Chl a to chlorophyllide and disturb the microenvironment of pigments.