Charge Equilibrium Reactions S2 and S3 States of Photosystem II with Cyt b559 and Tyrosine YD

Abstract

Electron transfer from the reduced tyrosine YD and cytochrome b559 (Cyt b559) to the S2 and S3 states of photosystem II was investigated at the temperature of 195 K. Electron transfer reactions were followed by measuring EPR signals of tyrosine YD ·, oxidized Cyt b559 and the S2-state multiline signal. Long term incubation (∼90 days) at 195 K causes decay of the majority of S2 centers up to ∼40% of initial value, while in this time scale the intensity of YD· radical increases less than 10%. Samples advanced to S3 state demonstrates an increasing behavior of the S2-state multiline signal intensity in the beginning of incubation (∼20 days) and slow decay up to 40% of maximal amplitude during further incubation of the samples. Similarly to the S2 sample, small increase in YD· radical signal was observed during the S3 decay. However, in both types of samples prepared in S2 and S3 states after 90 days of incubation the signal of oxidized Cyt b559 is increased from 45%–50% up to 100% maximal intensity. The results obtained in this study support the conclusion of our early investigations which claimed the reduced Cyt b559 as electron source for the S2 and S3 states.