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

Abstract

Electron transfer from the reduced tyrosine Y_D 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 Y_D ^·, 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 Y_D· 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 Y_D· 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.