An ESEEM study of the oxidized electron donor of plant photosystem II: Evidence that D is a neutral tyrosine radical

Abstract

Electron spin-echo envelope modulation (ESEEM) has been performed on the tyrosyl radical D ▪ in photosystem II of plants and algae. This radical was investigated in PS II-enriched subchloroplast particles, and in perdeuterated and 15N-substituted Chlorella vulgaris algae. From the ESEEM analysis in the time and frequency domain, two proton hyperfine interactions were determined. One of these protons has hyperfine parameters a iso=27.2 MHz, T 11=−3.1 MHz and T 22/ T 11=0.5. The second proton, which is easily exchangeable by deuterium, has parameters a iso=±1.6 MHz, T 11=∓8.4 MHz and T 22/ T 11=1. The hyperfine parameters of the first proton are characteristic of a β-methylene proton, whereas those of the second indicate that it is a proton hydrogen bonded to the tyrosyl oxygen. We conclude that the oxidized tyrosyl donor is a neutral hydrogen-bonded radical.