ESEEM study of spin-spin interactions in spin-polarised P+QA− pairs in the photosynthetic purple bacterium Rhodobacter sphaeroides R26

Electron spin echo envelope modulation (ESEEM) of the transient spin-polarised P+QA− pairs has been studied at 20 and 220 K. The observed strong out-of-phase modulation is interpreted as resulting from electron-electron dipolar and exchange interactions. Fourier-transformed echo envelopes are consistent with the theory developed recently by Tang, Thurnauer and Norris and allow us to obtain readily the values of dipolar and exchange couplings. The results are insensitive to 15N enrichment.