Heliobacteria are anaerobic phototrophic bacteria with a type I homodimeric reaction center (RC) using bacteriochlorophyll g (BChl g ). H. modesticaldum , the model organism used for heliobacteria, has been well studied and is understood to convert BChl g into 8 1 -OH-chlorophyll a F (BChl a F ) in the presence of light and dioxygen. Conversion of BChl g results in the loss of the light-driven charge separation. Previous work work shows that once both of the BChl g ’ molecules of the primary donor P 800 have been converted to BChl a F ’, the RC can no longer perform electron transfer. We show that a partially converted P 800 can exist by exposing the RC to dioxygen and demonstrate the presence of a BChl g ’/ BChl a F ’ heterodimer by Q-band 1 H ENDOR, 14 N HYSCORE spectroscopy, and DFT methods. The DFT calculations of a BChl g ’/ BChl g ’ homodimeric primary donor predict that the unpaired electron spin of P 800 + will be evenly delocalized across both of the BChl g ’ molecules, which is in excellent agreement with experimental hyperfine couplings of the anaerobic samples. Exposure to dioxygen drastically changes the experimental hyperfine interactions of the RC, which displays greater localization of the unpaired electron spin in P 800 + . In agreement with the experimental hyperfine couplings, DFT calculations using a computational model of the heterodimeric primary donor P 800 obtained by replacing one of the BChl g ’ with BChl a F ’ shows significant localization of the electron spin density on the BChl g ’ molecule in the heterodimer.
Landry et al. (Sun,) studied this question.