Triplet Charge Recombination in Heliobacterial Reaction Centers Does Not Produce a Spin-Polarized EPR Spectrum

In photosynthetic reaction centers, reduction of the secondary acceptors leads to triplet charge recombination of the primary radical pair (RP). This process is spin selective and in a magnetic field it populates only the T₀ state of the donor triplet state. As a result, the triplet state of the donor has a distinctive spin polarization pattern that can be measured by transient electron paramagnetic resonance (TREPR) spectroscopy. In heliobacterial reaction centers (HbRCs), the primary donor, P₈₀₀, is composed of two bacteriochlorophyll g′ molecules and its triplet state has not been studied as extensively as those of other reaction centers. Here, we present TREPR and optically detected magnetic resonance (ODMR) data of ³P₈₀₀ and show that although it can be detected by ODMR it is not observed in the TREPR data. We demonstrate that the absence of the TREPR spectrum is a result of the fact that the zero-field splitting (ZFS) tensor of ³P₈₀₀ is maximally rhombic, which results in complete cancelation of the absorptive and emissive polarization in randomly oriented samples.