Two-dimensional Zn HYSCORE spectroscopy reveals that a Zn-bacteriochlorophyll a' dimer is the primary donor (P) in the type-1 reaction centers of Chloracidobacterium thermophilum.

Chloracidobacterium (C.) thermophilum is a microaerophilic, chlorophototrophic species in the phylum Acidobacteria that uses homodimeric type-1 reaction centers (RC) to convert light energy into chemical energy using (bacterio)chlorophyll ((B)Chl) cofactors. Pigment analyses show that these RCs contain BChl a, Chl a, and Zn-BChl a' in the approximate ratio 7.1 : 5.4 : 1. However, the functional roles of these three different Chl species are not yet fully understood. It was recently demonstrated that Chl a is the primary electron acceptor. Because Zn-(B)Chl a' is present at low abundance, it was suggested that the primary electron donor might be a dimer of Zn-BChl a' molecules. In this study, we utilize isotopic enrichment and high-resolution two-dimensional (2D) N and Zn hyperfine sublevel correlation (HYSCORE) spectroscopy to demonstrate that the primary donor cation, P, in the C. thermophilum RC is indeed a Zn-BChl a' dimer. Density functional theory (DFT) calculations and the measured electron-nuclear hyperfine parameters of P indicate that the electron spin density on P is distributed nearly symmetrically over two Zn-(B)Chl a' molecules as expected in a homodimeric RC. To our knowledge this is the only example of a photochemical RC in which the Chl molecules of the primary donor are metallated differently than those of the antenna.