Two-dimensional 67Zn HYSCORE spectroscopy reveals that a Zn-bacteriochlorophyll aP' dimer is the primary donor (P840) in the type-1 reaction centers of Chloracidobacterium thermophilum.

Abstract

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 aP, Chl aPD, and Zn2+-BChl aP' 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 aPD is the primary electron acceptor. Because Zn2+-(B)Chl aP' is present at low abundance, it was suggested that the primary electron donor might be a dimer of Zn2+-BChl aP' molecules. In this study, we utilize isotopic enrichment and high-resolution two-dimensional (2D) 14N and 67Zn hyperfine sublevel correlation (HYSCORE) spectroscopy to demonstrate that the primary donor cation, P840+, in the C. thermophilum RC is indeed a Zn2+-BChl aP' dimer. Density functional theory (DFT) calculations and the measured electron-nuclear hyperfine parameters of P840+ indicate that the electron spin density on P840+ is distributed nearly symmetrically over two Zn2+-(B)Chl aP' 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.