Relationship between the oxidation potential and electron spin density of the primary electron donor in reaction centers from Rhodobacter sphaeroides

Abstract

The primary electron donor in bacterial reaction centers is a dimer of bacteriochlorophyll a molecules, labeled L or M based on their proximity to the symmetryrelated protein subunits. The electronic structure of the bacteriochlorophyll dimer was probed by introducing small systematic variations in the bacteriochlorophyll-protein interactions by a series of site-directed mutations that replaced residue Leu M160 with histidine, tyrosine, glutamic acid, glutamine, aspartic acid, asparagine, lysine, and serine. The midpoint potentials for oxidation of the dimer in the mutants showed an almost continuous increase up to Ϸ60 mV compared with wild type. The spin density distribution of the unpaired electron in the cation radical state of the dimer was determined by electron-nuclear-nuclear triple resonance spectroscopy in solution. The ratio of the spin density on the L side of the dimer to the M side varied from Ϸ2:1 to Ϸ5:1 in the mutants compared with Ϸ2:1 for wild type. The correlation between the midpoint potential and spin density distribution was described using a simple molecular orbital model, in which the major effect of the mutations is assumed to be a change in the energy of the M half of the dimer, providing estimates for the coupling and energy levels of the orbitals in the dimer. These results demonstrate that the midpoint potential can be fine-tuned by electrostatic interactions with amino acids near the dimer and show that the properties of the electronic structure of a donor or acceptor in a protein complex can be directly related to functional properties such as the oxidation-reduction midpoint potential.The reaction center is the site of the primary process in photosynthesis, which is the conversion of light energy into a charge-separated state (for reviews, see ref. 1). The electron transfer process in the reaction center has the remarkable aspect that the quantum yield is near unity, that is, for every photon absorbed, one electron is transferred. The reaction center isolated from the purple bacterium Rhodobacter sphaeroides is particularly useful for the study of electron transfer because this complex has been well characterized biochemically and spectroscopically, and the three-dimensional structure has been determined by x-ray diffraction (2-5). The reaction center has two core subunits, L and M, that form the binding sites for the cofactors and are related by an Ϸ2-fold symmetry axis. The primary electron donor (P) consists of two symmetry-related bacteriochlorophyll (BChl) a molecules labeled P L and P M that overlap at the ring I position and are separated by Ϸ3.5 Å (Fig. 1). Light absorption causes the excitation of P, and within 3.5 ps, an electron is transferred to a bacteriopheophytin acceptor. Electron transfer proceeds to the primary quinone acceptor, Q A , in Ϸ200 ps and to the secondary quinone, Q B , in Ϸ200 s. After reduction of the donor by cytochrome c 2 , absorption of a second photon leads to the transfer of a second electron to Q B , which then leaves the protein carry...

Photochemically induced nuclear spin polarization in reaction centers of photosystem II observed by13C-solid-state NMR reveals a strongly asymmetric electronic structure of the P680.+ primary donor chlorophyll

“…Clearly, a regular hydrogen bond is not sufficient to provoke a shift of the spin density to the ring III. This is in line with the work on the bacterial RC (48), where it was found that hydrogen bonding and similarly weak interactions affect the redox potential only moderately by Ϸ50 mV, an order of magnitude less than the actual value reported for PS2. In this way our calculations imply that, if a protein-cofactor interaction at the ring V keto carbonyl oxygen is responsible for the increased redox potential, it should be very strong.…”

“…It has been shown that protein-cofactor interactions at the keto group of the BChl in the bacterial RC can increase the redox potential of the radical cations by stabilizing the orbital containing the unpaired electron (48). According to the photo-CIDNP, such a type of interaction may also be a predominant factor in generating the unusually high redox potential of 1.2 V of P 680 .…”

Asymmetric Electron Transfer in Reaction Centers of Purple Bacteria Strongly Depends on Different Electron Matrix Elements in the Active and Inactive Branches

“…For deriving the polarizability of the LUMO(+) of P, we used the simple Hückel approach as described by Artz et al 40 The LUMOs of the two monomers were considered to be two one-electron states having nondiagonal coupling matrix elements, . 22 for Rps.…”

Tuning electron transfer by ester‐group of chlorophylls in bacterial photosynthetic reaction center

“…The spin density distribution of the BChla monomers of SP corresponds approximately to the charge distribution. Mutational studies on residues in the neighborhood of SP showed that its spin density dependents on the H-bond pattern of the BChla monomers [11,12]. Electrostatic computations suggested that small conformational changes of substituents at BChla can shift the midpoint redox potential for SP (E m (SP)), which should also affect the charge distribution pattern of the SP [13,14].…”