Effects of Hydrogen Bonds on the Redox Potential and Electronic Structure of the Bacterial Primary Electron Donor

Abstract

The primary donor, P, of photosynthetic bacterial reaction centers (RCs) is a dimer of excitonically interacting bacteriochlorophyll (BChl) molecules. The two constituents are named PL and PM to designate their close association with the L- and M-subunits, respectively, of the RC protein. A series of site-directed mutants of RCs from Rhodobacter sphaeroides has been constructed in order to model the effects of hydrogen bonding on the redox midpoint potential and electronic structure of P. The leucine residue at position M160 was genetically replaced with eight other amino acid residues capable of donating a hydrogen bond to the C9 keto carbonyl group of the PM BChl a molecule of P. Fourier transform (FT) (pre)resonance Raman spectroscopy with 1064 nm excitation was used to (i) determine the formation and strengths of hydrogen bonds on this latter keto carbonyl group in the reduced, neutral state (PO), and (ii) determine the degree of localization of the positive charge on one of the two constituent BChl molecules of P in its oxidized, radical cation state (P*+). A correlation was observed between the strength of the hydrogen bond and the increase in PO/P*+ redox midpoint potential. This correlation is less pronounced than that observed for another series of RC mutants where hydrogen bonds to the four pi-conjugated carbonyl groups of P were broken or formed uniquely involving histidinyl residues [Mattioli, T. A., Lin, X., Allen, J. P. and Williams, J. C. (1995) Biochemistry 34, 6142-6152], indicating that histidinyl residues are more effective in raising the PO/P*+ redox midpoint potential via hydrogen bond formation than are other hydrogen bond-forming residues. In addition, an increase in positive charge localization is correlated with the strength of the hydrogen bond and with the PO/P*+ redox midpoint potential. This latter correlation was analyzed using an asymmetric bacteriochlorophyll dimer model based on Hückel-type molecular orbitals in order to obtain estimates of certain energetic parameters of the primary donor. Based on this model, the correlation is extrapolated to the case of complete localization of the positive charge on PL and gives a predicted value for the P/P+ redox midpoint potential similar to that experimentally determined for the Rb. sphaeroides HL(M202) heterodimer. The model yields parameters for the highest occupied molecular orbital energies of the two BChl a constituents of P which are typical for the oxidation potential of isolated BChl a in vitro, suggesting that the protein, as compared to many solvents, does not impart atypical redox properties to the BChl a constituents of P.

Structural, dynamic, and energetic aspects of long-range electron transfer in photosynthetic reaction centers

“…Tyrosine and histidine both donate hydrogen bonds of similar strength to P; yet, the effect of tyrosine on E m is much weaker. Histidine side chains, however, have a strong dipole moment and thus may raise E m by charge-dipole interaction (38,42). The increased polarity in the environment of the cofactor is also expected to lead to an increase of , as seen in Fig.…”

“…Stark effect spectroscopy (35), photochemically induced dynamic nuclear polarization (photo-CIDNP), 13 C solid-state NMR (36), EPR, ENDOR (electron-nuclear double resonance), and TRIPLE (electron-nuclear-nuclear triple resonance) studies (37,38) all have shown asymmetric electron distributions in the ground, excited, and ionized states of the special pair in wild-type RCs, and addition or removal of hydrogen bonds between the special pair and the protein matrix affects the spin distribution between the P L and P M BChl monomers (37).…”

Hydrogen Bonding in a Model Bacteriochlorophyll-binding Site Drives Assembly of Light Harvesting Complex

“…In the case of the acetyl group, hydrogen bonding to neighboring residues modulate the functional properties, specifically the excitation energies of BChl a in light harvesting (LH) complexes from purple bacteria (3,4). It has also been suggested that hydrogen bonding to the acetyl group contributes to the tuning of the redox energies of P870 in bacterial reaction centers (5), but this is still under some dispute (6). Hydrogen bonding to the C13 1 keto carbonyl at the isocyclic ring, which is common to all chlorophylls, seems to be widespread (6 -9) but appears to have less or no effect on the electronic properties of (B)Chl (6,9,10).…”

Structural Role of (Bacterio)chlorophyll Ligated in the Energetically Unfavorable β-Position

“…The carbonyl groups frequently interact with the surrounding polypeptide and/or neighboring pigments. In particular, the H-bonding state of the acetyl carbonyl of BChls has been recognized to modulate the spectral and, although still disputed, redox properties of these molecules (10). H-bonding to the C- 13 1 keto carbonyl at the isocyclic ring, which is common to all bacteriochlorophylls and chlorophylls, seems to be widespread (see e.g.…”