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The photosynthetic reaction center in purple bacteria

The photosynthetic reaction center in purple bacteria


Michael Hutter

School of Chemistry F11
University of Sydney


NSW 2006
Australia


Introduction:

The photosynthetic reaction center in Rhodospeudomonas viridis has four protein subunits: The C (grey), L (cyan), M (orange) and H (red) chain as well as four heme cytochromes.

1prc: the 4 chains 1prc: the secondary structure


The L and M chains are transmembrane proteins. The actual reaction center lies in the transmembrane part and consists of four bacteriochlorophyll b molecules, two bacteriopheophytin b molecules, a menaquinone, an ubiquinone, and a non-heme iron atom.

1prc: the reaction center

Upon light irradiation the central two bacteriochlorophylls are excited which form the special pair (P). The excited dimer (P*) then transfers an electron to the bacteriochlorphyll (BChlL) and the bacteriopheophytin (BPhL) on the L side of the branch within 3 ps. The resulting radical pair (P+BPhL-) decays in about 200 ps moving the electron to the menaquinone (QA) from where it is further transported to the ubiquinone (QB). 

excitation scheme

The reason why the L side is preferred to the M side despite the apparent (pseudo-C2) symmetry is not clear yet. 

Investigations:

Currently we are investigating the influence of the reaction centre protein onto the structure of the special pair and the other cofactors using a combined QM/MM-method. While the quantum mechanical part containing the chromophores is treated on the basis of semiempirical AM1 theory, atom centered point charges for the surrounding protein are used. The necessary Magnesium parameters for AM1 were developed recently in our group. QM/MM model


Modeling the bacterial photosynthetic reaction centre
2. A combined quantum mechanical/molecular mechanical study of the structure of the cofactors in the reaction centre of purple bacteria

M. C. Hutter, J. M. Hughes, J. R. Reimers, N. S. Hush

J. Phys. Chem. B 103 (1999) 4906-4915

Abstract:

Ab initio and other such computational studies of bacterial reaction centre cofactors are usually either performed at observed (low resolution) X-ray structures. Unfortunately, these geometries are quite crude and this can have drastic influences on calculated properties. For example, the total energies of the four bacteriochlorophylls vary over 160 kcal mol -1 ! Here, to overcome this problem, a combined quantum mechanical/ molecular mechanical (QM/MM) method is employed to optimize the structure of the special pair and other cofactors in the photosynthetic reaction centres of Rhodospeudomonas viridis and Rhodobacter sphaeroides. Specifically, these optimizations are performed using a semiempirical AM1-based formalism of the QM/MM-method with the coordinates of the surrounding protein frozen. After relaxation, the energies of the bacteriochlorophylls differ by only typical conformer energies, ca. 3 kcal mol-1. Another example of improved cofactor properties is the PL-PM interaction energy which has been predicted to be strongly repulsive at the X-ray structure but here is shown to be realistically attractive after optimization. After optimization, the distortions in the geometries of the cofactors are seen to be controlled by protein-cofactor interactions, and the cofactors on the L side are all seen to fit more snugly into the protein environment than do their M-side counterparts. Also, the 2a-acetyl group of PM for Rb. sphaeroides, for which hydrogen bonding to the protein is restricted, is predicted to form a weakly-bound sixth ligand to the magnesium of PL; this is consistent with, but not obvious from, the X-ray structure.


Nature of the Special-Pair Radical Cation in Bacterial Photosynthesis


J. R. Reimers, M.C. Hutter, J. M. Hughes, N. S. Hush
Int. J. Quant. Chem. 80 (2000) 1224-1243

Abstract:

Primary charge separation in bacterial photosynthesis occurs at the "special pair", a bacteriochlorophyll dimer that, on optical excitation, ejects an electron to become the special-pair radical cation. Understanding the nature of this species is important to both the charge separation process itself and details of subsequent steps including charge recombination. Electron spin resonance (ESR)-based studies have led to the conclusion that the positive charge is delocalized over both bacteriochlorophyll monomers, the degree of delocalization being affected by site-directed mutagenesis. However, Breton et al. have observed charge-transfer electronic absorption spectra (centred at ca. 2500 cm-1), which, when interpreted using standard electron-transfer theory, indicate strong charge localization on just one of the bacteriochlorophylls. We present a complex computational strategy aimed at resolving this issue through vibronic coupling analysis of the high- and low-resolution spectra using a priori computed vibrational analyses, quantum chemical calculation of the strengths of a variety of key interactions, quantum mechanical/molecular mechanical (QM/MM) calculation of the structure of over 20 mutant reaction centers, calculation and interpretation of the observed midpoint potentials, analyses relevant to ESR and related spectroscopies, etc. The current state of progress is described, leading to a consistent picture that includes almost all available experimental data.
under construction

bio hazardous

pictures are made with the down loadable WebLabTM Viewer from MSI.


M. Hutter November, 20th 2000

Last update: November 2000 MCH © MPI Biophysics

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