Photosynthesis: Antennae and Reaction Centers

Amesz, J.

doi:10.1007/978-3-642-73023-8_6

Cited by 3 publications

(3 citation statements)

References 41 publications

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“…Within the photosynthetic reaction centers of plants and bacteria, magnesium-containing porphyrin systems play a central role in the initial step of generating chemical energy from light irradiation. , The actual system that has to be considered in quantum mechanical calculations of these processes is, however, quite large. Experimental and theoretical studies on a variety of purple bacteria have shown that there are many chlorophyll and pheophytin molecules involved, prohibiting high-level ab initio calculations thus far. − There are, however, a number of existing semiempirical methods designed to handle larger molecular systems than is possible ab initio.…”

Section: Introductionmentioning

confidence: 99%

Modeling the Bacterial Photosynthetic Reaction Center. 1. Magnesium Parameters for the Semiempirical AM1 Method Developed Using a Genetic Algorithm

1998

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Magnesium parameters for use with the semiempirical AM1 method are developed using a specially designed genetic algorithm. Parametrization priorities included development of a robust parametrization capable of describing a wide range of properties in diverse chemical environments, with emphasis on structural features of biologically relevant systems, e.g., chlorophylls. Specifically, the test data set included a selection of the heats of formation, geometric properties, dipole moment, and ionization energies evaluated for 32 compounds including halides, oxides, hypervalent compounds, organometallics, and porphyrins. In addition, calculated properties for an additional 27 molecules are used as an independent test on the quality of the parametrization obtained. Reference data are taken from eitherexperiment or previous ab initio calculations or evaluated using ab initio or density functional theory. For comparison, analogous results for all 59 molecules are obtained using the semiempirical PM3 and MNDO/d methods. Both AM1 and MNDO/d are found to be robust and widely applicable for magnesium compounds while the applicability of PM3 appears significantly restricted. MNDO/d appears the method of choice for ionization potentials and heats of formation while, reflecting our parametrization priorities, AM1 appears the method of choice for geometrical properties, especially those of magnesium porphyrins.

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Section: Introductionmentioning

confidence: 99%

Modeling the Bacterial Photosynthetic Reaction Center. 1. Magnesium Parameters for the Semiempirical AM1 Method Developed Using a Genetic Algorithm

1998

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“…Electronic interactions between the π systems of two or more porphyrin molecules in close proximity are believed to be important in defining the properties of the supramolecular ensemble. Notable examples of such interacting porphyrin π systems are those found in photosynthetic proteins and in linearly stacked arrays that are organic conductors . The photosynthetic proteins include the light-harvesting chlorophyll arrays and the photosynthetic reaction center (RC) special pairs .…”

Section: Introductionmentioning

confidence: 99%

Metalloporphyrin Mixed-Valence π-Cation Radicals: Solution Stability and Properties

1997

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Solution equilibria and optical spectra of several metallooctaethylporphyrin π-cation radicals have been examined in methylene chloride solution. The several π-cation radical species, [M(OEP•)]Y (M = Cu, Ni, Zn, Pd, or VO; Y = ClO4 - or SbCl6 -), are found to dimerize to form [M(OEP•)]2 2+. These dimeric species are characterized by the appearance of a new, strongly concentration-dependent near-infrared absorption band. This band is found in the region of 900−960 nm for all compounds except for the vanadyl complex which absorbs farther to the red at 1375 nm. The equilibrium constants for the dimerization reaction (2[M(OEP•)]+ (K D) [M(OEP•)]2 2+) have been evaluated from multiple-wavelength, concentration-dependent absorption data. Equimolar solutions of these π-cation radicals and their analogous neutral M(OEP) derivatives react to form new binuclear species, [M(OEP•/2)]2 +, in which the single radical electron is delocalized over both porphyrin rings. These new species, for which we suggest the term “mixed-valence” π-cation radical and which bear a formal relationship to the oxidized special pair of photosynthetic reaction centers, also display a near-infrared absorption band. The near-IR band is however distinctly different from that of the π-cation radical dimers. Equilibrium constants for the formation of these new species (M(OEP) + [M(OEP•)]+ (K MV) [M(OEP•/2)]2 +), have also been determined. The values of K MV are 10−100-fold larger than the analogous dimerization constant K D. Values of ΔH and ΔS have been obtained for both equilibrium processes. The variations in K MV and K D values are largely the consequence of entropy differences that probably result from a much stronger solvation of the π-cation radical compared to the neutral porphyrin.

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“…Electronic interactions between the π-systems of two or more porphyrin molecules in close proximity are believed to be important in defining the properties of the supramolecular ensemble. Notable examples of such interacting porphyrin π systems are those found in photosynthetic proteins and in linearly stacked arrays that are organic conductors . The photosynthetic proteins include the light-harvesting chlorophyll arrays and the photosynthetic reaction center (RC) special pairs .…”

Section: Introductionmentioning

confidence: 99%

Metalloporphyrin Mixed-Valence π-Cation Radicals: Solid-State Structures

et al. 1996

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The synthesis and structural characterization of three mixed-valence π-cation radical derivatives of (octaethylporphinato)metal(II) species, [M(OEP•/2)]2 +, (M = Ni, Cu, and (Zn,H2)), are reported. The species thus have the same formal ring oxidation level as the photooxidized special pair of photosynthetic reaction centers. The complexes display unique binuclear structures with cofacial porphyrin rings with mean plane separations of 3.20−3.32 Å, ring center to ring center distances of 3.50−3.55 Å, and lateral shifts between the two rings of 1.16−1.53 Å. These inter-ring geometries are distinctly different from either those of “classical” π-cation radical derivatives or those of dimers formed by neutral species. The different inter-ring geometries suggests that there are inherent inter-ring geometries that depend principally on the oxidation level of the porphyrin rings.

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Photosynthesis: Antennae and Reaction Centers

Cited by 3 publications

References 41 publications

Modeling the Bacterial Photosynthetic Reaction Center. 1. Magnesium Parameters for the Semiempirical AM1 Method Developed Using a Genetic Algorithm

Modeling the Bacterial Photosynthetic Reaction Center. 1. Magnesium Parameters for the Semiempirical AM1 Method Developed Using a Genetic Algorithm

Metalloporphyrin Mixed-Valence π-Cation Radicals: Solution Stability and Properties

Metalloporphyrin Mixed-Valence π-Cation Radicals: Solid-State Structures

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