| 1. |
- Blomberg, Margareta R A, et al.
(författare)
-
A Quantum Chemical Study of Hydrogen Abstraction from Manganese-Coordinated Water by a Tyrosyl Radical: A Model for Water Oxidation in Photosystem II
- 1997
-
Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 119:35, s. 8285-8292
-
Tidskriftsartikel (refereegranskat)abstract
- Recently, water oxidation in photosystem II was proposed to involve direct abstraction of hydrogen atoms from water molecules terminally ligated to manganese ions in the oxygen-evolving complex by the oxidized tyrosine radical, TyrZ. This model is tested here by performing quantum chemical calculations. An empirically parametrized hybrid density functional method is used, and both monomeric and dimeric manganese model systems are studied. It is found that, by coordination to a manganese center, the first O-H bond strength of water is lowered from 113.4 to 84.3 kcal/mol. This O-H bond strength is only 2.8 kcal/mol stronger than that in tyrosine. Using an extended basis set, we find that this difference decreases still further. The second hydrogen abstraction energy is quite similar. Since thermoneutrality in the reaction (or a weak exothermicity) is a requirement for the hydrogen abstraction model, the present calculations support this model. Possible functions of a coordinated chloride and a nearby calcium complex are suggested. Five- or six-coordination and ferro- or antiferromagnetic spin couplings of the manganese centers are discussed.
|
|
| 2. |
- Magnuson, Ann, et al.
(författare)
-
Mimicking electron transfer reactions in photosystem II : synthesis and photochemical characterization of a ruthenium(II) tris(bipyridyl) complex with a covalently linked tyrosine.
- 1997
-
Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 119:44, s. 10720-10725
-
Tidskriftsartikel (refereegranskat)abstract
- In the natural photosynthetic reaction center photosystem II, absorption of a photon leads to photooxidationof the primary electron donor P680, which subsequently retrieves electrons from a tyrosyl residue, functioning as aninterface to the oxygen-evolving manganese complex. In a first step toward mimicking these reactions, we havemade a Ru(II)-polypyridine complex with an attached tyrosyl moiety. The photoexcited ruthenium complex playedthe role of P680and was first oxidized by external acceptors. Combined transient absorbance and EPR studies provided evidence that the Ru(III) formed was reduced by intramolecular electron transfer from the attached tyrosine, with a rate constant of 5104s-1. Thus we show that a tyrosine radical could be formed by light-induced electrontransfer reactions, and we indicate future directions for developing a closer analogy with the photosystem II reactions.
|
|
| 3. |
|
|
| 4. |
- Schuettpelz, Eric, et al.
(författare)
-
A community-derived classification for extant lycophytes and ferns
- 2016
-
Ingår i: Journal of Systematics and Evolution. - : Wiley. - 1674-4918 .- 1759-6831. ; 54:6, s. 563-603
-
Tidskriftsartikel (refereegranskat)abstract
- Phylogeny has long informed pteridophyte classification. As our ability to infer evolutionary trees has improved, classifications aimed at recognizing natural groups have become increasingly predictive and stable. Here, we provide a modern, comprehensive classification for lycophytes and ferns, down to the genus level, utilizing a community-based approach. We use monophyly as the primary criterion for the recognition of taxa, but also aim to preserve existing taxa and circumscriptions that are both widely accepted and consistent with our understanding of pteridophyte phylogeny. In total, this classification treats an estimated 11 916 species in 337 genera, 51 families, 14 orders, and two classes. This classification is not intended as the final word on lycophyte and fern taxonomy, but rather a summary statement of current hypotheses, derived from the best available data and shaped by those most familiar with the plants in question. We hope that it will serve as a resource for those wanting references to the recent literature on pteridophyte phylogeny and classification, a framework for guiding future investigations, and a stimulus to further discourse.
|
|
| 5. |
|
|
