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Which Factors Determine the Acidity of the Phytochromobilin Chromophore of Plant Phytochrome?
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Physical and Analytical Chemistry, Quantum Chemistry.
2008 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, Vol. 10, no 18, 2528-2537 p.Article in journal (Refereed) Published
Abstract [en]

Quantum chemical calculations aimed at identifying the factors controlling the acidity of phytochromobilin, the tetrapyrrole chromophore of the plant photoreceptor phytochrome, are reported. Phytochrome is converted from an inactive (Pr) to an active form (Pfr) through a series of events initiated by a Z --> E photoisomerization of phytochromobilin, forming the Lumi-R intermediate, and much controversy exists as to whether the protonation state of the chromophore (cationic in Pr with all nitrogens protonated) changes during the photoactivation. Here, relative ground (S0) and excited-state (S1) pKa s of all four pyrrole moieties of phytochromobilin in all 64 possible configurations with respect to the three methine bridges are calculated in a protein-like environment, using a recently benchmarked level of theory. Accordingly, the relationships between acidity and chromophore geometry and charge distribution, hydrogen bonding, and light absorption are investigated in some detail, and discussed in terms of possible mechanisms making a proton transfer reaction more probable along the Pr --> Pfr reaction than in the parent cationic Pr state. It is found that charge distribution in the cationic species, intra-molecular hydrogen bonding in the neutral, and hydrogen bonding with two highly conserved aspartate and histidine residues have a significant effect on the acidity, while overall chromophore geometry and electronic state are less important factors. Furthermore, based on the calculations, two processes that may facilitate a proton transfer by substantially lowering the pKa s relative to their Pr values are identified: (i) a thermal Z,anti --> Z,syn isomerization at C5, occurring after formation of Lumi-R; (ii) a perturbation of the hydrogen bonding network which in Pr comprises the nitrogens of pyrroles A, B and C and the two aspartate and histidine residues.

Place, publisher, year, edition, pages
2008. Vol. 10, no 18, 2528-2537 p.
National Category
Chemical Sciences
URN: urn:nbn:se:uu:diva-96765DOI: 10.1039/b719190aISI: 000255449200011PubMedID: 18446253OAI: oai:DiVA.org:uu-96765DiVA: diva2:171446
Available from: 2008-02-26 Created: 2008-02-26 Last updated: 2010-01-07Bibliographically approved
In thesis
1. Theoretical Photochemistry: Halogenated Arenes, Phytochromobilin, Ru(II)polypyridyl complexes and 6-4 photoadducts
Open this publication in new window or tab >>Theoretical Photochemistry: Halogenated Arenes, Phytochromobilin, Ru(II)polypyridyl complexes and 6-4 photoadducts
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis presents Quantum Chemical calculations on the Photochemistry of Halogenated benzenes, Phytochromobilin, Ruthenium Polypyridyl complexes and 6-4 photoadducts in DNA. The work is focused on improving the understanding of a number of experimentally observed photochemical processes in these systems. New results regarding the mechanism of photodissociation of halogenated arenes, photointerconversion of phytochromobilin are presented, as well as of the photoprocesses of Ruthenium Polypyridyl complexes and new mechanistic insights in the repair of 6-4 photoadducts in DNA.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2008. 61 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 394
Quantum chemistry, Quantum Chemistry, Halogenated Arenes, Phytochromobilin, Phytochrome, Ruthenium, 6-4 photoadducts, Photochemistry, Kvantkemi
National Category
Theoretical Chemistry
urn:nbn:se:uu:diva-8469 (URN)978-91-554-7096-8 (ISBN)
Public defence
2008-03-19, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Available from: 2008-02-26 Created: 2008-02-26 Last updated: 2011-03-14Bibliographically approved

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