Open AccessStructural, Photophysical, and Photochemical Characterization of Zinc Protoporphyrin IX in a Dimeric Variant of an Iron Storage Protein: Insights into the Mechanism of Photosensitized H2 Generation
Author(s) -
Brenda S. Benavides,
Rajendra Acharya,
Emily R. Clark,
Priyanka Basak,
Michael J. Maroney,
Judith M. Nocek,
Kirk S. Schanze,
Donald M. Kurtz
Publication year - 2019
Publication title -
the journal of physical chemistry. b
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.864
H-Index - 392
eISSN - 1520-6106
pISSN - 1520-5207
DOI - 10.1021/acs.jpcb.9b04817
Subject(s) - chemistry , protoporphyrin ix , porphyrin , photochemistry , photosensitizer , dimer , quenching (fluorescence) , electron transfer , protoporphyrin , heme , fluorescence , photodynamic therapy , biochemistry , organic chemistry , physics , quantum mechanics , enzyme
Some of us have previously reported the preparation of a dimeric form of the iron storage protein, bacterioferritin (Bfr), in which the native heme b is substituted with the photosensitizer, Zn(II)-protoporphyrin IX (ZnPP-Bfr dimer). We further showed that the ZnPP-Bfr dimer can serve as a photosensitizer for platinum-catalyzed H 2 generation in aqueous solution without the usually added electron relay between photosensitizer and platinum ( Clark , E. R. , Inorg. Chem. 2017 , 56 , 4584 - 4593 ). We proposed reductive or oxidative quenching pathways involving the ZnPP anion radical (ZnPP •- ) or the ZnPP cation radical, (ZnPP •+ ), respectively. The present report describes structural, photophysical, and photochemical properties of the ZnPP in the ZnPP-Bfr dimer. X-ray absorption spectroscopic studies at 10 K showed a mixture of five- and six-coordinated Zn centers with axial coordination by one long Zn-SγMet distance of ∼2.8 Å and ∼40% having an additional shorter Zn-S distance of ∼2.4 Å, in addition to the expected 4 nitrogen atom coordination from the porphyrin. The ZnPP in ZnPP-Bfr dimer was prone to photosensitized oxidation to ZnPP •+ . The ZnPP •+ was rapidly reduced by ascorbic acid, which we previously determined was essential for photosensitized H 2 production in this system. These results are consistent with an oxidative quenching pathway involving electron transfer from 3 ZnPP* to platinum, which may be assisted by a flexible ZnPP axial coordination sphere. However, the low quantum yield for H 2 production (∼1%) in this system could make reductive quenching difficult to detect, and can, therefore, not be completely ruled out. The ZnPP-Bfr dimer provides a simple but versatile framework for mechanistic assessment and optimization of porphyrin-photosensitized H 2 generation without an electron relay between porphyrin and the platinum catalyst.