Electron Nuclear Double Resonance of the Chlorophyll Triplet State in the Water-Soluble Chlorophyll Protein from Brassic
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Applied Magnetic Resonance
ORIGINAL PAPER
Electron Nuclear Double Resonance of the Chlorophyll Triplet State in the Water‑Soluble Chlorophyll Protein from Brassica oleracea: Investigation of the Effect of the Binding Site on the Hyperfine Couplings Alessandro Agostini1,2 · Daniel M. Palm2 · Harald Paulsen2 · Marilena Di Valentin1 · Donatella Carbonera1 Received: 10 July 2020 / Revised: 3 August 2020 © The Author(s) 2020
Abstract An investigation of the photoexcited triplet state of chlorophyll (Chl) a in the watersoluble chlorophyll protein (WSCP) of Brassica oleracea has been carried out by means of electron-nuclear double resonance (ENDOR), achieving a complete assignment of the observed hyperfine couplings corresponding to methine protons and methyl groups of Chl a triplet state. The triplet-state properties, and in particular the hyperfine couplings, were found to be similar to those previously reported for Chl a in the WSCP of Lepidium virginicum. Therefore, the porphyrin ring deformation observed in Brassica oleracea WSCP seems to only slightly affect the spin density of 3Chl a. This may be relevant when considering the robustness of triplet–triplet energy transfer mechanisms, relying on wavefunction overlap, in systems, such as the photosynthetic light-harvesting complexes, in which Chl triplet states with distorted geometries are involved.
1 Introduction The water-soluble chlorophyll protein (WSCP) [1] has emerged in recent years as a promising model system for the investigation of tetrapyrroles in a protein environment [2], by virtue of its structural simplicity [3–5], high stability [6–8], and capability to be reconstituted in vitro with different tetrapyrroles [9–11]. Upon * Alessandro Agostini [email protected] * Donatella Carbonera [email protected] 1
Department of Chemical Sciences, University of Padova, Via Marzolo 1, 35131 Padua, Italy
2
Institute of Molecular Physiology, Johannes Gutenberg-University, Johannes‑von‑Müller‑Weg 6, 55128 Mainz, Germany
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tetramerization of four identical protein monomers, each binding one tetrapyrrole, a homotetrameric complex in which the tetrapyrroles are organized into a “dimer of dimers” configuration is obtained (Fig. 1A) [3–6]. The four symmetry-related chlorophyll (Chl) binding sites are identical, and therefore, the four bound chromophores experience identical protein surroundings and are spectroscopically equivalent, contrarily to chlorophyll (Chl)-binding complexes involved in photosynthesis, in which tens to hundreds of chromophores are bound in finely tuned individual binding sites [12], resulting in highly complex spectra. It follows that WSCP is an ideal model system for detailed spectroscopic investigation focused on understanding Chl–protein and Chl–Chl interactions [5, 13–20]. Brassicaceae WSCPs are categorized into two subclasses, IIa and IIb, according to their different Chl a/b ratio [21, 22]. Key amino acids in the protein sequence of class IIa and IIb WSPCs have been l
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