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          Institute: MPI für medizinische Forschung     Collection: Abteilung Biophysik     Display Documents

ID: 23102.0, MPI für medizinische Forschung / Abteilung Biophysik
Orientation and Protein-Cofactor Interactions of Monosubstituted n-Alkyl Naphthoquinones in the A1 Binding Site of Photosystem I
Translation of Title:Orientation and Protein-Cofactor Interactions of Monosubstituted n-Alkyl Naphthoquinones in the A<sub>1</sub> Binding Site of Photosystem I
Authors:Pushkar, Yulia N.; Zech, Stephan G.; Stehlik, Dietmar; Brown, Sarah; van der Est, Art; Zimmermann, Herbert
Date of Publication (YYYY-MM-DD):2002-10-24
Title of Journal:Journal of Physical Chemistry B
Journal Abbrev.:J. Phys. Chem. B
Issue / Number:46
Start Page:12052
End Page:12058
Review Status:Peer-review
Audience:Experts Only
Intended Educational Use:No
Abstract / Description:1,4-Naphthoquinone (NQ) and its monosubstituted derivatives, 2-methyl, 2-ethyl, and 2-butyl-1,4-naphthoquinone, have been incorporated into the A1 binding site of photosystem I (PS I) after organic solvent extraction of the native phylloquinone. The charge separated state P700+NQ- has been studied by multifrequency transient EPR. The Q-band (35 GHz) spectra of fully deuterated 2-methyl-1,4-NQ-d8 and 2-ethyl-1,4-NQ-d10 show sufficient spectral resolution for the orientation of the quinone g tensor and thus the headgroup of the molecule to be determined. All orientation parameters of the substituted NQs are found to be the same as those established for native phylloquinone in PS I. However, for 2-ethyl-1,4-NQ and 2-butyl-1,4-NQ the X- and Q-band spectra exhibit a well resolved 1:2:1 hyperfine splitting. From the fact that it is absent when the first methylene group of the side chain is selectively deuterated, the splitting is assigned to the hyperfine coupling of the methylene protons. The principal values of the axially symmetric hyperfine coupling tensor are determined to be Axx = 12.2 MHz, Ayy = 16.8 MHz, Azz = 12.2 MHz, and aiso = 13.7 MHz. The large methylene proton hyperfine coupling arises from a high spin density on the ring carbon atom to which the alkyl tail is attached. This in turn suggests that only one of the carbonyl groups of 2-alkyl-1,4-NQ is H-bonded to the protein and that the alkyl tail must be in the ortho position relative to the carbonyl group with the H bond. This implies that the alkyl side chain of the substituted NQs resides in the space normally occupied by the methyl group of phylloquinone and not that of the phytyl tail, which is meta to the H-bonded carbonyl group according to the X-ray structure. In addition, the hyperfine tensor indicates that the first two C-C bonds of the alkyl tail must be coplanar with the aromatic ring. However, the X-ray structure of PS I shows, for the native phylloquinone, that the phytyl tail is bent out of the quinone plane with the second C-C bond.
Comment of the Author/Creator:This work was supported by grants from the Deutsche Forschungsgemeinschaft (SFB 498, TPA3 to D.S. and S.Z.) and the Natural Sciences and Engineering Research Council, The Canada Foundation for Innovation, and The Ontario Innovation Trust (to A.v.d.E.).
External Publication Status:published
Document Type:Article
Communicated by:Wulf Kaiser
Affiliations:MPI für medizinische Forschung/Abteilung Biophysik
External Affiliations:Institut für Experimentalphysik, Freie Universität Berlin, Arnimallee 14, D-14195 Berlin, Germany; Department of Chemistry, Brock University, 500 Glenridge Ave., St. Catharines, Ontario, L2S 3A1, Canada
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