A water-soluble chlorophyll-binding protein (WSCP) is the single known instance of a putative chlorophyll (Chl) carrier in green plants. Recently the photoprotective function of WSCP has been demonstrated by EPR measurements; the light-induced singlet-oxygen formation of Chl in the WSCP tetramer is about four times lower than that of unbound Chl. This paper describes the crystal structure of the WSCP-Chl complex purified from leaves of Lepidium virginicum (Virginia pepperweed) to clarify the mechanism of its photoprotective function. The WSCP-Chl complex is a homotetramer comprising four protein chains of 180 amino acids and four Chl molecules. At the center of the complex one hydrophobic cavity is formed in which all of the four Chl molecules are tightly packed and isolated from bulk solvent. With reference to the novel Chl-binding mode, we propose that the photoprotection mechanism may be based on the inhibition of physical contact between the Chl molecules and molecular oxygen.Water-soluble chlorophyll-binding proteins (WSCPs), 4 which form a complex with chlorophyll (Chl), have been isolated from Amaranthaceae, Chenopodiaceae, and Polygonaceae (class-I) and from Brassicaceae (class-II). The two WSCP classes differ in that the protein-Chl complexes of the former change their absorption spectra upon illumination, whereas those of the latter show no photoconversion of their absorption behavior (2). The amino acid sequence of class-I WSCP shows no similarity to that of class-II WSCP.The class-II WSCPs (hereafter referred to as "WSCPs") are water-soluble proteins of ϳ20 kDa, which form a tetrameric assembly upon binding Chl molecules with a Chl-to-protein ratio of one or less, and exhibit high thermal and photostability (1, 3). The physiological function of WSCPs is still not known. Although WSCPs have a sequence similarity of ϳ30% with Künitz-type proteinase inhibitors, no significant protease inhibitor activity of WSCPs has yet been identified (4 -6). The low Chl content per protein makes it unlikely that WSCPs are involved in the light reaction of photosynthesis. Meanwhile, it has been speculated (7, 8) that WSCP acts as a scavenger of free Chl, by transporting it from the thylakoid membrane to the chloroplast envelope, where Chlase, the enzyme that initiates the Chl catabolism, is thought to reside (9).An in vitro binding assay with the recombinant apo-WSCP cloned from cauliflower (Brassica oleralcea var. Botrys) and Chl or its derivatives showed that the central Mg 2ϩ ion and the phytyl tail of Chl were essential for protein-pigment binding and tetrameric assembly, respectively (1). Furthermore, the recombinant apo-WSCP was able to remove Chl from the thylakoid membrane in vitro and organize the tetrameric WSCPChl complex (3). The absorption spectrum of the reconstituted complex is very similar to that of the native WSCP-Chl complex purified from fresh leaves (1, 3).The WSCP-Chl complex retains its fresh green color under dim light for months, enough for a long term crystallization experiment. Free Chl, on t...

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The crystal structure of bis-(2-hydroxyacetophenone) trimethylenediiminocopper(II), [Cu-(hapt)], has been determined on the basis of the X-ray diffraction data. The crystals are monoclinic. The cell dimensions are: a=17.746, b=9.467, c=10.646 Å, and β=114.06°. The space group is Cc, with four formula units in the unit cell. The structure has been refined from threedimensional data by the least-squares method with anisotropic temperature factors to an R-value of 0.093. The copper atom is four-covalent, with the two planar 2-hydroxyacetophenone groups oriented so as to give a flattened tetrahedral co-ordination geometry. The six-membered coppertrimethylenediamine ring takes a “twisted boat” configuration. The Cu–O bonds are 1.88 and 1.89 Å in length, while the Cu–N bonds are 1.94 and 1.97 Å. The angle between the mean planes of the 2-hydroxyacetophenone moieties is 46.2°.

The molecular structure of thioxanthone has been determined from gas electron-diffraction data. The molecular parameter values and their uncertainties are rg(C–C)=1.401±0.001 Å, rg(C–H)=1.099±0.007 Å, rg(C–S)=1.751±0.002 Å, rg(C–Cc)=1.498±0.004 Å, rg(Cc–O)=1.232±0.006 Å, ∠CSC=103.4±0.3°, ∠CCcC=119.4±0.6°, and θ(dihedral angle)=169.0±1.6°, where the angle parameters are rα parameters. The dihedral angle of thioxanthone is much larger than those of related molecules.

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